Bug 876029 - Make Gonk memory pressure by-pass the event queue. r=jlebar

widget/gonk/GonkMemoryPressureMonitoring.cpp

@@ -11,6 +11,7 @@
 #include "mozilla/Services.h"
 #include "nsIObserver.h"
 #include "nsIObserverService.h"
+#include "nsMemoryPressure.h"
 #include "nsThreadUtils.h"
 #include <errno.h>
 #include <fcntl.h>
@@ -24,37 +25,6 @@ using namespace mozilla;
 
 namespace {
 
-class MemoryPressureRunnable : public nsRunnable
-{
-  const char *mTopic;
-  const PRUnichar *mData;
-public:
-  MemoryPressureRunnable(const char *aTopic, const PRUnichar *aData) :
-    mTopic(aTopic), mData(aData)
-  {
-  }
-
-  NS_IMETHOD Run()
-  {
-    MOZ_ASSERT(NS_IsMainThread());
-    LOG("Dispatching low-memory memory-pressure event");
-
-    nsCOMPtr<nsIObserverService> os = services::GetObserverService();
-    if (os) {
-      os->NotifyObservers(nullptr, mTopic, mData);
-    }
-    return NS_OK;
-  }
-};
-
-static void
-Dispatch(const char *aTopic, const PRUnichar *aData)
-{
-  nsRefPtr<MemoryPressureRunnable> memoryPressureRunnable =
-    new MemoryPressureRunnable(aTopic, aData);
-  NS_DispatchToMainThread(memoryPressureRunnable);
-}
-
 /**
  * MemoryPressureWatcher watches sysfs from its own thread to notice when the
  * system is under memory pressure.  When we observe memory pressure, we use
@@ -186,8 +156,8 @@ public:
 
       // We use low-memory-no-forward because each process has its own watcher
       // and thus there is no need for the main process to forward this event.
-      Dispatch("memory-pressure",
-               NS_LITERAL_STRING("low-memory-no-forward").get());
+      rv = NS_DispatchMemoryPressure(MemPressure_New);
+      NS_ENSURE_SUCCESS(rv, rv);
 
       // Manually check lowMemFd until we observe that memory pressure is over.
       // We won't fire any more low-memory events until we observe that
@@ -219,8 +189,8 @@ public:
         NS_ENSURE_SUCCESS(rv, rv);
 
         if (memoryPressure) {
-          Dispatch("memory-pressure",
-                   NS_LITERAL_STRING("low-memory-ongoing-no-forward").get());
+          rv = NS_DispatchMemoryPressure(MemPressure_Ongoing);
+          NS_ENSURE_SUCCESS(rv, rv);
           continue;
         }
       } while (false);

xpcom/base/AvailableMemoryTracker.cpp

@@ -15,6 +15,7 @@
 #include "nsIObserverService.h"
 #include "nsIRunnable.h"
 #include "nsISupports.h"
+#include "nsMemoryPressure.h"
 #include "nsPrintfCString.h"
 #include "nsThread.h"
 
@@ -187,7 +188,7 @@ bool MaybeScheduleMemoryPressureEvent()
   sLastLowMemoryNotificationTime = PR_IntervalNow();
 
   LOG("Scheduling memory pressure notification.");
-  ScheduleMemoryPressureEvent();
+  NS_DispatchEventualMemoryPressure(MemPressure_New);
   return true;
 }
 
@@ -212,7 +213,7 @@ void CheckMemAvailable()
       // so don't worry about firing this notification too often.
       LOG("Detected low virtual memory.");
       PR_ATOMIC_INCREMENT(&sNumLowVirtualMemEvents);
-      ScheduleMemoryPressureEvent();
+      NS_DispatchEventualMemoryPressure(MemPressure_New);
     }
     else if (stat.ullAvailPageFile < sLowCommitSpaceThreshold * 1024 * 1024) {
       LOG("Detected low available page file space.");

xpcom/threads/moz.build

@@ -24,6 +24,7 @@ MODULE = 'xpcom'
 
 EXPORTS += [
     'nsEventQueue.h',
+    'nsMemoryPressure.h',
     'nsProcess.h',
     'nsThread.h',
 ]
@@ -40,6 +41,7 @@ CPP_SOURCES += [
     'TimerThread.cpp',
     'nsEnvironment.cpp',
     'nsEventQueue.cpp',
+    'nsMemoryPressure.cpp',
     'nsProcessCommon.cpp',
     'nsThread.cpp',
     'nsThreadManager.cpp',

xpcom/threads/nsMemoryPressure.cpp

@@ -0,0 +1,48 @@
+#include "nsMemoryPressure.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/Atomics.h"
+
+#include "nsThreadUtils.h"
+
+using namespace mozilla;
+
+static Atomic<int32_t, Relaxed> sMemoryPressurePending;
+MOZ_STATIC_ASSERT(MemPressure_None == 0,
+                  "Bad static initialization with the default constructor.");
+
+MemoryPressureState
+NS_GetPendingMemoryPressure()
+{
+  int32_t value = sMemoryPressurePending.exchange(MemPressure_None);
+  return MemoryPressureState(value);
+}
+
+void
+NS_DispatchEventualMemoryPressure(MemoryPressureState state)
+{
+  /*
+   * A new memory pressure event erases an ongoing memory pressure, but an
+   * existing "new" memory pressure event takes precedence over a new "ongoing"
+   * memory pressure event.
+   */
+  switch (state) {
+    case MemPressure_None:
+      sMemoryPressurePending = MemPressure_None;
+      break;
+    case MemPressure_New:
+      sMemoryPressurePending = MemPressure_New;
+      break;
+    case MemPressure_Ongoing:
+      sMemoryPressurePending.compareExchange(MemPressure_None, MemPressure_Ongoing);
+      break;
+  }
+}
+
+nsresult
+NS_DispatchMemoryPressure(MemoryPressureState state)
+{
+  NS_DispatchEventualMemoryPressure(state);
+  nsCOMPtr<nsIRunnable> event = new nsRunnable;
+  NS_ENSURE_TRUE(event, NS_ERROR_OUT_OF_MEMORY);
+  return NS_DispatchToMainThread(event);
+}

xpcom/threads/nsMemoryPressure.h

@@ -0,0 +1,76 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef nsMemoryPressure_h__
+#define nsMemoryPressure_h__
+
+#include "nscore.h"
+
+enum MemoryPressureState {
+  /*
+   * No memory pressure.
+   */
+  MemPressure_None = 0,
+
+  /*
+   * New memory pressure deteced.
+   *
+   * On a new memory pressure, we stop everything to start cleaning
+   * aggresively the memory used, in order to free as much memory as
+   * possible.
+   */
+  MemPressure_New,
+
+  /*
+   * Repeated memory pressure.
+   *
+   * A repeated memory pressure implies to clean softly recent allocations.
+   * It is supposed to happen after a new memory pressure which already
+   * cleaned aggressivley.  So there is no need to damage the reactivity of
+   * Gecko by stopping the world again.
+   *
+   * In case of conflict with an new memory pressue, the new memory pressure
+   * takes precedence over an ongoing memory pressure.  The reason being
+   * that if no events are processed between 2 notifications (new followed
+   * by ongoing, or ongoing followed by a new) we want to be as aggresive as
+   * possible on the clean-up of the memory.  After all, we are trying to
+   * keep Gecko alive as long as possible.
+   */
+  MemPressure_Ongoing
+};
+
+/**
+ * Return and erase the latest state of the memory pressure event set by any of
+ * the corresponding dispatch function.
+ */
+MemoryPressureState
+NS_GetPendingMemoryPressure();
+
+/**
+ * This function causes the main thread to fire a memory pressure event
+ * before processing the next event, but if there are no events pending in
+ * the main thread's event queue, the memory pressure event would not be
+ * dispatched until one is enqueued. It is infallible and does not allocate
+ * any memory.
+ *
+ * You may call this function from any thread.
+ */
+void
+NS_DispatchEventualMemoryPressure(MemoryPressureState state);
+
+/**
+ * This function causes the main thread to fire a memory pressure event
+ * before processing the next event. We wake up the main thread by adding a
+ * dummy event to its event loop, so, unlike with
+ * NS_DispatchEventualMemoryPressure, this memory-pressure event is always
+ * fired relatively quickly, even if the event loop is otherwise empty.
+ *
+ * You may call this function from any thread.
+ */
+nsresult
+NS_DispatchMemoryPressure(MemoryPressureState state);
+
+#endif // nsMemoryPressure_h__

xpcom/threads/nsThread.cpp

@@ -5,6 +5,7 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #include "mozilla/ReentrantMonitor.h"
+#include "nsMemoryPressure.h"
 #include "nsThread.h"
 #include "nsThreadManager.h"
 #include "nsIClassInfoImpl.h"
@@ -57,23 +58,6 @@ NS_DECL_CI_INTERFACE_GETTER(nsThread)
 
 nsIThreadObserver* nsThread::sMainThreadObserver = nullptr;
 
-namespace mozilla {
-
-// Fun fact: Android's GCC won't convert bool* to int32_t*, so we can't
-// PR_ATOMIC_SET a bool.
-static int32_t sMemoryPressurePending = 0;
-
-/*
- * It's important that this function not acquire any locks, nor do anything
- * which might cause malloc to run.
- */
-void ScheduleMemoryPressureEvent()
-{
-  PR_ATOMIC_SET(&sMemoryPressurePending, 1);
-}
-
-} // namespace mozilla
-
 //-----------------------------------------------------------------------------
 // Because we do not have our own nsIFactory, we have to implement nsIClassInfo
 // somewhat manually.
@@ -580,14 +564,20 @@ nsThread::ProcessNextEvent(bool mayWait, bool *result)
   // Fire a memory pressure notification, if we're the main thread and one is
   // pending.
   if (MAIN_THREAD == mIsMainThread && !ShuttingDown()) {
-    bool mpPending = PR_ATOMIC_SET(&sMemoryPressurePending, 0);
-    if (mpPending) {
+    MemoryPressureState mpPending = NS_GetPendingMemoryPressure();
+    if (mpPending != MemPressure_None) {
       nsCOMPtr<nsIObserverService> os = services::GetObserverService();
+
+      // Use no-forward to prevent the notifications from being transferred to
+      // the children of this process.
+      NS_NAMED_LITERAL_STRING(lowMem, "low-memory-no-forward");
+      NS_NAMED_LITERAL_STRING(lowMemOngoing, "low-memory-ongoing-no-forward");
+
       if (os) {
         os->NotifyObservers(nullptr, "memory-pressure",
-                            NS_LITERAL_STRING("low-memory").get());
-      }
-      else {
+                            mpPending == MemPressure_New ? lowMem.get() :
+                                                           lowMemOngoing.get());
+      } else {
         NS_WARNING("Can't get observer service!");
       }
     }

xpcom/threads/nsThread.h

@@ -128,16 +128,4 @@ private:
   nsresult mResult;
 };
 
-namespace mozilla {
-
-/**
- * This function causes the main thread to fire a memory pressure event at its
- * next available opportunity.
- *
- * You may call this function from any thread.
- */
-void ScheduleMemoryPressureEvent();
-
-} // namespace mozilla
-
 #endif  // nsThread_h__