[PartitionAlloc] Make the thread cache threshold explicit.

Bug: 998048
Change-Id: I9c8cdbec7b7748d6d4f9b473affb6d3948a374ac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2428896
Commit-Queue: Benoit L <lizeb@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810977}

base/allocator/partition_allocator/partition_alloc.cc

@@ -272,16 +272,6 @@ void PartitionRoot<thread_safe>::Init(PartitionOptions opts) {
   // If alignment needs to be enforced, disallow adding cookies and/or tags at
   // the beginning of the slot.
   allow_extras = (opts.alignment != PartitionOptions::Alignment::kAlignedAlloc);
-#if !defined(OS_POSIX)
-  // TLS in ThreadCache not supported on other OSes.
-  with_thread_cache = false;
-#else
-  with_thread_cache =
-      (opts.thread_cache == PartitionOptions::ThreadCache::kEnabled);
-
-  if (with_thread_cache)
-    internal::ThreadCache::Init(this);
-#endif  // !defined(OS_POSIX)
 
   // We mark the sentinel bucket/page as free to make sure it is skipped by our
   // logic to find a new active page.
@@ -320,6 +310,17 @@ void PartitionRoot<thread_safe>::Init(PartitionOptions opts) {
   // this operation is idempotent, so there is no harm.
   InitBucketIndexLookup(this);
 
+#if !defined(OS_POSIX)
+  // TLS in ThreadCache not supported on other OSes.
+  with_thread_cache = false;
+#else
+  with_thread_cache =
+      (opts.thread_cache == PartitionOptions::ThreadCache::kEnabled);
+
+  if (with_thread_cache)
+    internal::ThreadCache::Init(this);
+#endif  // !defined(OS_POSIX)
+
   initialized = true;
 }
 

base/allocator/partition_allocator/thread_cache.cc

@@ -105,6 +105,8 @@ void ThreadCacheRegistry::PurgeAll() {
 
 // static
 void ThreadCache::Init(PartitionRoot<ThreadSafe>* root) {
+  PA_CHECK(root->buckets[kBucketCount - 1].slot_size == kSizeThreshold);
+
   bool ok = PartitionTlsCreate(&g_thread_cache_key, DeleteThreadCache);
   PA_CHECK(ok);
 

base/allocator/partition_allocator/thread_cache.h

@@ -17,6 +17,7 @@
 #include "base/no_destructor.h"
 #include "base/partition_alloc_buildflags.h"
 #include "base/synchronization/lock.h"
+#include "build/build_config.h"
 
 namespace base {
 
@@ -166,9 +167,15 @@ class BASE_EXPORT ThreadCache {
     PartitionFreelistEntry* freelist_head;
   };
 
-  // TODO(lizeb): Optimize the threshold, and define it as an allocation size
-  // rather than a bucket index.
-  static constexpr size_t kBucketCount = 40;
+  // TODO(lizeb): Optimize the threshold.
+#if defined(ARCH_CPU_64_BITS)
+  static constexpr size_t kBucketCount = 41;
+#else
+  static constexpr size_t kBucketCount = 49;
+#endif
+  // Checked in ThreadCache::Init(), not with static_assert() as the size is not
+  // set at compile-time.
+  static constexpr size_t kSizeThreshold = 512;
   static_assert(
       kBucketCount < kNumBuckets,
       "Cannot have more cached buckets than what the allocator supports");