cache: improve coherence handling of writebacks

If we write back an exclusive copy, we now mark it
as such, so the cache receiving the writeback can
mark its copy as exclusive.  This avoids some
unnecessary upgrade requests when a cache later
tries to re-acquire exclusive access to the block.

diff --git a/src/mem/cache/cache_impl.hh b/src/mem/cache/cache_impl.hh
index 4f4d720b6ccb2b75f2888fa977b0645128c579d1..86bf79b7b77174ac1e8f53ffd5d4554281fe7b9d 100644 (file)
--- a/src/mem/cache/cache_impl.hh
+++ b/src/mem/cache/cache_impl.hh
@@ -360,6 +360,9 @@ Cache<TagStore>::access(PacketPtr pkt, BlkType *&blk,
         }
         std::memcpy(blk->data, pkt->getPtr<uint8_t>(), blkSize);
         blk->status |= BlkDirty;
+        if (pkt->isSupplyExclusive()) {
+            blk->status |= BlkWritable;
+        }
         // nothing else to do; writeback doesn't expect response
         assert(!pkt->needsResponse());
         incHitCount(pkt, id);
@@ -1007,6 +1010,9 @@ Cache<TagStore>::writebackBlk(BlkType *blk)
     Request *writebackReq =
         new Request(tags->regenerateBlkAddr(blk->tag, blk->set), blkSize, 0);
     PacketPtr writeback = new Packet(writebackReq, MemCmd::Writeback, -1);
+    if (blk->isWritable()) {
+        writeback->setSupplyExclusive();
+    }
     writeback->allocate();
     std::memcpy(writeback->getPtr<uint8_t>(), blk->data, blkSize);
 
@@ -1307,6 +1313,8 @@ Cache<TagStore>::snoopTiming(PacketPtr pkt)
             pkt->assertMemInhibit();
             if (!pkt->needsExclusive()) {
                 pkt->assertShared();
+                // the writeback is no longer the exclusive copy in the system
+                wb_pkt->clearSupplyExclusive();
             } else {
                 // if we're not asserting the shared line, we need to
                 // invalidate our copy.  we'll do that below as long as

diff --git a/src/mem/packet.hh b/src/mem/packet.hh
index cefb7c2edaa14596c47bf54b07d0076afa1189f6..48b80879e6b72ea495e0bde149f394af967bfbe7 100644 (file)
--- a/src/mem/packet.hh
+++ b/src/mem/packet.hh
@@ -422,6 +422,7 @@ class Packet : public FastAlloc, public Printable
     void setExpressSnoop()      { flags.set(EXPRESS_SNOOP); }
     bool isExpressSnoop()       { return flags.isSet(EXPRESS_SNOOP); }
     void setSupplyExclusive()   { flags.set(SUPPLY_EXCLUSIVE); }
+    void clearSupplyExclusive() { flags.clear(SUPPLY_EXCLUSIVE); }
     bool isSupplyExclusive()    { return flags.isSet(SUPPLY_EXCLUSIVE); }
 
     // Network error conditions... encapsulate them as methods since