base/allocator: Record a synthetic field trial: PAE-and-PCScan

In order to perform a tri-arm experiment of
PartitionAlloc-Everywhere and PCScan, adds a new synthetic
field trial.

Bug: 1121427
Change-Id: I70a1b8384cd066cea2d92000364877e965d6b82c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2573560
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Bartek Nowierski <bartekn@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#834706}

base/allocator/partition_allocator/partition_alloc_features.cc

@@ -25,5 +25,11 @@ const Feature kPartitionAllocGigaCage{"PartitionAllocGigaCage32bit",
 const Feature kPartitionAllocPCScan{"PartitionAllocPCScan",
                                     FEATURE_DISABLED_BY_DEFAULT};
 
+#if BUILDFLAG(USE_PARTITION_ALLOC_AS_MALLOC)
+// If enabled, PCScan is turned on only for the browser's malloc partition.
+const Feature kPartitionAllocPCScanBrowserOnly{
+    "PartitionAllocPCScanBrowserOnly", FEATURE_DISABLED_BY_DEFAULT};
+#endif
+
 }  // namespace features
 }  // namespace base

base/allocator/partition_allocator/partition_alloc_features.h

@@ -27,6 +27,9 @@ namespace features {
 
 extern const BASE_EXPORT Feature kPartitionAllocGigaCage;
 extern const BASE_EXPORT Feature kPartitionAllocPCScan;
+#if BUILDFLAG(USE_PARTITION_ALLOC_AS_MALLOC)
+extern const BASE_EXPORT Feature kPartitionAllocPCScanBrowserOnly;
+#endif
 
 ALWAYS_INLINE bool IsPartitionAllocGigaCageEnabled() {
 #if defined(PA_HAS_64_BITS_POINTERS) && defined(OS_WIN)
@@ -61,12 +64,23 @@ ALWAYS_INLINE bool IsPartitionAllocGigaCageEnabled() {
 }
 
 ALWAYS_INLINE bool IsPartitionAllocPCScanEnabled() {
-#if !defined(PA_HAS_64_BITS_POINTERS)
-  return false;
-#endif  // !PA_HAS_64_BITS_POINTERS
+#if defined(PA_HAS_64_BITS_POINTERS)
   return FeatureList::IsEnabled(kPartitionAllocPCScan);
+#else  // PA_HAS_64_BITS_POINTERS
+  return false;
+#endif
 }
 
+#if BUILDFLAG(USE_PARTITION_ALLOC_AS_MALLOC)
+ALWAYS_INLINE bool IsPartitionAllocPCScanBrowserOnlyEnabled() {
+#if defined(PA_HAS_64_BITS_POINTERS)
+  return FeatureList::IsEnabled(kPartitionAllocPCScanBrowserOnly);
+#else  // PA_HAS_64_BITS_POINTERS
+  return false;
+#endif
+}
+#endif
+
 }  // namespace features
 }  // namespace base
 

chrome/browser/metrics/chrome_browser_main_extra_parts_metrics.cc

@@ -7,6 +7,7 @@
 #include <cmath>
 #include <string>
 
+#include "base/allocator/partition_allocator/partition_alloc_features.h"
 #include "base/bind.h"
 #include "base/command_line.h"
 #include "base/cpu.h"
@@ -546,11 +547,14 @@ void ChromeBrowserMainExtraPartsMetrics::PreBrowserStart() {
 
 #endif  // defined(OS_WIN)
 
-  // Records whether or not PartitionAlloc is used as the default allocator.
+  // Records whether or not PartitionAlloc is used as the default allocator,
+  // plus whether or not PCScan is enabled.
   ChromeMetricsServiceAccessor::RegisterSyntheticFieldTrial(
-      "PartitionAllocEverywhere",
+      "PartitionAllocEverywhereAndPCScan",
 #if BUILDFLAG(USE_PARTITION_ALLOC_EVERYWHERE)
-      "Enabled"
+      base::features::IsPartitionAllocPCScanBrowserOnlyEnabled()
+          ? "EnabledWithPCScan"
+          : "EnabledWithoutPCScan"
 #else
       "Disabled"
 #endif

content/app/content_main_runner_impl.cc

@@ -17,6 +17,7 @@
 #include "base/allocator/allocator_extension.h"
 #include "base/allocator/allocator_shim.h"
 #include "base/allocator/buildflags.h"
+#include "base/allocator/partition_allocator/partition_alloc_features.h"
 #include "base/at_exit.h"
 #include "base/base_switches.h"
 #include "base/bind.h"
@@ -246,12 +247,9 @@ void EnablePCScanForMallocPartitionsIfNeeded() {
 }
 
 void EnablePCScanForMallocPartitionsInBrowserProcessIfNeeded() {
-  static const base::Feature kPartitionAllocPCScanBrowserOnly{
-      "PartitionAllocPCScanBrowserOnly", base::FEATURE_DISABLED_BY_DEFAULT};
-  (void)kPartitionAllocPCScanBrowserOnly;
 #if BUILDFLAG(USE_PARTITION_ALLOC_AS_MALLOC)
   CHECK(base::FeatureList::GetInstance());
-  if (base::FeatureList::IsEnabled(kPartitionAllocPCScanBrowserOnly)) {
+  if (base::features::IsPartitionAllocPCScanBrowserOnlyEnabled()) {
     base::allocator::EnablePCScan();
   }
 #endif