runtime: Use runtime_m to get m value after call to runtime_mcall.

[gcc.git] / libgo / runtime / proc.c

// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

#include <limits.h>
#include <signal.h>
#include <stdlib.h>
#include <pthread.h>
#include <unistd.h>

#include "config.h"

#ifdef HAVE_DL_ITERATE_PHDR
#include <link.h>
#endif

#include "runtime.h"
#include "arch.h"
#include "defs.h"
#include "malloc.h"
#include "race.h"
#include "go-type.h"
#include "go-defer.h"

#ifdef USING_SPLIT_STACK

/* FIXME: These are not declared anywhere.  */

extern void __splitstack_getcontext(void *context[10]);

extern void __splitstack_setcontext(void *context[10]);

extern void *__splitstack_makecontext(size_t, void *context[10], size_t *);

extern void * __splitstack_resetcontext(void *context[10], size_t *);

extern void *__splitstack_find(void *, void *, size_t *, void **, void **,
                               void **);

extern void __splitstack_block_signals (int *, int *);

extern void __splitstack_block_signals_context (void *context[10], int *,
                                                int *);

#endif

#ifndef PTHREAD_STACK_MIN
# define PTHREAD_STACK_MIN 8192
#endif

#if defined(USING_SPLIT_STACK) && defined(LINKER_SUPPORTS_SPLIT_STACK)
# define StackMin PTHREAD_STACK_MIN
#else
# define StackMin 2 * 1024 * 1024
#endif

uintptr runtime_stacks_sys;

static void gtraceback(G*);

#ifdef __rtems__
#define __thread
#endif

static __thread G *g;
static __thread M *m;

#ifndef SETCONTEXT_CLOBBERS_TLS

static inline void
initcontext(void)
{
}

static inline void
fixcontext(ucontext_t *c __attribute__ ((unused)))
{
}

#else

# if defined(__x86_64__) && defined(__sun__)

// x86_64 Solaris 10 and 11 have a bug: setcontext switches the %fs
// register to that of the thread which called getcontext.  The effect
// is that the address of all __thread variables changes.  This bug
// also affects pthread_self() and pthread_getspecific.  We work
// around it by clobbering the context field directly to keep %fs the
// same.

static __thread greg_t fs;

static inline void
initcontext(void)
{
        ucontext_t c;

        getcontext(&c);
        fs = c.uc_mcontext.gregs[REG_FSBASE];
}

static inline void
fixcontext(ucontext_t* c)
{
        c->uc_mcontext.gregs[REG_FSBASE] = fs;
}

# elif defined(__NetBSD__)

// NetBSD has a bug: setcontext clobbers tlsbase, we need to save
// and restore it ourselves.

static __thread __greg_t tlsbase;

static inline void
initcontext(void)
{
        ucontext_t c;

        getcontext(&c);
        tlsbase = c.uc_mcontext._mc_tlsbase;
}

static inline void
fixcontext(ucontext_t* c)
{
        c->uc_mcontext._mc_tlsbase = tlsbase;
}

# else

#  error unknown case for SETCONTEXT_CLOBBERS_TLS

# endif

#endif

// We can not always refer to the TLS variables directly.  The
// compiler will call tls_get_addr to get the address of the variable,
// and it may hold it in a register across a call to schedule.  When
// we get back from the call we may be running in a different thread,
// in which case the register now points to the TLS variable for a
// different thread.  We use non-inlinable functions to avoid this
// when necessary.

G* runtime_g(void) __attribute__ ((noinline, no_split_stack));

G*
runtime_g(void)
{
        return g;
}

M* runtime_m(void) __attribute__ ((noinline, no_split_stack));

M*
runtime_m(void)
{
        return m;
}

// Set m and g.
void
runtime_setmg(M* mp, G* gp)
{
        m = mp;
        g = gp;
}

// The static TLS size.  See runtime_newm.
static int tlssize;

// Start a new thread.
static void
runtime_newosproc(M *mp)
{
        pthread_attr_t attr;
        size_t stacksize;
        sigset_t clear, old;
        pthread_t tid;
        int ret;

        if(pthread_attr_init(&attr) != 0)
                runtime_throw("pthread_attr_init");
        if(pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_DETACHED) != 0)
                runtime_throw("pthread_attr_setdetachstate");

        stacksize = PTHREAD_STACK_MIN;

        // With glibc before version 2.16 the static TLS size is taken
        // out of the stack size, and we get an error or a crash if
        // there is not enough stack space left.  Add it back in if we
        // can, in case the program uses a lot of TLS space.  FIXME:
        // This can be disabled in glibc 2.16 and later, if the bug is
        // indeed fixed then.
        stacksize += tlssize;

        if(pthread_attr_setstacksize(&attr, stacksize) != 0)
                runtime_throw("pthread_attr_setstacksize");

        // Block signals during pthread_create so that the new thread
        // starts with signals disabled.  It will enable them in minit.
        sigfillset(&clear);

#ifdef SIGTRAP
        // Blocking SIGTRAP reportedly breaks gdb on Alpha GNU/Linux.
        sigdelset(&clear, SIGTRAP);
#endif

        sigemptyset(&old);
        sigprocmask(SIG_BLOCK, &clear, &old);
        ret = pthread_create(&tid, &attr, runtime_mstart, mp);
        sigprocmask(SIG_SETMASK, &old, nil);

        if (ret != 0)
                runtime_throw("pthread_create");
}

// First function run by a new goroutine.  This replaces gogocall.
static void
kickoff(void)
{
        void (*fn)(void*);

        if(g->traceback != nil)
                gtraceback(g);

        fn = (void (*)(void*))(g->entry);
        fn(g->param);
        runtime_goexit();
}

// Switch context to a different goroutine.  This is like longjmp.
void runtime_gogo(G*) __attribute__ ((noinline));
void
runtime_gogo(G* newg)
{
#ifdef USING_SPLIT_STACK
        __splitstack_setcontext(&newg->stack_context[0]);
#endif
        g = newg;
        newg->fromgogo = true;
        fixcontext(&newg->context);
        setcontext(&newg->context);
        runtime_throw("gogo setcontext returned");
}

// Save context and call fn passing g as a parameter.  This is like
// setjmp.  Because getcontext always returns 0, unlike setjmp, we use
// g->fromgogo as a code.  It will be true if we got here via
// setcontext.  g == nil the first time this is called in a new m.
void runtime_mcall(void (*)(G*)) __attribute__ ((noinline));
void
runtime_mcall(void (*pfn)(G*))
{
        M *mp;
        G *gp;
#ifndef USING_SPLIT_STACK
        int i;
#endif

        // Ensure that all registers are on the stack for the garbage
        // collector.
        __builtin_unwind_init();

        mp = m;
        gp = g;
        if(gp == mp->g0)
                runtime_throw("runtime: mcall called on m->g0 stack");

        if(gp != nil) {

#ifdef USING_SPLIT_STACK
                __splitstack_getcontext(&g->stack_context[0]);
#else
                gp->gcnext_sp = &i;
#endif
                gp->fromgogo = false;
                getcontext(&gp->context);

                // When we return from getcontext, we may be running
                // in a new thread.  That means that m and g may have
                // changed.  They are global variables so we will
                // reload them, but the addresses of m and g may be
                // cached in our local stack frame, and those
                // addresses may be wrong.  Call functions to reload
                // the values for this thread.
                mp = runtime_m();
                gp = runtime_g();

                if(gp->traceback != nil)
                        gtraceback(gp);
        }
        if (gp == nil || !gp->fromgogo) {
#ifdef USING_SPLIT_STACK
                __splitstack_setcontext(&mp->g0->stack_context[0]);
#endif
                mp->g0->entry = (byte*)pfn;
                mp->g0->param = gp;

                // It's OK to set g directly here because this case
                // can not occur if we got here via a setcontext to
                // the getcontext call just above.
                g = mp->g0;

                fixcontext(&mp->g0->context);
                setcontext(&mp->g0->context);
                runtime_throw("runtime: mcall function returned");
        }
}

#ifdef HAVE_DL_ITERATE_PHDR

// Called via dl_iterate_phdr.

static int
addtls(struct dl_phdr_info* info, size_t size __attribute__ ((unused)), void *data)
{
        size_t *total = (size_t *)data;
        unsigned int i;

        for(i = 0; i < info->dlpi_phnum; ++i) {
                if(info->dlpi_phdr[i].p_type == PT_TLS)
                        *total += info->dlpi_phdr[i].p_memsz;
        }
        return 0;
}

// Set the total TLS size.

static void
inittlssize()
{
        size_t total = 0;

        dl_iterate_phdr(addtls, (void *)&total);
        tlssize = total;
}

#else

static void
inittlssize()
{
}

#endif

// Goroutine scheduler
// The scheduler's job is to distribute ready-to-run goroutines over worker threads.
//
// The main concepts are:
// G - goroutine.
// M - worker thread, or machine.
// P - processor, a resource that is required to execute Go code.
//     M must have an associated P to execute Go code, however it can be
//     blocked or in a syscall w/o an associated P.
//
// Design doc at http://golang.org/s/go11sched.

typedef struct Sched Sched;
struct Sched {
        Lock;

        uint64  goidgen;
        M*      midle;   // idle m's waiting for work
        int32   nmidle;  // number of idle m's waiting for work
        int32   nmidlelocked; // number of locked m's waiting for work
        int32   mcount;  // number of m's that have been created
        int32   maxmcount;      // maximum number of m's allowed (or die)

        P*      pidle;  // idle P's
        uint32  npidle;
        uint32  nmspinning;

        // Global runnable queue.
        G*      runqhead;
        G*      runqtail;
        int32   runqsize;

        // Global cache of dead G's.
        Lock    gflock;
        G*      gfree;

        uint32  gcwaiting;      // gc is waiting to run
        int32   stopwait;
        Note    stopnote;
        uint32  sysmonwait;
        Note    sysmonnote;
        uint64  lastpoll;

        int32   profilehz;      // cpu profiling rate
};

// The max value of GOMAXPROCS.
// There are no fundamental restrictions on the value.
enum { MaxGomaxprocs = 1<<8 };

Sched   runtime_sched;
int32   runtime_gomaxprocs;
uint32  runtime_needextram = 1;
bool    runtime_iscgo = true;
M       runtime_m0;
G       runtime_g0;      // idle goroutine for m0
G*      runtime_allg;
G*      runtime_lastg;
M*      runtime_allm;
P**     runtime_allp;
M*      runtime_extram;
int8*   runtime_goos;
int32   runtime_ncpu;
bool    runtime_precisestack;
static int32    newprocs;

void* runtime_mstart(void*);
static void runqput(P*, G*);
static G* runqget(P*);
static void runqgrow(P*);
static G* runqsteal(P*, P*);
static void mput(M*);
static M* mget(void);
static void mcommoninit(M*);
static void schedule(void);
static void procresize(int32);
static void acquirep(P*);
static P* releasep(void);
static void newm(void(*)(void), P*);
static void stopm(void);
static void startm(P*, bool);
static void handoffp(P*);
static void wakep(void);
static void stoplockedm(void);
static void startlockedm(G*);
static void sysmon(void);
static uint32 retake(int64);
static void incidlelocked(int32);
static void checkdead(void);
static void exitsyscall0(G*);
static void park0(G*);
static void goexit0(G*);
static void gfput(P*, G*);
static G* gfget(P*);
static void gfpurge(P*);
static void globrunqput(G*);
static G* globrunqget(P*, int32);
static P* pidleget(void);
static void pidleput(P*);
static void injectglist(G*);
static bool preemptall(void);
static bool exitsyscallfast(void);

// The bootstrap sequence is:
//
//      call osinit
//      call schedinit
//      make & queue new G
//      call runtime_mstart
//
// The new G calls runtime_main.
void
runtime_schedinit(void)
{
        int32 n, procs;
        const byte *p;
        Eface i;

        m = &runtime_m0;
        g = &runtime_g0;
        m->g0 = g;
        m->curg = g;
        g->m = m;

        initcontext();
        inittlssize();

        runtime_sched.maxmcount = 10000;
        runtime_precisestack = 0;

        runtime_mprofinit();
        runtime_mallocinit();
        mcommoninit(m);
        
        // Initialize the itable value for newErrorCString,
        // so that the next time it gets called, possibly
        // in a fault during a garbage collection, it will not
        // need to allocated memory.
        runtime_newErrorCString(0, &i);

        runtime_goargs();
        runtime_goenvs();
        runtime_parsedebugvars();

        runtime_sched.lastpoll = runtime_nanotime();
        procs = 1;
        p = runtime_getenv("GOMAXPROCS");
        if(p != nil && (n = runtime_atoi(p)) > 0) {
                if(n > MaxGomaxprocs)
                        n = MaxGomaxprocs;
                procs = n;
        }
        runtime_allp = runtime_malloc((MaxGomaxprocs+1)*sizeof(runtime_allp[0]));
        procresize(procs);

        // Can not enable GC until all roots are registered.
        // mstats.enablegc = 1;

        // if(raceenabled)
        //      g->racectx = runtime_raceinit();
}

extern void main_init(void) __asm__ (GOSYM_PREFIX "__go_init_main");
extern void main_main(void) __asm__ (GOSYM_PREFIX "main.main");

static void
initDone(void *arg __attribute__ ((unused))) {
        runtime_unlockOSThread();
};

// The main goroutine.
void
runtime_main(void* dummy __attribute__((unused)))
{
        Defer d;
        _Bool frame;
        
        newm(sysmon, nil);

        // Lock the main goroutine onto this, the main OS thread,
        // during initialization.  Most programs won't care, but a few
        // do require certain calls to be made by the main thread.
        // Those can arrange for main.main to run in the main thread
        // by calling runtime.LockOSThread during initialization
        // to preserve the lock.
        runtime_lockOSThread();
        
        // Defer unlock so that runtime.Goexit during init does the unlock too.
        d.__pfn = initDone;
        d.__next = g->defer;
        d.__arg = (void*)-1;
        d.__panic = g->panic;
        d.__retaddr = nil;
        d.__frame = &frame;
        g->defer = &d;

        if(m != &runtime_m0)
                runtime_throw("runtime_main not on m0");
        __go_go(runtime_MHeap_Scavenger, nil);
        main_init();

        if(g->defer != &d || d.__pfn != initDone)
                runtime_throw("runtime: bad defer entry after init");
        g->defer = d.__next;
        runtime_unlockOSThread();

        // For gccgo we have to wait until after main is initialized
        // to enable GC, because initializing main registers the GC
        // roots.
        mstats.enablegc = 1;

        main_main();
        if(raceenabled)
                runtime_racefini();

        // Make racy client program work: if panicking on
        // another goroutine at the same time as main returns,
        // let the other goroutine finish printing the panic trace.
        // Once it does, it will exit. See issue 3934.
        if(runtime_panicking)
                runtime_park(nil, nil, "panicwait");

        runtime_exit(0);
        for(;;)
                *(int32*)0 = 0;
}

void
runtime_goroutineheader(G *gp)
{
        const char *status;

        switch(gp->status) {
        case Gidle:
                status = "idle";
                break;
        case Grunnable:
                status = "runnable";
                break;
        case Grunning:
                status = "running";
                break;
        case Gsyscall:
                status = "syscall";
                break;
        case Gwaiting:
                if(gp->waitreason)
                        status = gp->waitreason;
                else
                        status = "waiting";
                break;
        default:
                status = "???";
                break;
        }
        runtime_printf("goroutine %D [%s]:\n", gp->goid, status);
}

void
runtime_printcreatedby(G *g)
{
        if(g != nil && g->gopc != 0 && g->goid != 1) {
                String fn;
                String file;
                intgo line;

                if(__go_file_line(g->gopc - 1, &fn, &file, &line)) {
                        runtime_printf("created by %S\n", fn);
                        runtime_printf("\t%S:%D\n", file, (int64) line);
                }
        }
}

struct Traceback
{
        G* gp;
        Location locbuf[100];
        int32 c;
};

void
runtime_tracebackothers(G * volatile me)
{
        G * volatile gp;
        Traceback tb;
        int32 traceback;

        tb.gp = me;
        traceback = runtime_gotraceback(nil);
        
        // Show the current goroutine first, if we haven't already.
        if((gp = m->curg) != nil && gp != me) {
                runtime_printf("\n");
                runtime_goroutineheader(gp);
                gp->traceback = &tb;

#ifdef USING_SPLIT_STACK
                __splitstack_getcontext(&me->stack_context[0]);
#endif
                getcontext(&me->context);

                if(gp->traceback != nil) {
                  runtime_gogo(gp);
                }

                runtime_printtrace(tb.locbuf, tb.c, false);
                runtime_printcreatedby(gp);
        }

        for(gp = runtime_allg; gp != nil; gp = gp->alllink) {
                if(gp == me || gp == m->curg || gp->status == Gdead)
                        continue;
                if(gp->issystem && traceback < 2)
                        continue;
                runtime_printf("\n");
                runtime_goroutineheader(gp);

                // Our only mechanism for doing a stack trace is
                // _Unwind_Backtrace.  And that only works for the
                // current thread, not for other random goroutines.
                // So we need to switch context to the goroutine, get
                // the backtrace, and then switch back.

                // This means that if g is running or in a syscall, we
                // can't reliably print a stack trace.  FIXME.

                if(gp->status == Grunning) {
                        runtime_printf("\tgoroutine running on other thread; stack unavailable\n");
                        runtime_printcreatedby(gp);
                } else if(gp->status == Gsyscall) {
                        runtime_printf("\tgoroutine in C code; stack unavailable\n");
                        runtime_printcreatedby(gp);
                } else {
                        gp->traceback = &tb;

#ifdef USING_SPLIT_STACK
                        __splitstack_getcontext(&me->stack_context[0]);
#endif
                        getcontext(&me->context);

                        if(gp->traceback != nil) {
                                runtime_gogo(gp);
                        }

                        runtime_printtrace(tb.locbuf, tb.c, false);
                        runtime_printcreatedby(gp);
                }
        }
}

static void
checkmcount(void)
{
        // sched lock is held
        if(runtime_sched.mcount > runtime_sched.maxmcount) {
                runtime_printf("runtime: program exceeds %d-thread limit\n", runtime_sched.maxmcount);
                runtime_throw("thread exhaustion");
        }
}

// Do a stack trace of gp, and then restore the context to
// gp->dotraceback.

static void
gtraceback(G* gp)
{
        Traceback* traceback;

        traceback = gp->traceback;
        gp->traceback = nil;
        traceback->c = runtime_callers(1, traceback->locbuf,
                sizeof traceback->locbuf / sizeof traceback->locbuf[0]);
        runtime_gogo(traceback->gp);
}

static void
mcommoninit(M *mp)
{
        // If there is no mcache runtime_callers() will crash,
        // and we are most likely in sysmon thread so the stack is senseless anyway.
        if(m->mcache)
                runtime_callers(1, mp->createstack, nelem(mp->createstack));

        mp->fastrand = 0x49f6428aUL + mp->id + runtime_cputicks();

        runtime_lock(&runtime_sched);
        mp->id = runtime_sched.mcount++;
        checkmcount();
        runtime_mpreinit(mp);

        // Add to runtime_allm so garbage collector doesn't free m
        // when it is just in a register or thread-local storage.
        mp->alllink = runtime_allm;
        // runtime_NumCgoCall() iterates over allm w/o schedlock,
        // so we need to publish it safely.
        runtime_atomicstorep(&runtime_allm, mp);
        runtime_unlock(&runtime_sched);
}

// Mark gp ready to run.
void
runtime_ready(G *gp)
{
        // Mark runnable.
        m->locks++;  // disable preemption because it can be holding p in a local var
        if(gp->status != Gwaiting) {
                runtime_printf("goroutine %D has status %d\n", gp->goid, gp->status);
                runtime_throw("bad g->status in ready");
        }
        gp->status = Grunnable;
        runqput(m->p, gp);
        if(runtime_atomicload(&runtime_sched.npidle) != 0 && runtime_atomicload(&runtime_sched.nmspinning) == 0)  // TODO: fast atomic
                wakep();
        m->locks--;
}

int32
runtime_gcprocs(void)
{
        int32 n;

        // Figure out how many CPUs to use during GC.
        // Limited by gomaxprocs, number of actual CPUs, and MaxGcproc.
        runtime_lock(&runtime_sched);
        n = runtime_gomaxprocs;
        if(n > runtime_ncpu)
                n = runtime_ncpu > 0 ? runtime_ncpu : 1;
        if(n > MaxGcproc)
                n = MaxGcproc;
        if(n > runtime_sched.nmidle+1) // one M is currently running
                n = runtime_sched.nmidle+1;
        runtime_unlock(&runtime_sched);
        return n;
}

static bool
needaddgcproc(void)
{
        int32 n;

        runtime_lock(&runtime_sched);
        n = runtime_gomaxprocs;
        if(n > runtime_ncpu)
                n = runtime_ncpu;
        if(n > MaxGcproc)
                n = MaxGcproc;
        n -= runtime_sched.nmidle+1; // one M is currently running
        runtime_unlock(&runtime_sched);
        return n > 0;
}

void
runtime_helpgc(int32 nproc)
{
        M *mp;
        int32 n, pos;

        runtime_lock(&runtime_sched);
        pos = 0;
        for(n = 1; n < nproc; n++) {  // one M is currently running
                if(runtime_allp[pos]->mcache == m->mcache)
                        pos++;
                mp = mget();
                if(mp == nil)
                        runtime_throw("runtime_gcprocs inconsistency");
                mp->helpgc = n;
                mp->mcache = runtime_allp[pos]->mcache;
                pos++;
                runtime_notewakeup(&mp->park);
        }
        runtime_unlock(&runtime_sched);
}

// Similar to stoptheworld but best-effort and can be called several times.
// There is no reverse operation, used during crashing.
// This function must not lock any mutexes.
void
runtime_freezetheworld(void)
{
        int32 i;

        if(runtime_gomaxprocs == 1)
                return;
        // stopwait and preemption requests can be lost
        // due to races with concurrently executing threads,
        // so try several times
        for(i = 0; i < 5; i++) {
                // this should tell the scheduler to not start any new goroutines
                runtime_sched.stopwait = 0x7fffffff;
                runtime_atomicstore((uint32*)&runtime_sched.gcwaiting, 1);
                // this should stop running goroutines
                if(!preemptall())
                        break;  // no running goroutines
                runtime_usleep(1000);
        }
        // to be sure
        runtime_usleep(1000);
        preemptall();
        runtime_usleep(1000);
}

void
runtime_stoptheworld(void)
{
        int32 i;
        uint32 s;
        P *p;
        bool wait;

        runtime_lock(&runtime_sched);
        runtime_sched.stopwait = runtime_gomaxprocs;
        runtime_atomicstore((uint32*)&runtime_sched.gcwaiting, 1);
        preemptall();
        // stop current P
        m->p->status = Pgcstop;
        runtime_sched.stopwait--;
        // try to retake all P's in Psyscall status
        for(i = 0; i < runtime_gomaxprocs; i++) {
                p = runtime_allp[i];
                s = p->status;
                if(s == Psyscall && runtime_cas(&p->status, s, Pgcstop))
                        runtime_sched.stopwait--;
        }
        // stop idle P's
        while((p = pidleget()) != nil) {
                p->status = Pgcstop;
                runtime_sched.stopwait--;
        }
        wait = runtime_sched.stopwait > 0;
        runtime_unlock(&runtime_sched);

        // wait for remaining P's to stop voluntarily
        if(wait) {
                runtime_notesleep(&runtime_sched.stopnote);
                runtime_noteclear(&runtime_sched.stopnote);
        }
        if(runtime_sched.stopwait)
                runtime_throw("stoptheworld: not stopped");
        for(i = 0; i < runtime_gomaxprocs; i++) {
                p = runtime_allp[i];
                if(p->status != Pgcstop)
                        runtime_throw("stoptheworld: not stopped");
        }
}

static void
mhelpgc(void)
{
        m->helpgc = -1;
}

void
runtime_starttheworld(void)
{
        P *p, *p1;
        M *mp;
        G *gp;
        bool add;

        m->locks++;  // disable preemption because it can be holding p in a local var
        gp = runtime_netpoll(false);  // non-blocking
        injectglist(gp);
        add = needaddgcproc();
        runtime_lock(&runtime_sched);
        if(newprocs) {
                procresize(newprocs);
                newprocs = 0;
        } else
                procresize(runtime_gomaxprocs);
        runtime_sched.gcwaiting = 0;

        p1 = nil;
        while((p = pidleget()) != nil) {
                // procresize() puts p's with work at the beginning of the list.
                // Once we reach a p without a run queue, the rest don't have one either.
                if(p->runqhead == p->runqtail) {
                        pidleput(p);
                        break;
                }
                p->m = mget();
                p->link = p1;
                p1 = p;
        }
        if(runtime_sched.sysmonwait) {
                runtime_sched.sysmonwait = false;
                runtime_notewakeup(&runtime_sched.sysmonnote);
        }
        runtime_unlock(&runtime_sched);

        while(p1) {
                p = p1;
                p1 = p1->link;
                if(p->m) {
                        mp = p->m;
                        p->m = nil;
                        if(mp->nextp)
                                runtime_throw("starttheworld: inconsistent mp->nextp");
                        mp->nextp = p;
                        runtime_notewakeup(&mp->park);
                } else {
                        // Start M to run P.  Do not start another M below.
                        newm(nil, p);
                        add = false;
                }
        }

        if(add) {
                // If GC could have used another helper proc, start one now,
                // in the hope that it will be available next time.
                // It would have been even better to start it before the collection,
                // but doing so requires allocating memory, so it's tricky to
                // coordinate.  This lazy approach works out in practice:
                // we don't mind if the first couple gc rounds don't have quite
                // the maximum number of procs.
                newm(mhelpgc, nil);
        }
        m->locks--;
}

// Called to start an M.
void*
runtime_mstart(void* mp)
{
        m = (M*)mp;
        g = m->g0;

        initcontext();

        g->entry = nil;
        g->param = nil;

        // Record top of stack for use by mcall.
        // Once we call schedule we're never coming back,
        // so other calls can reuse this stack space.
#ifdef USING_SPLIT_STACK
        __splitstack_getcontext(&g->stack_context[0]);
#else
        g->gcinitial_sp = &mp;
        // Setting gcstack_size to 0 is a marker meaning that gcinitial_sp
        // is the top of the stack, not the bottom.
        g->gcstack_size = 0;
        g->gcnext_sp = &mp;
#endif
        getcontext(&g->context);

        if(g->entry != nil) {
                // Got here from mcall.
                void (*pfn)(G*) = (void (*)(G*))g->entry;
                G* gp = (G*)g->param;
                pfn(gp);
                *(int*)0x21 = 0x21;
        }
        runtime_minit();

#ifdef USING_SPLIT_STACK
        {
                int dont_block_signals = 0;
                __splitstack_block_signals(&dont_block_signals, nil);
        }
#endif

        // Install signal handlers; after minit so that minit can
        // prepare the thread to be able to handle the signals.
        if(m == &runtime_m0)
                runtime_initsig();
        
        if(m->mstartfn)
                m->mstartfn();

        if(m->helpgc) {
                m->helpgc = 0;
                stopm();
        } else if(m != &runtime_m0) {
                acquirep(m->nextp);
                m->nextp = nil;
        }
        schedule();

        // TODO(brainman): This point is never reached, because scheduler
        // does not release os threads at the moment. But once this path
        // is enabled, we must remove our seh here.

        return nil;
}

typedef struct CgoThreadStart CgoThreadStart;
struct CgoThreadStart
{
        M *m;
        G *g;
        void (*fn)(void);
};

// Allocate a new m unassociated with any thread.
// Can use p for allocation context if needed.
M*
runtime_allocm(P *p, int32 stacksize, byte** ret_g0_stack, size_t* ret_g0_stacksize)
{
        M *mp;

        m->locks++;  // disable GC because it can be called from sysmon
        if(m->p == nil)
                acquirep(p);  // temporarily borrow p for mallocs in this function
#if 0
        if(mtype == nil) {
                Eface e;
                runtime_gc_m_ptr(&e);
                mtype = ((const PtrType*)e.__type_descriptor)->__element_type;
        }
#endif

        mp = runtime_mal(sizeof *mp);
        mcommoninit(mp);
        mp->g0 = runtime_malg(stacksize, ret_g0_stack, ret_g0_stacksize);

        if(p == m->p)
                releasep();
        m->locks--;

        return mp;
}

static M* lockextra(bool nilokay);
static void unlockextra(M*);

// needm is called when a cgo callback happens on a
// thread without an m (a thread not created by Go).
// In this case, needm is expected to find an m to use
// and return with m, g initialized correctly.
// Since m and g are not set now (likely nil, but see below)
// needm is limited in what routines it can call. In particular
// it can only call nosplit functions (textflag 7) and cannot
// do any scheduling that requires an m.
//
// In order to avoid needing heavy lifting here, we adopt
// the following strategy: there is a stack of available m's
// that can be stolen. Using compare-and-swap
// to pop from the stack has ABA races, so we simulate
// a lock by doing an exchange (via casp) to steal the stack
// head and replace the top pointer with MLOCKED (1).
// This serves as a simple spin lock that we can use even
// without an m. The thread that locks the stack in this way
// unlocks the stack by storing a valid stack head pointer.
//
// In order to make sure that there is always an m structure
// available to be stolen, we maintain the invariant that there
// is always one more than needed. At the beginning of the
// program (if cgo is in use) the list is seeded with a single m.
// If needm finds that it has taken the last m off the list, its job
// is - once it has installed its own m so that it can do things like
// allocate memory - to create a spare m and put it on the list.
//
// Each of these extra m's also has a g0 and a curg that are
// pressed into service as the scheduling stack and current
// goroutine for the duration of the cgo callback.
//
// When the callback is done with the m, it calls dropm to
// put the m back on the list.
//
// Unlike the gc toolchain, we start running on curg, since we are
// just going to return and let the caller continue.
void
runtime_needm(void)
{
        M *mp;

        if(runtime_needextram) {
                // Can happen if C/C++ code calls Go from a global ctor.
                // Can not throw, because scheduler is not initialized yet.
                runtime_write(2, "fatal error: cgo callback before cgo call\n",
                        sizeof("fatal error: cgo callback before cgo call\n")-1);
                runtime_exit(1);
        }

        // Lock extra list, take head, unlock popped list.
        // nilokay=false is safe here because of the invariant above,
        // that the extra list always contains or will soon contain
        // at least one m.
        mp = lockextra(false);

        // Set needextram when we've just emptied the list,
        // so that the eventual call into cgocallbackg will
        // allocate a new m for the extra list. We delay the
        // allocation until then so that it can be done
        // after exitsyscall makes sure it is okay to be
        // running at all (that is, there's no garbage collection
        // running right now).
        mp->needextram = mp->schedlink == nil;
        unlockextra(mp->schedlink);

        // Install m and g (= m->curg).
        runtime_setmg(mp, mp->curg);

        // Initialize g's context as in mstart.
        initcontext();
        g->status = Gsyscall;
        g->entry = nil;
        g->param = nil;
#ifdef USING_SPLIT_STACK
        __splitstack_getcontext(&g->stack_context[0]);
#else
        g->gcinitial_sp = &mp;
        g->gcstack_size = 0;
        g->gcnext_sp = &mp;
#endif
        getcontext(&g->context);

        if(g->entry != nil) {
                // Got here from mcall.
                void (*pfn)(G*) = (void (*)(G*))g->entry;
                G* gp = (G*)g->param;
                pfn(gp);
                *(int*)0x22 = 0x22;
        }

        // Initialize this thread to use the m.
        runtime_minit();

#ifdef USING_SPLIT_STACK
        {
                int dont_block_signals = 0;
                __splitstack_block_signals(&dont_block_signals, nil);
        }
#endif
}

// newextram allocates an m and puts it on the extra list.
// It is called with a working local m, so that it can do things
// like call schedlock and allocate.
void
runtime_newextram(void)
{
        M *mp, *mnext;
        G *gp;
        byte *g0_sp, *sp;
        size_t g0_spsize, spsize;

        // Create extra goroutine locked to extra m.
        // The goroutine is the context in which the cgo callback will run.
        // The sched.pc will never be returned to, but setting it to
        // runtime.goexit makes clear to the traceback routines where
        // the goroutine stack ends.
        mp = runtime_allocm(nil, StackMin, &g0_sp, &g0_spsize);
        gp = runtime_malg(StackMin, &sp, &spsize);
        gp->status = Gdead;
        mp->curg = gp;
        mp->locked = LockInternal;
        mp->lockedg = gp;
        gp->lockedm = mp;
        gp->goid = runtime_xadd64(&runtime_sched.goidgen, 1);
        // put on allg for garbage collector
        runtime_lock(&runtime_sched);
        if(runtime_lastg == nil)
                runtime_allg = gp;
        else
                runtime_lastg->alllink = gp;
        runtime_lastg = gp;
        runtime_unlock(&runtime_sched);
        gp->goid = runtime_xadd64(&runtime_sched.goidgen, 1);

        // The context for gp will be set up in runtime_needm.  But
        // here we need to set up the context for g0.
        getcontext(&mp->g0->context);
        mp->g0->context.uc_stack.ss_sp = g0_sp;
#ifdef MAKECONTEXT_STACK_TOP
        mp->g0->context.uc_stack.ss_sp += g0_spsize;
#endif
        mp->g0->context.uc_stack.ss_size = g0_spsize;
        makecontext(&mp->g0->context, kickoff, 0);

        // Add m to the extra list.
        mnext = lockextra(true);
        mp->schedlink = mnext;
        unlockextra(mp);
}

// dropm is called when a cgo callback has called needm but is now
// done with the callback and returning back into the non-Go thread.
// It puts the current m back onto the extra list.
//
// The main expense here is the call to signalstack to release the
// m's signal stack, and then the call to needm on the next callback
// from this thread. It is tempting to try to save the m for next time,
// which would eliminate both these costs, but there might not be
// a next time: the current thread (which Go does not control) might exit.
// If we saved the m for that thread, there would be an m leak each time
// such a thread exited. Instead, we acquire and release an m on each
// call. These should typically not be scheduling operations, just a few
// atomics, so the cost should be small.
//
// TODO(rsc): An alternative would be to allocate a dummy pthread per-thread
// variable using pthread_key_create. Unlike the pthread keys we already use
// on OS X, this dummy key would never be read by Go code. It would exist
// only so that we could register at thread-exit-time destructor.
// That destructor would put the m back onto the extra list.
// This is purely a performance optimization. The current version,
// in which dropm happens on each cgo call, is still correct too.
// We may have to keep the current version on systems with cgo
// but without pthreads, like Windows.
void
runtime_dropm(void)
{
        M *mp, *mnext;

        // Undo whatever initialization minit did during needm.
        runtime_unminit();

        // Clear m and g, and return m to the extra list.
        // After the call to setmg we can only call nosplit functions.
        mp = m;
        runtime_setmg(nil, nil);

        mp->curg->status = Gdead;

        mnext = lockextra(true);
        mp->schedlink = mnext;
        unlockextra(mp);
}

#define MLOCKED ((M*)1)

// lockextra locks the extra list and returns the list head.
// The caller must unlock the list by storing a new list head
// to runtime.extram. If nilokay is true, then lockextra will
// return a nil list head if that's what it finds. If nilokay is false,
// lockextra will keep waiting until the list head is no longer nil.
static M*
lockextra(bool nilokay)
{
        M *mp;
        void (*yield)(void);

        for(;;) {
                mp = runtime_atomicloadp(&runtime_extram);
                if(mp == MLOCKED) {
                        yield = runtime_osyield;
                        yield();
                        continue;
                }
                if(mp == nil && !nilokay) {
                        runtime_usleep(1);
                        continue;
                }
                if(!runtime_casp(&runtime_extram, mp, MLOCKED)) {
                        yield = runtime_osyield;
                        yield();
                        continue;
                }
                break;
        }
        return mp;
}

static void
unlockextra(M *mp)
{
        runtime_atomicstorep(&runtime_extram, mp);
}

static int32
countextra()
{
        M *mp, *mc;
        int32 c;

        for(;;) {
                mp = runtime_atomicloadp(&runtime_extram);
                if(mp == MLOCKED) {
                        runtime_osyield();
                        continue;
                }
                if(!runtime_casp(&runtime_extram, mp, MLOCKED)) {
                        runtime_osyield();
                        continue;
                }
                c = 0;
                for(mc = mp; mc != nil; mc = mc->schedlink)
                        c++;
                runtime_atomicstorep(&runtime_extram, mp);
                return c;
        }
}

// Create a new m.  It will start off with a call to fn, or else the scheduler.
static void
newm(void(*fn)(void), P *p)
{
        M *mp;

        mp = runtime_allocm(p, -1, nil, nil);
        mp->nextp = p;
        mp->mstartfn = fn;

        runtime_newosproc(mp);
}

// Stops execution of the current m until new work is available.
// Returns with acquired P.
static void
stopm(void)
{
        if(m->locks)
                runtime_throw("stopm holding locks");
        if(m->p)
                runtime_throw("stopm holding p");
        if(m->spinning) {
                m->spinning = false;
                runtime_xadd(&runtime_sched.nmspinning, -1);
        }

retry:
        runtime_lock(&runtime_sched);
        mput(m);
        runtime_unlock(&runtime_sched);
        runtime_notesleep(&m->park);
        runtime_noteclear(&m->park);
        if(m->helpgc) {
                runtime_gchelper();
                m->helpgc = 0;
                m->mcache = nil;
                goto retry;
        }
        acquirep(m->nextp);
        m->nextp = nil;
}

static void
mspinning(void)
{
        m->spinning = true;
}

// Schedules some M to run the p (creates an M if necessary).
// If p==nil, tries to get an idle P, if no idle P's returns false.
static void
startm(P *p, bool spinning)
{
        M *mp;
        void (*fn)(void);

        runtime_lock(&runtime_sched);
        if(p == nil) {
                p = pidleget();
                if(p == nil) {
                        runtime_unlock(&runtime_sched);
                        if(spinning)
                                runtime_xadd(&runtime_sched.nmspinning, -1);
                        return;
                }
        }
        mp = mget();
        runtime_unlock(&runtime_sched);
        if(mp == nil) {
                fn = nil;
                if(spinning)
                        fn = mspinning;
                newm(fn, p);
                return;
        }
        if(mp->spinning)
                runtime_throw("startm: m is spinning");
        if(mp->nextp)
                runtime_throw("startm: m has p");
        mp->spinning = spinning;
        mp->nextp = p;
        runtime_notewakeup(&mp->park);
}

// Hands off P from syscall or locked M.
static void
handoffp(P *p)
{
        // if it has local work, start it straight away
        if(p->runqhead != p->runqtail || runtime_sched.runqsize) {
                startm(p, false);
                return;
        }
        // no local work, check that there are no spinning/idle M's,
        // otherwise our help is not required
        if(runtime_atomicload(&runtime_sched.nmspinning) + runtime_atomicload(&runtime_sched.npidle) == 0 &&  // TODO: fast atomic
                runtime_cas(&runtime_sched.nmspinning, 0, 1)) {
                startm(p, true);
                return;
        }
        runtime_lock(&runtime_sched);
        if(runtime_sched.gcwaiting) {
                p->status = Pgcstop;
                if(--runtime_sched.stopwait == 0)
                        runtime_notewakeup(&runtime_sched.stopnote);
                runtime_unlock(&runtime_sched);
                return;
        }
        if(runtime_sched.runqsize) {
                runtime_unlock(&runtime_sched);
                startm(p, false);
                return;
        }
        // If this is the last running P and nobody is polling network,
        // need to wakeup another M to poll network.
        if(runtime_sched.npidle == (uint32)runtime_gomaxprocs-1 && runtime_atomicload64(&runtime_sched.lastpoll) != 0) {
                runtime_unlock(&runtime_sched);
                startm(p, false);
                return;
        }
        pidleput(p);
        runtime_unlock(&runtime_sched);
}

// Tries to add one more P to execute G's.
// Called when a G is made runnable (newproc, ready).
static void
wakep(void)
{
        // be conservative about spinning threads
        if(!runtime_cas(&runtime_sched.nmspinning, 0, 1))
                return;
        startm(nil, true);
}

// Stops execution of the current m that is locked to a g until the g is runnable again.
// Returns with acquired P.
static void
stoplockedm(void)
{
        P *p;

        if(m->lockedg == nil || m->lockedg->lockedm != m)
                runtime_throw("stoplockedm: inconsistent locking");
        if(m->p) {
                // Schedule another M to run this p.
                p = releasep();
                handoffp(p);
        }
        incidlelocked(1);
        // Wait until another thread schedules lockedg again.
        runtime_notesleep(&m->park);
        runtime_noteclear(&m->park);
        if(m->lockedg->status != Grunnable)
                runtime_throw("stoplockedm: not runnable");
        acquirep(m->nextp);
        m->nextp = nil;
}

// Schedules the locked m to run the locked gp.
static void
startlockedm(G *gp)
{
        M *mp;
        P *p;

        mp = gp->lockedm;
        if(mp == m)
                runtime_throw("startlockedm: locked to me");
        if(mp->nextp)
                runtime_throw("startlockedm: m has p");
        // directly handoff current P to the locked m
        incidlelocked(-1);
        p = releasep();
        mp->nextp = p;
        runtime_notewakeup(&mp->park);
        stopm();
}

// Stops the current m for stoptheworld.
// Returns when the world is restarted.
static void
gcstopm(void)
{
        P *p;

        if(!runtime_sched.gcwaiting)
                runtime_throw("gcstopm: not waiting for gc");
        if(m->spinning) {
                m->spinning = false;
                runtime_xadd(&runtime_sched.nmspinning, -1);
        }
        p = releasep();
        runtime_lock(&runtime_sched);
        p->status = Pgcstop;
        if(--runtime_sched.stopwait == 0)
                runtime_notewakeup(&runtime_sched.stopnote);
        runtime_unlock(&runtime_sched);
        stopm();
}

// Schedules gp to run on the current M.
// Never returns.
static void
execute(G *gp)
{
        int32 hz;

        if(gp->status != Grunnable) {
                runtime_printf("execute: bad g status %d\n", gp->status);
                runtime_throw("execute: bad g status");
        }
        gp->status = Grunning;
        m->p->schedtick++;
        m->curg = gp;
        gp->m = m;

        // Check whether the profiler needs to be turned on or off.
        hz = runtime_sched.profilehz;
        if(m->profilehz != hz)
                runtime_resetcpuprofiler(hz);

        runtime_gogo(gp);
}

// Finds a runnable goroutine to execute.
// Tries to steal from other P's, get g from global queue, poll network.
static G*
findrunnable(void)
{
        G *gp;
        P *p;
        int32 i;

top:
        if(runtime_sched.gcwaiting) {
                gcstopm();
                goto top;
        }
        // local runq
        gp = runqget(m->p);
        if(gp)
                return gp;
        // global runq
        if(runtime_sched.runqsize) {
                runtime_lock(&runtime_sched);
                gp = globrunqget(m->p, 0);
                runtime_unlock(&runtime_sched);
                if(gp)
                        return gp;
        }
        // poll network
        gp = runtime_netpoll(false);  // non-blocking
        if(gp) {
                injectglist(gp->schedlink);
                gp->status = Grunnable;
                return gp;
        }
        // If number of spinning M's >= number of busy P's, block.
        // This is necessary to prevent excessive CPU consumption
        // when GOMAXPROCS>>1 but the program parallelism is low.
        if(!m->spinning && 2 * runtime_atomicload(&runtime_sched.nmspinning) >= runtime_gomaxprocs - runtime_atomicload(&runtime_sched.npidle))  // TODO: fast atomic
                goto stop;
        if(!m->spinning) {
                m->spinning = true;
                runtime_xadd(&runtime_sched.nmspinning, 1);
        }
        // random steal from other P's
        for(i = 0; i < 2*runtime_gomaxprocs; i++) {
                if(runtime_sched.gcwaiting)
                        goto top;
                p = runtime_allp[runtime_fastrand1()%runtime_gomaxprocs];
                if(p == m->p)
                        gp = runqget(p);
                else
                        gp = runqsteal(m->p, p);
                if(gp)
                        return gp;
        }
stop:
        // return P and block
        runtime_lock(&runtime_sched);
        if(runtime_sched.gcwaiting) {
                runtime_unlock(&runtime_sched);
                goto top;
        }
        if(runtime_sched.runqsize) {
                gp = globrunqget(m->p, 0);
                runtime_unlock(&runtime_sched);
                return gp;
        }
        p = releasep();
        pidleput(p);
        runtime_unlock(&runtime_sched);
        if(m->spinning) {
                m->spinning = false;
                runtime_xadd(&runtime_sched.nmspinning, -1);
        }
        // check all runqueues once again
        for(i = 0; i < runtime_gomaxprocs; i++) {
                p = runtime_allp[i];
                if(p && p->runqhead != p->runqtail) {
                        runtime_lock(&runtime_sched);
                        p = pidleget();
                        runtime_unlock(&runtime_sched);
                        if(p) {
                                acquirep(p);
                                goto top;
                        }
                        break;
                }
        }
        // poll network
        if(runtime_xchg64(&runtime_sched.lastpoll, 0) != 0) {
                if(m->p)
                        runtime_throw("findrunnable: netpoll with p");
                if(m->spinning)
                        runtime_throw("findrunnable: netpoll with spinning");
                gp = runtime_netpoll(true);  // block until new work is available
                runtime_atomicstore64(&runtime_sched.lastpoll, runtime_nanotime());
                if(gp) {
                        runtime_lock(&runtime_sched);
                        p = pidleget();
                        runtime_unlock(&runtime_sched);
                        if(p) {
                                acquirep(p);
                                injectglist(gp->schedlink);
                                gp->status = Grunnable;
                                return gp;
                        }
                        injectglist(gp);
                }
        }
        stopm();
        goto top;
}

static void
resetspinning(void)
{
        int32 nmspinning;

        if(m->spinning) {
                m->spinning = false;
                nmspinning = runtime_xadd(&runtime_sched.nmspinning, -1);
                if(nmspinning < 0)
                        runtime_throw("findrunnable: negative nmspinning");
        } else
                nmspinning = runtime_atomicload(&runtime_sched.nmspinning);

        // M wakeup policy is deliberately somewhat conservative (see nmspinning handling),
        // so see if we need to wakeup another P here.
        if (nmspinning == 0 && runtime_atomicload(&runtime_sched.npidle) > 0)
                wakep();
}

// Injects the list of runnable G's into the scheduler.
// Can run concurrently with GC.
static void
injectglist(G *glist)
{
        int32 n;
        G *gp;

        if(glist == nil)
                return;
        runtime_lock(&runtime_sched);
        for(n = 0; glist; n++) {
                gp = glist;
                glist = gp->schedlink;
                gp->status = Grunnable;
                globrunqput(gp);
        }
        runtime_unlock(&runtime_sched);

        for(; n && runtime_sched.npidle; n--)
                startm(nil, false);
}

// One round of scheduler: find a runnable goroutine and execute it.
// Never returns.
static void
schedule(void)
{
        G *gp;
        uint32 tick;

        if(m->locks)
                runtime_throw("schedule: holding locks");

top:
        if(runtime_sched.gcwaiting) {
                gcstopm();
                goto top;
        }

        gp = nil;
        // Check the global runnable queue once in a while to ensure fairness.
        // Otherwise two goroutines can completely occupy the local runqueue
        // by constantly respawning each other.
        tick = m->p->schedtick;
        // This is a fancy way to say tick%61==0,
        // it uses 2 MUL instructions instead of a single DIV and so is faster on modern processors.
        if(tick - (((uint64)tick*0x4325c53fu)>>36)*61 == 0 && runtime_sched.runqsize > 0) {
                runtime_lock(&runtime_sched);
                gp = globrunqget(m->p, 1);
                runtime_unlock(&runtime_sched);
                if(gp)
                        resetspinning();
        }
        if(gp == nil) {
                gp = runqget(m->p);
                if(gp && m->spinning)
                        runtime_throw("schedule: spinning with local work");
        }
        if(gp == nil) {
                gp = findrunnable();  // blocks until work is available
                resetspinning();
        }

        if(gp->lockedm) {
                // Hands off own p to the locked m,
                // then blocks waiting for a new p.
                startlockedm(gp);
                goto top;
        }

        execute(gp);
}

// Puts the current goroutine into a waiting state and unlocks the lock.
// The goroutine can be made runnable again by calling runtime_ready(gp).
void
runtime_park(void(*unlockf)(Lock*), Lock *lock, const char *reason)
{
        m->waitlock = lock;
        m->waitunlockf = unlockf;
        g->waitreason = reason;
        runtime_mcall(park0);
}

// runtime_park continuation on g0.
static void
park0(G *gp)
{
        gp->status = Gwaiting;
        gp->m = nil;
        m->curg = nil;
        if(m->waitunlockf) {
                m->waitunlockf(m->waitlock);
                m->waitunlockf = nil;
                m->waitlock = nil;
        }
        if(m->lockedg) {
                stoplockedm();
                execute(gp);  // Never returns.
        }
        schedule();
}

// Scheduler yield.
void
runtime_gosched(void)
{
        runtime_mcall(runtime_gosched0);
}

// runtime_gosched continuation on g0.
void
runtime_gosched0(G *gp)
{
        gp->status = Grunnable;
        gp->m = nil;
        m->curg = nil;
        runtime_lock(&runtime_sched);
        globrunqput(gp);
        runtime_unlock(&runtime_sched);
        if(m->lockedg) {
                stoplockedm();
                execute(gp);  // Never returns.
        }
        schedule();
}

// Finishes execution of the current goroutine.
// Need to mark it as nosplit, because it runs with sp > stackbase (as runtime_lessstack).
// Since it does not return it does not matter.  But if it is preempted
// at the split stack check, GC will complain about inconsistent sp.
void
runtime_goexit(void)
{
        if(raceenabled)
                runtime_racegoend();
        runtime_mcall(goexit0);
}

// runtime_goexit continuation on g0.
static void
goexit0(G *gp)
{
        gp->status = Gdead;
        gp->entry = nil;
        gp->m = nil;
        gp->lockedm = nil;
        m->curg = nil;
        m->lockedg = nil;
        if(m->locked & ~LockExternal) {
                runtime_printf("invalid m->locked = %d\n", m->locked);
                runtime_throw("internal lockOSThread error");
        }       
        m->locked = 0;
        gfput(m->p, gp);
        schedule();
}

// The goroutine g is about to enter a system call.
// Record that it's not using the cpu anymore.
// This is called only from the go syscall library and cgocall,
// not from the low-level system calls used by the runtime.
//
// Entersyscall cannot split the stack: the runtime_gosave must
// make g->sched refer to the caller's stack segment, because
// entersyscall is going to return immediately after.

void runtime_entersyscall(void) __attribute__ ((no_split_stack));

void
runtime_entersyscall()
{
        // Disable preemption because during this function g is in Gsyscall status,
        // but can have inconsistent g->sched, do not let GC observe it.
        m->locks++;

        // Leave SP around for GC and traceback.
#ifdef USING_SPLIT_STACK
        g->gcstack = __splitstack_find(nil, nil, &g->gcstack_size,
                                       &g->gcnext_segment, &g->gcnext_sp,
                                       &g->gcinitial_sp);
#else
        {
                uint32 v;

                g->gcnext_sp = (byte *) &v;
        }
#endif

        // Save the registers in the g structure so that any pointers
        // held in registers will be seen by the garbage collector.
        getcontext(&g->gcregs);

        g->status = Gsyscall;

        if(runtime_atomicload(&runtime_sched.sysmonwait)) {  // TODO: fast atomic
                runtime_lock(&runtime_sched);
                if(runtime_atomicload(&runtime_sched.sysmonwait)) {
                        runtime_atomicstore(&runtime_sched.sysmonwait, 0);
                        runtime_notewakeup(&runtime_sched.sysmonnote);
                }
                runtime_unlock(&runtime_sched);
        }

        m->mcache = nil;
        m->p->m = nil;
        runtime_atomicstore(&m->p->status, Psyscall);
        if(runtime_sched.gcwaiting) {
                runtime_lock(&runtime_sched);
                if (runtime_sched.stopwait > 0 && runtime_cas(&m->p->status, Psyscall, Pgcstop)) {
                        if(--runtime_sched.stopwait == 0)
                                runtime_notewakeup(&runtime_sched.stopnote);
                }
                runtime_unlock(&runtime_sched);
        }

        m->locks--;
}

// The same as runtime_entersyscall(), but with a hint that the syscall is blocking.
void
runtime_entersyscallblock(void)
{
        P *p;

        m->locks++;  // see comment in entersyscall

        // Leave SP around for GC and traceback.
#ifdef USING_SPLIT_STACK
        g->gcstack = __splitstack_find(nil, nil, &g->gcstack_size,
                                       &g->gcnext_segment, &g->gcnext_sp,
                                       &g->gcinitial_sp);
#else
        g->gcnext_sp = (byte *) &p;
#endif

        // Save the registers in the g structure so that any pointers
        // held in registers will be seen by the garbage collector.
        getcontext(&g->gcregs);

        g->status = Gsyscall;

        p = releasep();
        handoffp(p);
        if(g->isbackground)  // do not consider blocked scavenger for deadlock detection
                incidlelocked(1);

        m->locks--;
}

// The goroutine g exited its system call.
// Arrange for it to run on a cpu again.
// This is called only from the go syscall library, not
// from the low-level system calls used by the runtime.
void
runtime_exitsyscall(void)
{
        G *gp;

        m->locks++;  // see comment in entersyscall

        gp = g;
        if(gp->isbackground)  // do not consider blocked scavenger for deadlock detection
                incidlelocked(-1);

        if(exitsyscallfast()) {
                // There's a cpu for us, so we can run.
                m->p->syscalltick++;
                gp->status = Grunning;
                // Garbage collector isn't running (since we are),
                // so okay to clear gcstack and gcsp.
#ifdef USING_SPLIT_STACK
                gp->gcstack = nil;
#endif
                gp->gcnext_sp = nil;
                runtime_memclr(&gp->gcregs, sizeof gp->gcregs);
                m->locks--;
                return;
        }

        m->locks--;

        // Call the scheduler.
        runtime_mcall(exitsyscall0);

        // Scheduler returned, so we're allowed to run now.
        // Delete the gcstack information that we left for
        // the garbage collector during the system call.
        // Must wait until now because until gosched returns
        // we don't know for sure that the garbage collector
        // is not running.
#ifdef USING_SPLIT_STACK
        gp->gcstack = nil;
#endif
        gp->gcnext_sp = nil;
        runtime_memclr(&gp->gcregs, sizeof gp->gcregs);

        // Don't refer to m again, we might be running on a different
        // thread after returning from runtime_mcall.
        runtime_m()->p->syscalltick++;
}

static bool
exitsyscallfast(void)
{
        P *p;

        // Freezetheworld sets stopwait but does not retake P's.
        if(runtime_sched.stopwait) {
                m->p = nil;
                return false;
        }

        // Try to re-acquire the last P.
        if(m->p && m->p->status == Psyscall && runtime_cas(&m->p->status, Psyscall, Prunning)) {
                // There's a cpu for us, so we can run.
                m->mcache = m->p->mcache;
                m->p->m = m;
                return true;
        }
        // Try to get any other idle P.
        m->p = nil;
        if(runtime_sched.pidle) {
                runtime_lock(&runtime_sched);
                p = pidleget();
                if(p && runtime_atomicload(&runtime_sched.sysmonwait)) {
                        runtime_atomicstore(&runtime_sched.sysmonwait, 0);
                        runtime_notewakeup(&runtime_sched.sysmonnote);
                }
                runtime_unlock(&runtime_sched);
                if(p) {
                        acquirep(p);
                        return true;
                }
        }
        return false;
}

// runtime_exitsyscall slow path on g0.
// Failed to acquire P, enqueue gp as runnable.
static void
exitsyscall0(G *gp)
{
        P *p;

        gp->status = Grunnable;
        gp->m = nil;
        m->curg = nil;
        runtime_lock(&runtime_sched);
        p = pidleget();
        if(p == nil)
                globrunqput(gp);
        else if(runtime_atomicload(&runtime_sched.sysmonwait)) {
                runtime_atomicstore(&runtime_sched.sysmonwait, 0);
                runtime_notewakeup(&runtime_sched.sysmonnote);
        }
        runtime_unlock(&runtime_sched);
        if(p) {
                acquirep(p);
                execute(gp);  // Never returns.
        }
        if(m->lockedg) {
                // Wait until another thread schedules gp and so m again.
                stoplockedm();
                execute(gp);  // Never returns.
        }
        stopm();
        schedule();  // Never returns.
}

// Called from syscall package before fork.
void syscall_runtime_BeforeFork(void)
  __asm__(GOSYM_PREFIX "syscall.runtime_BeforeFork");
void
syscall_runtime_BeforeFork(void)
{
        // Fork can hang if preempted with signals frequently enough (see issue 5517).
        // Ensure that we stay on the same M where we disable profiling.
        m->locks++;
        if(m->profilehz != 0)
                runtime_resetcpuprofiler(0);
}

// Called from syscall package after fork in parent.
void syscall_runtime_AfterFork(void)
  __asm__(GOSYM_PREFIX "syscall.runtime_AfterFork");
void
syscall_runtime_AfterFork(void)
{
        int32 hz;

        hz = runtime_sched.profilehz;
        if(hz != 0)
                runtime_resetcpuprofiler(hz);
        m->locks--;
}

// Allocate a new g, with a stack big enough for stacksize bytes.
G*
runtime_malg(int32 stacksize, byte** ret_stack, size_t* ret_stacksize)
{
        G *newg;

        newg = runtime_malloc(sizeof(G));
        if(stacksize >= 0) {
#if USING_SPLIT_STACK
                int dont_block_signals = 0;

                *ret_stack = __splitstack_makecontext(stacksize,
                                                      &newg->stack_context[0],
                                                      ret_stacksize);
                __splitstack_block_signals_context(&newg->stack_context[0],
                                                   &dont_block_signals, nil);
#else
                *ret_stack = runtime_mallocgc(stacksize, 0, FlagNoProfiling|FlagNoGC);
                *ret_stacksize = stacksize;
                newg->gcinitial_sp = *ret_stack;
                newg->gcstack_size = stacksize;
                runtime_xadd(&runtime_stacks_sys, stacksize);
#endif
        }
        return newg;
}

/* For runtime package testing.  */


// Create a new g running fn with siz bytes of arguments.
// Put it on the queue of g's waiting to run.
// The compiler turns a go statement into a call to this.
// Cannot split the stack because it assumes that the arguments
// are available sequentially after &fn; they would not be
// copied if a stack split occurred.  It's OK for this to call
// functions that split the stack.
void runtime_testing_entersyscall(void)
  __asm__ (GOSYM_PREFIX "runtime.entersyscall");
void
runtime_testing_entersyscall()
{
        runtime_entersyscall();
}

void runtime_testing_exitsyscall(void)
  __asm__ (GOSYM_PREFIX "runtime.exitsyscall");

void
runtime_testing_exitsyscall()
{
        runtime_exitsyscall();
}

G*
__go_go(void (*fn)(void*), void* arg)
{
        byte *sp;
        size_t spsize;
        G *newg;

//runtime_printf("newproc1 %p %p narg=%d nret=%d\n", fn->fn, argp, narg, nret);
        m->locks++;  // disable preemption because it can be holding p in a local var

        if((newg = gfget(m->p)) != nil) {
#ifdef USING_SPLIT_STACK
                int dont_block_signals = 0;

                sp = __splitstack_resetcontext(&newg->stack_context[0],
                                               &spsize);
                __splitstack_block_signals_context(&newg->stack_context[0],
                                                   &dont_block_signals, nil);
#else
                sp = newg->gcinitial_sp;
                spsize = newg->gcstack_size;
                if(spsize == 0)
                        runtime_throw("bad spsize in __go_go");
                newg->gcnext_sp = sp;
#endif
        } else {
                newg = runtime_malg(StackMin, &sp, &spsize);
                runtime_lock(&runtime_sched);
                if(runtime_lastg == nil)
                        runtime_allg = newg;
                else
                        runtime_lastg->alllink = newg;
                runtime_lastg = newg;
                runtime_unlock(&runtime_sched);
        }

        newg->entry = (byte*)fn;
        newg->param = arg;
        newg->gopc = (uintptr)__builtin_return_address(0);
        newg->status = Grunnable;
        newg->goid = runtime_xadd64(&runtime_sched.goidgen, 1);

        {
                // Avoid warnings about variables clobbered by
                // longjmp.
                byte * volatile vsp = sp;
                size_t volatile vspsize = spsize;
                G * volatile vnewg = newg;

                getcontext(&vnewg->context);
                vnewg->context.uc_stack.ss_sp = vsp;
#ifdef MAKECONTEXT_STACK_TOP
                vnewg->context.uc_stack.ss_sp += vspsize;
#endif
                vnewg->context.uc_stack.ss_size = vspsize;
                makecontext(&vnewg->context, kickoff, 0);

                runqput(m->p, vnewg);

                if(runtime_atomicload(&runtime_sched.npidle) != 0 && runtime_atomicload(&runtime_sched.nmspinning) == 0 && fn != runtime_main)  // TODO: fast atomic
                        wakep();
                m->locks--;
                return vnewg;
        }
}

// Put on gfree list.
// If local list is too long, transfer a batch to the global list.
static void
gfput(P *p, G *gp)
{
        gp->schedlink = p->gfree;
        p->gfree = gp;
        p->gfreecnt++;
        if(p->gfreecnt >= 64) {
                runtime_lock(&runtime_sched.gflock);
                while(p->gfreecnt >= 32) {
                        p->gfreecnt--;
                        gp = p->gfree;
                        p->gfree = gp->schedlink;
                        gp->schedlink = runtime_sched.gfree;
                        runtime_sched.gfree = gp;
                }
                runtime_unlock(&runtime_sched.gflock);
        }
}

// Get from gfree list.
// If local list is empty, grab a batch from global list.
static G*
gfget(P *p)
{
        G *gp;

retry:
        gp = p->gfree;
        if(gp == nil && runtime_sched.gfree) {
                runtime_lock(&runtime_sched.gflock);
                while(p->gfreecnt < 32 && runtime_sched.gfree) {
                        p->gfreecnt++;
                        gp = runtime_sched.gfree;
                        runtime_sched.gfree = gp->schedlink;
                        gp->schedlink = p->gfree;
                        p->gfree = gp;
                }
                runtime_unlock(&runtime_sched.gflock);
                goto retry;
        }
        if(gp) {
                p->gfree = gp->schedlink;
                p->gfreecnt--;
        }
        return gp;
}

// Purge all cached G's from gfree list to the global list.
static void
gfpurge(P *p)
{
        G *gp;

        runtime_lock(&runtime_sched.gflock);
        while(p->gfreecnt) {
                p->gfreecnt--;
                gp = p->gfree;
                p->gfree = gp->schedlink;
                gp->schedlink = runtime_sched.gfree;
                runtime_sched.gfree = gp;
        }
        runtime_unlock(&runtime_sched.gflock);
}

void
runtime_Breakpoint(void)
{
        runtime_breakpoint();
}

void runtime_Gosched (void) __asm__ (GOSYM_PREFIX "runtime.Gosched");

void
runtime_Gosched(void)
{
        runtime_gosched();
}

// Implementation of runtime.GOMAXPROCS.
// delete when scheduler is even stronger
int32
runtime_gomaxprocsfunc(int32 n)
{
        int32 ret;

        if(n > MaxGomaxprocs)
                n = MaxGomaxprocs;
        runtime_lock(&runtime_sched);
        ret = runtime_gomaxprocs;
        if(n <= 0 || n == ret) {
                runtime_unlock(&runtime_sched);
                return ret;
        }
        runtime_unlock(&runtime_sched);

        runtime_semacquire(&runtime_worldsema, false);
        m->gcing = 1;
        runtime_stoptheworld();
        newprocs = n;
        m->gcing = 0;
        runtime_semrelease(&runtime_worldsema);
        runtime_starttheworld();

        return ret;
}

// lockOSThread is called by runtime.LockOSThread and runtime.lockOSThread below
// after they modify m->locked. Do not allow preemption during this call,
// or else the m might be different in this function than in the caller.
static void
lockOSThread(void)
{
        m->lockedg = g;
        g->lockedm = m;
}

void    runtime_LockOSThread(void) __asm__ (GOSYM_PREFIX "runtime.LockOSThread");
void
runtime_LockOSThread(void)
{
        m->locked |= LockExternal;
        lockOSThread();
}

void
runtime_lockOSThread(void)
{
        m->locked += LockInternal;
        lockOSThread();
}


// unlockOSThread is called by runtime.UnlockOSThread and runtime.unlockOSThread below
// after they update m->locked. Do not allow preemption during this call,
// or else the m might be in different in this function than in the caller.
static void
unlockOSThread(void)
{
        if(m->locked != 0)
                return;
        m->lockedg = nil;
        g->lockedm = nil;
}

void    runtime_UnlockOSThread(void) __asm__ (GOSYM_PREFIX "runtime.UnlockOSThread");

void
runtime_UnlockOSThread(void)
{
        m->locked &= ~LockExternal;
        unlockOSThread();
}

void
runtime_unlockOSThread(void)
{
        if(m->locked < LockInternal)
                runtime_throw("runtime: internal error: misuse of lockOSThread/unlockOSThread");
        m->locked -= LockInternal;
        unlockOSThread();
}

bool
runtime_lockedOSThread(void)
{
        return g->lockedm != nil && m->lockedg != nil;
}

// for testing of callbacks

_Bool runtime_golockedOSThread(void)
  __asm__ (GOSYM_PREFIX "runtime.golockedOSThread");

_Bool
runtime_golockedOSThread(void)
{
        return runtime_lockedOSThread();
}

intgo runtime_NumGoroutine (void)
  __asm__ (GOSYM_PREFIX "runtime.NumGoroutine");

intgo
runtime_NumGoroutine()
{
        return runtime_gcount();
}

int32
runtime_gcount(void)
{
        G *gp;
        int32 n, s;

        n = 0;
        runtime_lock(&runtime_sched);
        // TODO(dvyukov): runtime.NumGoroutine() is O(N).
        // We do not want to increment/decrement centralized counter in newproc/goexit,
        // just to make runtime.NumGoroutine() faster.
        // Compromise solution is to introduce per-P counters of active goroutines.
        for(gp = runtime_allg; gp; gp = gp->alllink) {
                s = gp->status;
                if(s == Grunnable || s == Grunning || s == Gsyscall || s == Gwaiting)
                        n++;
        }
        runtime_unlock(&runtime_sched);
        return n;
}

int32
runtime_mcount(void)
{
        return runtime_sched.mcount;
}

static struct {
        Lock;
        void (*fn)(uintptr*, int32);
        int32 hz;
        uintptr pcbuf[100];
        Location locbuf[100];
} prof;

static void
System(void)
{
}

// Called if we receive a SIGPROF signal.
void
runtime_sigprof()
{
        int32 n, i;
        bool traceback;

        if(prof.fn == nil || prof.hz == 0)
                return;
        traceback = true;
        // Windows does profiling in a dedicated thread w/o m.
        if(!Windows && (m == nil || m->mcache == nil))
                traceback = false;
        
        runtime_lock(&prof);
        if(prof.fn == nil) {
                runtime_unlock(&prof);
                return;
        }
        n = 0;
        if(traceback) {
                n = runtime_callers(0, prof.locbuf, nelem(prof.locbuf));
                for(i = 0; i < n; i++)
                        prof.pcbuf[i] = prof.locbuf[i].pc;
        }
        if (!traceback || n <= 0) {
                n = 2;
                prof.pcbuf[0] = (uintptr)runtime_getcallerpc(&n);
                prof.pcbuf[1] = (uintptr)System + 1;
        }
        prof.fn(prof.pcbuf, n);
        runtime_unlock(&prof);
}

// Arrange to call fn with a traceback hz times a second.
void
runtime_setcpuprofilerate(void (*fn)(uintptr*, int32), int32 hz)
{
        // Force sane arguments.
        if(hz < 0)
                hz = 0;
        if(hz == 0)
                fn = nil;
        if(fn == nil)
                hz = 0;

        // Disable preemption, otherwise we can be rescheduled to another thread
        // that has profiling enabled.
        m->locks++;

        // Stop profiler on this thread so that it is safe to lock prof.
        // if a profiling signal came in while we had prof locked,
        // it would deadlock.
        runtime_resetcpuprofiler(0);

        runtime_lock(&prof);
        prof.fn = fn;
        prof.hz = hz;
        runtime_unlock(&prof);
        runtime_lock(&runtime_sched);
        runtime_sched.profilehz = hz;
        runtime_unlock(&runtime_sched);

        if(hz != 0)
                runtime_resetcpuprofiler(hz);

        m->locks--;
}

// Change number of processors.  The world is stopped, sched is locked.
static void
procresize(int32 new)
{
        int32 i, old;
        G *gp;
        P *p;

        old = runtime_gomaxprocs;
        if(old < 0 || old > MaxGomaxprocs || new <= 0 || new >MaxGomaxprocs)
                runtime_throw("procresize: invalid arg");
        // initialize new P's
        for(i = 0; i < new; i++) {
                p = runtime_allp[i];
                if(p == nil) {
                        p = (P*)runtime_mallocgc(sizeof(*p), 0, FlagNoInvokeGC);
                        p->id = i;
                        p->status = Pgcstop;
                        runtime_atomicstorep(&runtime_allp[i], p);
                }
                if(p->mcache == nil) {
                        if(old==0 && i==0)
                                p->mcache = m->mcache;  // bootstrap
                        else
                                p->mcache = runtime_allocmcache();
                }
                if(p->runq == nil) {
                        p->runqsize = 128;
                        p->runq = (G**)runtime_mallocgc(p->runqsize*sizeof(G*), 0, FlagNoInvokeGC);
                }
        }

        // redistribute runnable G's evenly
        for(i = 0; i < old; i++) {
                p = runtime_allp[i];
                while((gp = runqget(p)) != nil)
                        globrunqput(gp);
        }
        // start at 1 because current M already executes some G and will acquire allp[0] below,
        // so if we have a spare G we want to put it into allp[1].
        for(i = 1; runtime_sched.runqhead; i++) {
                gp = runtime_sched.runqhead;
                runtime_sched.runqhead = gp->schedlink;
                runqput(runtime_allp[i%new], gp);
        }
        runtime_sched.runqtail = nil;
        runtime_sched.runqsize = 0;

        // free unused P's
        for(i = new; i < old; i++) {
                p = runtime_allp[i];
                runtime_freemcache(p->mcache);
                p->mcache = nil;
                gfpurge(p);
                p->status = Pdead;
                // can't free P itself because it can be referenced by an M in syscall
        }

        if(m->p)
                m->p->m = nil;
        m->p = nil;
        m->mcache = nil;
        p = runtime_allp[0];
        p->m = nil;
        p->status = Pidle;
        acquirep(p);
        for(i = new-1; i > 0; i--) {
                p = runtime_allp[i];
                p->status = Pidle;
                pidleput(p);
        }
        runtime_atomicstore((uint32*)&runtime_gomaxprocs, new);
}

// Associate p and the current m.
static void
acquirep(P *p)
{
        if(m->p || m->mcache)
                runtime_throw("acquirep: already in go");
        if(p->m || p->status != Pidle) {
                runtime_printf("acquirep: p->m=%p(%d) p->status=%d\n", p->m, p->m ? p->m->id : 0, p->status);
                runtime_throw("acquirep: invalid p state");
        }
        m->mcache = p->mcache;
        m->p = p;
        p->m = m;
        p->status = Prunning;
}

// Disassociate p and the current m.
static P*
releasep(void)
{
        P *p;

        if(m->p == nil || m->mcache == nil)
                runtime_throw("releasep: invalid arg");
        p = m->p;
        if(p->m != m || p->mcache != m->mcache || p->status != Prunning) {
                runtime_printf("releasep: m=%p m->p=%p p->m=%p m->mcache=%p p->mcache=%p p->status=%d\n",
                        m, m->p, p->m, m->mcache, p->mcache, p->status);
                runtime_throw("releasep: invalid p state");
        }
        m->p = nil;
        m->mcache = nil;
        p->m = nil;
        p->status = Pidle;
        return p;
}

static void
incidlelocked(int32 v)
{
        runtime_lock(&runtime_sched);
        runtime_sched.nmidlelocked += v;
        if(v > 0)
                checkdead();
        runtime_unlock(&runtime_sched);
}

// Check for deadlock situation.
// The check is based on number of running M's, if 0 -> deadlock.
static void
checkdead(void)
{
        G *gp;
        int32 run, grunning, s;

        // -1 for sysmon
        run = runtime_sched.mcount - runtime_sched.nmidle - runtime_sched.nmidlelocked - 1 - countextra();
        if(run > 0)
                return;
        if(run < 0) {
                runtime_printf("checkdead: nmidle=%d nmidlelocked=%d mcount=%d\n",
                        runtime_sched.nmidle, runtime_sched.nmidlelocked, runtime_sched.mcount);
                runtime_throw("checkdead: inconsistent counts");
        }
        grunning = 0;
        for(gp = runtime_allg; gp; gp = gp->alllink) {
                if(gp->isbackground)
                        continue;
                s = gp->status;
                if(s == Gwaiting)
                        grunning++;
                else if(s == Grunnable || s == Grunning || s == Gsyscall) {
                        runtime_printf("checkdead: find g %D in status %d\n", gp->goid, s);
                        runtime_throw("checkdead: runnable g");
                }
        }
        if(grunning == 0)  // possible if main goroutine calls runtime_Goexit()
                runtime_exit(0);
        m->throwing = -1;  // do not dump full stacks
        runtime_throw("all goroutines are asleep - deadlock!");
}

static void
sysmon(void)
{
        uint32 idle, delay;
        int64 now, lastpoll, lasttrace;
        G *gp;

        lasttrace = 0;
        idle = 0;  // how many cycles in succession we had not wokeup somebody
        delay = 0;
        for(;;) {
                if(idle == 0)  // start with 20us sleep...
                        delay = 20;
                else if(idle > 50)  // start doubling the sleep after 1ms...
                        delay *= 2;
                if(delay > 10*1000)  // up to 10ms
                        delay = 10*1000;
                runtime_usleep(delay);
                if(runtime_debug.schedtrace <= 0 &&
                        (runtime_sched.gcwaiting || runtime_atomicload(&runtime_sched.npidle) == (uint32)runtime_gomaxprocs)) {  // TODO: fast atomic
                        runtime_lock(&runtime_sched);
                        if(runtime_atomicload(&runtime_sched.gcwaiting) || runtime_atomicload(&runtime_sched.npidle) == (uint32)runtime_gomaxprocs) {
                                runtime_atomicstore(&runtime_sched.sysmonwait, 1);
                                runtime_unlock(&runtime_sched);
                                runtime_notesleep(&runtime_sched.sysmonnote);
                                runtime_noteclear(&runtime_sched.sysmonnote);
                                idle = 0;
                                delay = 20;
                        } else
                                runtime_unlock(&runtime_sched);
                }
                // poll network if not polled for more than 10ms
                lastpoll = runtime_atomicload64(&runtime_sched.lastpoll);
                now = runtime_nanotime();
                if(lastpoll != 0 && lastpoll + 10*1000*1000 < now) {
                        runtime_cas64(&runtime_sched.lastpoll, lastpoll, now);
                        gp = runtime_netpoll(false);  // non-blocking
                        if(gp) {
                                // Need to decrement number of idle locked M's
                                // (pretending that one more is running) before injectglist.
                                // Otherwise it can lead to the following situation:
                                // injectglist grabs all P's but before it starts M's to run the P's,
                                // another M returns from syscall, finishes running its G,
                                // observes that there is no work to do and no other running M's
                                // and reports deadlock.
                                incidlelocked(-1);
                                injectglist(gp);
                                incidlelocked(1);
                        }
                }
                // retake P's blocked in syscalls
                // and preempt long running G's
                if(retake(now))
                        idle = 0;
                else
                        idle++;

                if(runtime_debug.schedtrace > 0 && lasttrace + runtime_debug.schedtrace*1000000ll <= now) {
                        lasttrace = now;
                        runtime_schedtrace(runtime_debug.scheddetail);
                }
        }
}

typedef struct Pdesc Pdesc;
struct Pdesc
{
        uint32  schedtick;
        int64   schedwhen;
        uint32  syscalltick;
        int64   syscallwhen;
};
static Pdesc pdesc[MaxGomaxprocs];

static uint32
retake(int64 now)
{
        uint32 i, s, n;
        int64 t;
        P *p;
        Pdesc *pd;

        n = 0;
        for(i = 0; i < (uint32)runtime_gomaxprocs; i++) {
                p = runtime_allp[i];
                if(p==nil)
                        continue;
                pd = &pdesc[i];
                s = p->status;
                if(s == Psyscall) {
                        // Retake P from syscall if it's there for more than 1 sysmon tick (20us).
                        // But only if there is other work to do.
                        t = p->syscalltick;
                        if(pd->syscalltick != t) {
                                pd->syscalltick = t;
                                pd->syscallwhen = now;
                                continue;
                        }
                        if(p->runqhead == p->runqtail &&
                                runtime_atomicload(&runtime_sched.nmspinning) + runtime_atomicload(&runtime_sched.npidle) > 0)
                                continue;
                        // Need to decrement number of idle locked M's
                        // (pretending that one more is running) before the CAS.
                        // Otherwise the M from which we retake can exit the syscall,
                        // increment nmidle and report deadlock.
                        incidlelocked(-1);
                        if(runtime_cas(&p->status, s, Pidle)) {
                                n++;
                                handoffp(p);
                        }
                        incidlelocked(1);
                } else if(s == Prunning) {
                        // Preempt G if it's running for more than 10ms.
                        t = p->schedtick;
                        if(pd->schedtick != t) {
                                pd->schedtick = t;
                                pd->schedwhen = now;
                                continue;
                        }
                        if(pd->schedwhen + 10*1000*1000 > now)
                                continue;
                        // preemptone(p);
                }
        }
        return n;
}

// Tell all goroutines that they have been preempted and they should stop.
// This function is purely best-effort.  It can fail to inform a goroutine if a
// processor just started running it.
// No locks need to be held.
// Returns true if preemption request was issued to at least one goroutine.
static bool
preemptall(void)
{
        return false;
}

void
runtime_schedtrace(bool detailed)
{
        static int64 starttime;
        int64 now;
        int64 id1, id2, id3;
        int32 i, q, t, h, s;
        const char *fmt;
        M *mp, *lockedm;
        G *gp, *lockedg;
        P *p;

        now = runtime_nanotime();
        if(starttime == 0)
                starttime = now;

        runtime_lock(&runtime_sched);
        runtime_printf("SCHED %Dms: gomaxprocs=%d idleprocs=%d threads=%d idlethreads=%d runqueue=%d",
                (now-starttime)/1000000, runtime_gomaxprocs, runtime_sched.npidle, runtime_sched.mcount,
                runtime_sched.nmidle, runtime_sched.runqsize);
        if(detailed) {
                runtime_printf(" gcwaiting=%d nmidlelocked=%d nmspinning=%d stopwait=%d sysmonwait=%d\n",
                        runtime_sched.gcwaiting, runtime_sched.nmidlelocked, runtime_sched.nmspinning,
                        runtime_sched.stopwait, runtime_sched.sysmonwait);
        }
        // We must be careful while reading data from P's, M's and G's.
        // Even if we hold schedlock, most data can be changed concurrently.
        // E.g. (p->m ? p->m->id : -1) can crash if p->m changes from non-nil to nil.
        for(i = 0; i < runtime_gomaxprocs; i++) {
                p = runtime_allp[i];
                if(p == nil)
                        continue;
                mp = p->m;
                t = p->runqtail;
                h = p->runqhead;
                s = p->runqsize;
                q = t - h;
                if(q < 0)
                        q += s;
                if(detailed)
                        runtime_printf("  P%d: status=%d schedtick=%d syscalltick=%d m=%d runqsize=%d/%d gfreecnt=%d\n",
                                i, p->status, p->schedtick, p->syscalltick, mp ? mp->id : -1, q, s, p->gfreecnt);
                else {
                        // In non-detailed mode format lengths of per-P run queues as:
                        // [len1 len2 len3 len4]
                        fmt = " %d";
                        if(runtime_gomaxprocs == 1)
                                fmt = " [%d]\n";
                        else if(i == 0)
                                fmt = " [%d";
                        else if(i == runtime_gomaxprocs-1)
                                fmt = " %d]\n";
                        runtime_printf(fmt, q);
                }
        }
        if(!detailed) {
                runtime_unlock(&runtime_sched);
                return;
        }
        for(mp = runtime_allm; mp; mp = mp->alllink) {
                p = mp->p;
                gp = mp->curg;
                lockedg = mp->lockedg;
                id1 = -1;
                if(p)
                        id1 = p->id;
                id2 = -1;
                if(gp)
                        id2 = gp->goid;
                id3 = -1;
                if(lockedg)
                        id3 = lockedg->goid;
                runtime_printf("  M%d: p=%D curg=%D mallocing=%d throwing=%d gcing=%d"
                        " locks=%d dying=%d helpgc=%d spinning=%d lockedg=%D\n",
                        mp->id, id1, id2,
                        mp->mallocing, mp->throwing, mp->gcing, mp->locks, mp->dying, mp->helpgc,
                        mp->spinning, id3);
        }
        for(gp = runtime_allg; gp; gp = gp->alllink) {
                mp = gp->m;
                lockedm = gp->lockedm;
                runtime_printf("  G%D: status=%d(%s) m=%d lockedm=%d\n",
                        gp->goid, gp->status, gp->waitreason, mp ? mp->id : -1,
                        lockedm ? lockedm->id : -1);
        }
        runtime_unlock(&runtime_sched);
}

// Put mp on midle list.
// Sched must be locked.
static void
mput(M *mp)
{
        mp->schedlink = runtime_sched.midle;
        runtime_sched.midle = mp;
        runtime_sched.nmidle++;
        checkdead();
}

// Try to get an m from midle list.
// Sched must be locked.
static M*
mget(void)
{
        M *mp;

        if((mp = runtime_sched.midle) != nil){
                runtime_sched.midle = mp->schedlink;
                runtime_sched.nmidle--;
        }
        return mp;
}

// Put gp on the global runnable queue.
// Sched must be locked.
static void
globrunqput(G *gp)
{
        gp->schedlink = nil;
        if(runtime_sched.runqtail)
                runtime_sched.runqtail->schedlink = gp;
        else
                runtime_sched.runqhead = gp;
        runtime_sched.runqtail = gp;
        runtime_sched.runqsize++;
}

// Try get a batch of G's from the global runnable queue.
// Sched must be locked.
static G*
globrunqget(P *p, int32 max)
{
        G *gp, *gp1;
        int32 n;

        if(runtime_sched.runqsize == 0)
                return nil;
        n = runtime_sched.runqsize/runtime_gomaxprocs+1;
        if(n > runtime_sched.runqsize)
                n = runtime_sched.runqsize;
        if(max > 0 && n > max)
                n = max;
        runtime_sched.runqsize -= n;
        if(runtime_sched.runqsize == 0)
                runtime_sched.runqtail = nil;
        gp = runtime_sched.runqhead;
        runtime_sched.runqhead = gp->schedlink;
        n--;
        while(n--) {
                gp1 = runtime_sched.runqhead;
                runtime_sched.runqhead = gp1->schedlink;
                runqput(p, gp1);
        }
        return gp;
}

// Put p to on pidle list.
// Sched must be locked.
static void
pidleput(P *p)
{
        p->link = runtime_sched.pidle;
        runtime_sched.pidle = p;
        runtime_xadd(&runtime_sched.npidle, 1);  // TODO: fast atomic
}

// Try get a p from pidle list.
// Sched must be locked.
static P*
pidleget(void)
{
        P *p;

        p = runtime_sched.pidle;
        if(p) {
                runtime_sched.pidle = p->link;
                runtime_xadd(&runtime_sched.npidle, -1);  // TODO: fast atomic
        }
        return p;
}

// Put g on local runnable queue.
// TODO(dvyukov): consider using lock-free queue.
static void
runqput(P *p, G *gp)
{
        int32 h, t, s;

        runtime_lock(p);
retry:
        h = p->runqhead;
        t = p->runqtail;
        s = p->runqsize;
        if(t == h-1 || (h == 0 && t == s-1)) {
                runqgrow(p);
                goto retry;
        }
        p->runq[t++] = gp;
        if(t == s)
                t = 0;
        p->runqtail = t;
        runtime_unlock(p);
}

// Get g from local runnable queue.
static G*
runqget(P *p)
{
        G *gp;
        int32 t, h, s;

        if(p->runqhead == p->runqtail)
                return nil;
        runtime_lock(p);
        h = p->runqhead;
        t = p->runqtail;
        s = p->runqsize;
        if(t == h) {
                runtime_unlock(p);
                return nil;
        }
        gp = p->runq[h++];
        if(h == s)
                h = 0;
        p->runqhead = h;
        runtime_unlock(p);
        return gp;
}

// Grow local runnable queue.
// TODO(dvyukov): consider using fixed-size array
// and transfer excess to the global list (local queue can grow way too big).
static void
runqgrow(P *p)
{
        G **q;
        int32 s, t, h, t2;

        h = p->runqhead;
        t = p->runqtail;
        s = p->runqsize;
        t2 = 0;
        q = runtime_malloc(2*s*sizeof(*q));
        while(t != h) {
                q[t2++] = p->runq[h++];
                if(h == s)
                        h = 0;
        }
        runtime_free(p->runq);
        p->runq = q;
        p->runqhead = 0;
        p->runqtail = t2;
        p->runqsize = 2*s;
}

// Steal half of elements from local runnable queue of p2
// and put onto local runnable queue of p.
// Returns one of the stolen elements (or nil if failed).
static G*
runqsteal(P *p, P *p2)
{
        G *gp, *gp1;
        int32 t, h, s, t2, h2, s2, c, i;

        if(p2->runqhead == p2->runqtail)
                return nil;
        // sort locks to prevent deadlocks
        if(p < p2)
                runtime_lock(p);
        runtime_lock(p2);
        if(p2->runqhead == p2->runqtail) {
                runtime_unlock(p2);
                if(p < p2)
                        runtime_unlock(p);
                return nil;
        }
        if(p >= p2)
                runtime_lock(p);
        // now we've locked both queues and know the victim is not empty
        h = p->runqhead;
        t = p->runqtail;
        s = p->runqsize;
        h2 = p2->runqhead;
        t2 = p2->runqtail;
        s2 = p2->runqsize;
        gp = p2->runq[h2++];  // return value
        if(h2 == s2)
                h2 = 0;
        // steal roughly half
        if(t2 > h2)
                c = (t2 - h2) / 2;
        else
                c = (s2 - h2 + t2) / 2;
        // copy
        for(i = 0; i != c; i++) {
                // the target queue is full?
                if(t == h-1 || (h == 0 && t == s-1))
                        break;
                // the victim queue is empty?
                if(t2 == h2)
                        break;
                gp1 = p2->runq[h2++];
                if(h2 == s2)
                        h2 = 0;
                p->runq[t++] = gp1;
                if(t == s)
                        t = 0;
        }
        p->runqtail = t;
        p2->runqhead = h2;
        runtime_unlock(p2);
        runtime_unlock(p);
        return gp;
}

void runtime_testSchedLocalQueue(void)
  __asm__("runtime.testSchedLocalQueue");

void
runtime_testSchedLocalQueue(void)
{
        P p;
        G gs[1000];
        int32 i, j;

        runtime_memclr((byte*)&p, sizeof(p));
        p.runqsize = 1;
        p.runqhead = 0;
        p.runqtail = 0;
        p.runq = runtime_malloc(p.runqsize*sizeof(*p.runq));

        for(i = 0; i < (int32)nelem(gs); i++) {
                if(runqget(&p) != nil)
                        runtime_throw("runq is not empty initially");
                for(j = 0; j < i; j++)
                        runqput(&p, &gs[i]);
                for(j = 0; j < i; j++) {
                        if(runqget(&p) != &gs[i]) {
                                runtime_printf("bad element at iter %d/%d\n", i, j);
                                runtime_throw("bad element");
                        }
                }
                if(runqget(&p) != nil)
                        runtime_throw("runq is not empty afterwards");
        }
}

void runtime_testSchedLocalQueueSteal(void)
  __asm__("runtime.testSchedLocalQueueSteal");

void
runtime_testSchedLocalQueueSteal(void)
{
        P p1, p2;
        G gs[1000], *gp;
        int32 i, j, s;

        runtime_memclr((byte*)&p1, sizeof(p1));
        p1.runqsize = 1;
        p1.runqhead = 0;
        p1.runqtail = 0;
        p1.runq = runtime_malloc(p1.runqsize*sizeof(*p1.runq));

        runtime_memclr((byte*)&p2, sizeof(p2));
        p2.runqsize = nelem(gs);
        p2.runqhead = 0;
        p2.runqtail = 0;
        p2.runq = runtime_malloc(p2.runqsize*sizeof(*p2.runq));

        for(i = 0; i < (int32)nelem(gs); i++) {
                for(j = 0; j < i; j++) {
                        gs[j].sig = 0;
                        runqput(&p1, &gs[j]);
                }
                gp = runqsteal(&p2, &p1);
                s = 0;
                if(gp) {
                        s++;
                        gp->sig++;
                }
                while((gp = runqget(&p2)) != nil) {
                        s++;
                        gp->sig++;
                }
                while((gp = runqget(&p1)) != nil)
                        gp->sig++;
                for(j = 0; j < i; j++) {
                        if(gs[j].sig != 1) {
                                runtime_printf("bad element %d(%d) at iter %d\n", j, gs[j].sig, i);
                                runtime_throw("bad element");
                        }
                }
                if(s != i/2 && s != i/2+1) {
                        runtime_printf("bad steal %d, want %d or %d, iter %d\n",
                                s, i/2, i/2+1, i);
                        runtime_throw("bad steal");
                }
        }
}

intgo runtime_debug_setMaxThreads(intgo)
     __asm__(GOSYM_PREFIX "runtime_debug.setMaxThreads");

intgo
runtime_debug_setMaxThreads(intgo in)
{
        intgo out;

        runtime_lock(&runtime_sched);
        out = runtime_sched.maxmcount;
        runtime_sched.maxmcount = in;
        checkmcount();
        runtime_unlock(&runtime_sched);
        return out;
}

void
runtime_proc_scan(void (*addroot)(Obj))
{
        addroot((Obj){(byte*)&runtime_sched, sizeof runtime_sched, 0});
}

// When a function calls a closure, it passes the closure value to
// __go_set_closure immediately before the function call.  When a
// function uses a closure, it calls __go_get_closure immediately on
// function entry.  This is a hack, but it will work on any system.
// It would be better to use the static chain register when there is
// one.  It is also worth considering expanding these functions
// directly in the compiler.

void
__go_set_closure(void* v)
{
        g->closure = v;
}

void *
__go_get_closure(void)
{
        return g->closure;
}

// Return whether we are waiting for a GC.  This gc toolchain uses
// preemption instead.
bool
runtime_gcwaiting(void)
{
        return runtime_sched.gcwaiting;
}