From 1ad3de988d2f41c72de66613c68ed78507a3abbd Mon Sep 17 00:00:00 2001 From: Pedro Alves Date: Tue, 24 May 2016 14:47:57 +0100 Subject: [PATCH] [Linux] Avoid refetching core-of-thread if thread hasn't run Hacking the gdb.threads/attach-many-short-lived-threads.exp test to spawn thousands of threads instead of dozens, I saw GDB having trouble keeping up with threads being spawned too fast, when it tried to stop them all. This was because while gdb is doing that, it updates the thread list to make sure no new thread has sneaked in that might need to be paused. It does this a few times until it sees no-new-threads twice in a row. The thread listing update itself is not that expensive, however, in the Linux backend, updating the threads list calls linux_common_core_of_thread for each LWP to record on which core each LWP was last seen running, which opens/reads/closes a /proc file for each LWP which becomes expensive when you need to do it for thousands of LWPs. perf shows gdb in linux_common_core_of_thread 44% of the time, in the stop_all_threads -> update_thread_list path in this use case. This patch simply makes linux_common_core_of_thread avoid updating the core the thread is bound to if the thread hasn't run since the last time we updated that info. This makes linux_common_core_of_thread disappear into the noise in the perf report. gdb/ChangeLog: 2016-05-24 Pedro Alves PR gdb/19828 * linux-nat.c (linux_resume_one_lwp_throw): Clear the LWP's core field. (linux_nat_update_thread_list): Don't fetch the core if already known. --- gdb/ChangeLog | 8 ++++++++ gdb/linux-nat.c | 9 ++++++++- 2 files changed, 16 insertions(+), 1 deletion(-) diff --git a/gdb/ChangeLog b/gdb/ChangeLog index dd6282404c0..ba4fc8f2ee7 100644 --- a/gdb/ChangeLog +++ b/gdb/ChangeLog @@ -1,3 +1,11 @@ +2016-05-24 Pedro Alves + + PR gdb/19828 + * linux-nat.c (linux_resume_one_lwp_throw): Clear the LWP's core + field. + (linux_nat_update_thread_list): Don't fetch the core if already + known. + 2016-05-24 Pedro Alves PR gdb/19828 diff --git a/gdb/linux-nat.c b/gdb/linux-nat.c index 19fe12f80a3..dbadf2900e0 100644 --- a/gdb/linux-nat.c +++ b/gdb/linux-nat.c @@ -1432,6 +1432,7 @@ linux_resume_one_lwp_throw (struct lwp_info *lp, int step, status. Note that we must not throw after this is cleared, otherwise handle_zombie_lwp_error would get confused. */ lp->stopped = 0; + lp->core = -1; lp->stop_reason = TARGET_STOPPED_BY_NO_REASON; registers_changed_ptid (lp->ptid); } @@ -3785,7 +3786,13 @@ linux_nat_update_thread_list (struct target_ops *ops) /* Update the processor core that each lwp/thread was last seen running on. */ ALL_LWPS (lwp) - lwp->core = linux_common_core_of_thread (lwp->ptid); + { + /* Avoid accessing /proc if the thread hasn't run since we last + time we fetched the thread's core. Accessing /proc becomes + noticeably expensive when we have thousands of LWPs. */ + if (lwp->core == -1) + lwp->core = linux_common_core_of_thread (lwp->ptid); + } } static char * -- 2.30.2