Bump field trial allocator size to 64kb and report usage

base/metrics/field_trial.cc

Index: base/metrics/field_trial.cc
diff --git a/base/metrics/field_trial.cc b/base/metrics/field_trial.cc
index dec24c9663b19d5577b6739abb67571acf54607f..f7c2924cd21ec6987960f123db6587ac8d15aa8b 100644
--- a/base/metrics/field_trial.cc
+++ b/base/metrics/field_trial.cc
@@ -12,7 +12,6 @@
 #include "base/command_line.h"
 #include "base/feature_list.h"
 #include "base/logging.h"
-#include "base/metrics/histogram_macros.h"
 #include "base/pickle.h"
 #include "base/process/memory.h"
 #include "base/rand_util.h"
@@ -42,16 +41,26 @@ const bool kUseSharedMemoryForFieldTrials = false;
 
 // Constants for the field trial allocator.
 const char kAllocatorName[] = "FieldTrialAllocator";
-const uint32_t kFieldTrialType = 0xABA17E13 + 1;  // SHA1(FieldTrialEntry) v1
+const uint32_t kFieldTrialType = 0xABA17E13 + 2;  // SHA1(FieldTrialEntry) v2
+
+// We allocate 64 KiB to hold all the field trial data. This should be enough,
+// as currently we use ~8KiB for the field trials, and ~10KiB for experiment
+// parameters (as of 9/11/2016). This also doesn't allocate all 64 KiB at once
+// -- the pages only get mapped to physical memory when they are touched. If the
+// size of the allocated field trials does get larger than 64 KiB, then we will
+// drop some field trials in child processes, leading to an inconsistent view
+// between browser and child processes and possibly causing crashes (see
+// crbug.com/661617).
 #if !defined(OS_NACL)
-const size_t kFieldTrialAllocationSize = 4 << 10;  // 4 KiB = one page
+const size_t kFieldTrialAllocationSize = 64 << 10;  // 64 KiB
 #endif
 
 // We create one FieldTrialEntry per field trial in shared memory, via
 // AddToAllocatorWhileLocked. The FieldTrialEntry is followed by a base::Pickle
-// object that we unpickle and read from.
+// object that we unpickle and read from. Any changes to this structure requires
+// a bump in kFieldTrialType id defined above.
 struct FieldTrialEntry {
-  bool activated;
+  uint32_t activated;

[Alexei Svitkine (slow), 2016/11/09 18:14:27]

Please add a comment above this field. Especially to explain that it's a
boolean.

[lawrencewu, 2016/11/09 18:20:48]

Somehow, this never actually gets done -- Done.

 
   // Size of the pickled structure, NOT the total size of this entry.
   uint32_t size;
@@ -728,8 +737,7 @@ void FieldTrialList::CopyFieldTrialStateToFlags(
                                      std::to_string(field_trial_length);
 
     cmd_line->AppendSwitchASCII(field_trial_handle_switch, field_trial_handle);
-    UMA_HISTOGRAM_COUNTS_10000("UMA.FieldTrialAllocator.Size",
-                               field_trial_length);
+    global_->field_trial_allocator_->UpdateTrackingHistograms();
     return;
   }
 #endif