Record if large discardable memory allocations are from IPC or Freelist

Bug: 1134257
Change-Id: I7a5da462acc3031fbe940345c907db1abfb1d11a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2441946Reviewed-by: Michael Thiessen <mthiesse@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Peng Huang <penghuang@chromium.org>
Commit-Queue: Thiabaud Engelbrecht <thiabaud@google.com>
Cr-Commit-Position: refs/heads/master@{#813753}

components/discardable_memory/client/client_discardable_shared_memory_manager.cc

@@ -298,6 +298,11 @@ ClientDiscardableSharedMemoryManager::AllocateLockedDiscardableMemory(
 
     free_span->set_is_locked(true);
 
+    if (pages >= allocation_pages) {
+      UMA_HISTOGRAM_BOOLEAN("Memory.Discardable.LargeAllocationFromFreelist",
+                            true);
+    }
+
     // Memory usage is guaranteed to have changed after having removed
     // at least one span from the free lists.
     MemoryUsageChanged(heap_->GetSize(), heap_->GetSizeOfFreeLists());
@@ -357,6 +362,11 @@ ClientDiscardableSharedMemoryManager::AllocateLockedDiscardableMemory(
     heap_->MergeIntoFreeLists(std::move(leftover));
   }
 
+  if (pages >= allocation_pages) {
+    UMA_HISTOGRAM_BOOLEAN("Memory.Discardable.LargeAllocationFromFreelist",
+                          false);
+  }
+
   MemoryUsageChanged(heap_->GetSize(), heap_->GetSizeOfFreeLists());
 
   auto discardable_memory =

tools/metrics/histograms/enums.xml

@@ -7930,6 +7930,11 @@ Called by update_bad_message_reasons.py.-->
   <int value="1" label="Known"/>
 </enum>
 
+<enum name="BooleanLargeAllocationFromFreelist">
+  <int value="0" label="Allocated via IPC"/>
+  <int value="1" label="Allocated via Freelist"/>
+</enum>
+
 <enum name="BooleanLatched">
   <int value="0" label="Not latched"/>
   <int value="1" label="Latched"/>

tools/metrics/histograms/histograms_xml/memory/histograms.xml

@@ -398,6 +398,16 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
   </summary>
 </histogram>
 
+<histogram name="Memory.Discardable.LargeAllocationFromFreelist"
+    enum="BooleanLargeAllocationFromFreelist" expires_after="2021-03-07">
+  <owner>thiabaud@google.com</owner>
+  <owner>lizeb@chromium.org</owner>
+  <summary>
+    Boolean metric which tracks whether or not large chunks of discardable
+    memory are allocated from the freelist or via IPC.
+  </summary>
+</histogram>
+
 <histogram name="Memory.Discardable.LockingSuccess"
     enum="BooleanLockingSuccess" expires_after="2021-03-07">
   <owner>thiabaud@google.com</owner>