mem-cache: prevent prefetcher from saturating the write buffer

When the write buffer is full, it still has space to store an additional
number of entries (reserve) equal to the number of MSHRs so that if any
of them requires a writeback this can be handled. Even if the slave port
is blocked, a prefetcher can generate new MSHR entries that may lead to
additional writebacks and eventually saturate the reserve space. This is
solved by checking if the cache is blocked for accesses before
prefetching data.

Change-Id: Iaad04dd6786a09eab7afae4a53d1b1299c341f33
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/29615
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>

diff --git a/src/mem/cache/base.cc b/src/mem/cache/base.cc
index 1e16008f694741e3991426f4270b2ece7f1e3e65..018770332e1dab2fe800d34a872e926180edbdb8 100644 (file)
--- a/src/mem/cache/base.cc
+++ b/src/mem/cache/base.cc
@@ -514,7 +514,7 @@ BaseCache::recvTimingResp(PacketPtr pkt)
 
         // Request the bus for a prefetch if this deallocation freed enough
         // MSHRs for a prefetch to take place
-        if (prefetcher && mshrQueue.canPrefetch()) {
+        if (prefetcher && mshrQueue.canPrefetch() && !isBlocked()) {
             Tick next_pf_time = std::max(prefetcher->nextPrefetchReadyTime(),
                                          clockEdge());
             if (next_pf_time != MaxTick)
@@ -764,7 +764,7 @@ BaseCache::getNextQueueEntry()
 
     // fall through... no pending requests.  Try a prefetch.
     assert(!miss_mshr && !wq_entry);
-    if (prefetcher && mshrQueue.canPrefetch()) {
+    if (prefetcher && mshrQueue.canPrefetch() && !isBlocked()) {
         // If we have a miss queue slot, we can try a prefetch
         PacketPtr pkt = prefetcher->getPacket();
         if (pkt) {
@@ -1643,7 +1643,7 @@ BaseCache::nextQueueReadyTime() const
 
     // Don't signal prefetch ready time if no MSHRs available
     // Will signal once enoguh MSHRs are deallocated
-    if (prefetcher && mshrQueue.canPrefetch()) {
+    if (prefetcher && mshrQueue.canPrefetch() && !isBlocked()) {
         nextReady = std::min(nextReady,
                              prefetcher->nextPrefetchReadyTime());
     }