Add Mac memory pressure statistic reporting and consolidate platform code

base/memory/memory_pressure_monitor_mac.cc

Index: base/memory/memory_pressure_monitor_mac.cc
diff --git a/base/memory/memory_pressure_monitor_mac.cc b/base/memory/memory_pressure_monitor_mac.cc
index d3430e94ca45a1ad9150c3cfe8f3342faf02ffe6..5ea381fd3bd4941987d7a7c471bca4b09b383723 100644
--- a/base/memory/memory_pressure_monitor_mac.cc
+++ b/base/memory/memory_pressure_monitor_mac.cc
@@ -8,6 +8,8 @@
 #include <stddef.h>
 #include <sys/sysctl.h>
 
+#include <cmath>
+
 #include "base/bind.h"
 #include "base/logging.h"
 #include "base/mac/mac_util.h"
@@ -33,29 +35,21 @@ MemoryPressureMonitor::MemoryPressureLevelForMacMemoryPressure(
   return MemoryPressureListener::MEMORY_PRESSURE_LEVEL_NONE;
 }
 
-void MemoryPressureMonitor::NotifyMemoryPressureChanged(
-    dispatch_source_s* event_source,
-    const MemoryPressureMonitor::DispatchCallback& dispatch_callback) {
-  int mac_memory_pressure = dispatch_source_get_data(event_source);
-  MemoryPressureListener::MemoryPressureLevel memory_pressure_level =
-      MemoryPressureLevelForMacMemoryPressure(mac_memory_pressure);
-  dispatch_callback.Run(memory_pressure_level);
-}
-
 MemoryPressureMonitor::MemoryPressureMonitor()
-    // The MemoryPressureListener doesn't want to know about transitions to
-    // MEMORY_PRESSURE_LEVEL_NONE so don't watch for
-    // DISPATCH_MEMORYPRESSURE_NORMAL notifications.
     : memory_level_event_source_(dispatch_source_create(
           DISPATCH_SOURCE_TYPE_MEMORYPRESSURE,
           0,
-          DISPATCH_MEMORYPRESSURE_WARN | DISPATCH_MEMORYPRESSURE_CRITICAL,
+          DISPATCH_MEMORYPRESSURE_WARN | DISPATCH_MEMORYPRESSURE_CRITICAL |
+              DISPATCH_MEMORYPRESSURE_NORMAL,
           dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0))),
       dispatch_callback_(
-          base::Bind(&MemoryPressureListener::NotifyMemoryPressure)) {
+          base::Bind(&MemoryPressureListener::NotifyMemoryPressure)),
+      last_pressure_change_(CFAbsoluteTimeGetCurrent()),
+      reporting_error_(0) {
+  last_pressure_level_ = GetCurrentPressureLevel();
   dispatch_source_set_event_handler(memory_level_event_source_, ^{
-    NotifyMemoryPressureChanged(memory_level_event_source_.get(),
-                                dispatch_callback_);
+    OnMemoryPressureChanged(memory_level_event_source_.get(),
+                            dispatch_callback_);
   });
   dispatch_resume(memory_level_event_source_);
 }
@@ -72,6 +66,35 @@ MemoryPressureMonitor::GetCurrentPressureLevel() const {
                &length, nullptr, 0);
   return MemoryPressureLevelForMacMemoryPressure(mac_memory_pressure);
 }
+void MemoryPressureMonitor::OnMemoryPressureChanged(
+    dispatch_source_s* event_source,
+    const MemoryPressureMonitor::DispatchCallback& dispatch_callback) {
+  int mac_memory_pressure = dispatch_source_get_data(event_source);
+  MemoryPressureListener::MemoryPressureLevel memory_pressure_level =
+      MemoryPressureLevelForMacMemoryPressure(mac_memory_pressure);
+  CFTimeInterval now = CFAbsoluteTimeGetCurrent();
+  CFTimeInterval since_last_change = now - last_pressure_change_;
+  last_pressure_change_ = now;
+
+  double ticks_to_report;
+  reporting_error_ =
+      modf(since_last_change + reporting_error_, &ticks_to_report);
+
+  // Sierra fails to call the handler when pressure returns to normal,
+  // which would skew our data. For example, if pressure went to 'warn'
+  // at T0, back to 'normal' at T1, then to 'critical' at T10, we would
+  // report 10 ticks of 'warn' instead of 1 tick of 'warn' and 9 ticks
+  // of 'normal'.
+  // This is rdar://29114314
+  if (mac::IsAtMostOS10_11())
+    RecordMemoryPressure(last_pressure_level_,
+                         static_cast<int>(ticks_to_report));
+
+  last_pressure_level_ = memory_pressure_level;
+  if (memory_pressure_level !=
+      MemoryPressureListener::MEMORY_PRESSURE_LEVEL_NONE)
+    dispatch_callback.Run(memory_pressure_level);
+}
 
 void MemoryPressureMonitor::SetDispatchCallback(
     const DispatchCallback& callback) {