tor-browser

RegisteredThread.h (6002B)

---

/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* 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 RegisteredThread_h
#define RegisteredThread_h

#include "platform.h"
#include "ThreadInfo.h"

namespace mozilla {
namespace baseprofiler {

// This class contains the state for a single thread that is accessible without
// protection from gPSMutex in platform.cpp. Because there is no external
// protection against data races, it must provide internal protection. Hence
// the "Racy" prefix.
//
class RacyRegisteredThread final {
public:
 explicit RacyRegisteredThread(BaseProfilerThreadId aThreadId)
     : mThreadId(aThreadId), mSleep(AWAKE), mIsBeingProfiled(false) {}

 ~RacyRegisteredThread() {}

 void SetIsBeingProfiled(bool aIsBeingProfiled) {
   mIsBeingProfiled = aIsBeingProfiled;
 }

 bool IsBeingProfiled() const { return mIsBeingProfiled; }

 // This is called on every profiler restart. Put things that should happen at
 // that time here.
 void ReinitializeOnResume() {
   // This is needed to cause an initial sample to be taken from sleeping
   // threads that had been observed prior to the profiler stopping and
   // restarting. Otherwise sleeping threads would not have any samples to
   // copy forward while sleeping.
   (void)mSleep.compareExchange(SLEEPING_OBSERVED, SLEEPING_NOT_OBSERVED);
 }

 // This returns true for the second and subsequent calls in each sleep cycle.
 bool CanDuplicateLastSampleDueToSleep() {
   if (mSleep == AWAKE) {
     return false;
   }

   if (mSleep.compareExchange(SLEEPING_NOT_OBSERVED, SLEEPING_OBSERVED)) {
     return false;
   }

   return true;
 }

 // Call this whenever the current thread sleeps. Calling it twice in a row
 // without an intervening setAwake() call is an error.
 void SetSleeping() {
   MOZ_ASSERT(mSleep == AWAKE);
   mSleep = SLEEPING_NOT_OBSERVED;
 }

 // Call this whenever the current thread wakes. Calling it twice in a row
 // without an intervening setSleeping() call is an error.
 void SetAwake() {
   MOZ_ASSERT(mSleep != AWAKE);
   mSleep = AWAKE;
 }

 bool IsSleeping() { return mSleep != AWAKE; }

 BaseProfilerThreadId ThreadId() const { return mThreadId; }

 class ProfilingStack& ProfilingStack() { return mProfilingStack; }
 const class ProfilingStack& ProfilingStack() const { return mProfilingStack; }

private:
 class ProfilingStack mProfilingStack;

 // mThreadId contains the thread ID of the current thread. It is safe to read
 // this from multiple threads concurrently, as it will never be mutated.
 const BaseProfilerThreadId mThreadId;

 // mSleep tracks whether the thread is sleeping, and if so, whether it has
 // been previously observed. This is used for an optimization: in some cases,
 // when a thread is asleep, we duplicate the previous sample, which is
 // cheaper than taking a new sample.
 //
 // mSleep is atomic because it is accessed from multiple threads.
 //
 // - It is written only by this thread, via setSleeping() and setAwake().
 //
 // - It is read by SamplerThread::Run().
 //
 // There are two cases where racing between threads can cause an issue.
 //
 // - If CanDuplicateLastSampleDueToSleep() returns false but that result is
 //   invalidated before being acted upon, we will take a full sample
 //   unnecessarily. This is additional work but won't cause any correctness
 //   issues. (In actual fact, this case is impossible. In order to go from
 //   CanDuplicateLastSampleDueToSleep() returning false to it returning true
 //   requires an intermediate call to it in order for mSleep to go from
 //   SLEEPING_NOT_OBSERVED to SLEEPING_OBSERVED.)
 //
 // - If CanDuplicateLastSampleDueToSleep() returns true but that result is
 //   invalidated before being acted upon -- i.e. the thread wakes up before
 //   DuplicateLastSample() is called -- we will duplicate the previous
 //   sample. This is inaccurate, but only slightly... we will effectively
 //   treat the thread as having slept a tiny bit longer than it really did.
 //
 // This latter inaccuracy could be avoided by moving the
 // CanDuplicateLastSampleDueToSleep() check within the thread-freezing code,
 // e.g. the section where Tick() is called. But that would reduce the
 // effectiveness of the optimization because more code would have to be run
 // before we can tell that duplication is allowed.
 //
 static const int AWAKE = 0;
 static const int SLEEPING_NOT_OBSERVED = 1;
 static const int SLEEPING_OBSERVED = 2;
 Atomic<int> mSleep;

 // Is this thread being profiled? (e.g., should markers be recorded?)
 Atomic<bool, MemoryOrdering::Relaxed> mIsBeingProfiled;
};

// This class contains information that's relevant to a single thread only
// while that thread is running and registered with the profiler, but
// regardless of whether the profiler is running. All accesses to it are
// protected by the profiler state lock.
class RegisteredThread final {
public:
 RegisteredThread(ThreadInfo* aInfo, void* aStackTop);
 ~RegisteredThread();

 class RacyRegisteredThread& RacyRegisteredThread() {
   return mRacyRegisteredThread;
 }
 const class RacyRegisteredThread& RacyRegisteredThread() const {
   return mRacyRegisteredThread;
 }

 PlatformData* GetPlatformData() const { return mPlatformData.get(); }
 const void* StackTop() const { return mStackTop; }

 size_t SizeOfIncludingThis(MallocSizeOf aMallocSizeOf) const;

 const RefPtr<ThreadInfo> Info() const { return mThreadInfo; }

private:
 class RacyRegisteredThread mRacyRegisteredThread;

 const UniquePlatformData mPlatformData;
 const void* mStackTop;

 const RefPtr<ThreadInfo> mThreadInfo;
};

}  // namespace baseprofiler
}  // namespace mozilla

#endif  // RegisteredThread_h