зеркало из https://github.com/mozilla/gecko-dev.git
Bug 1347274 (part 3) - Make some hot profiler functions lockless. r=mstange.
The functions are profiler_is_active(), profiler_feature_active(), profiler_tracing(), profiler_log(), profiler_add_marker(). This is achieved by adding RacyFeatures, which duplicates the features stored in ActivePS in a way that can be accessed locklessly. There's a chance of imprecision, but that doesn't matter for the way these functions are used; the worst that can happen is we might very occasionally miss adding a marker, or add one that we shouldn't. --HG-- extra : rebase_source : 278ebad8d643670e3f2e92c8eeec13b19f94b8ce
This commit is contained in:
Nicholas Nethercote
Родитель
e6fb672256
Коммит
6534e30922
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@ -479,6 +479,52 @@ uint32_t ActivePS::sNextGeneration = 0;
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// The mutex that guards accesses to CorePS and ActivePS.
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static PSMutex gPSMutex;
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// The preferred way to check profiler activeness and features is via
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// ActivePS(). However, that requires locking gPSMutex. There are some hot
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// operations where absolute precision isn't required, so we duplicate the
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// activeness/feature state in a lock-free manner in this class.
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class RacyFeatures
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{
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public:
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static void SetActive(uint32_t aFeatures)
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{
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sActiveAndFeatures = Active | aFeatures;
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}
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static void SetInactive() { sActiveAndFeatures = 0; }
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static bool IsActive() { return uint32_t(sActiveAndFeatures) & Active; }
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static bool IsActiveWithFeature(uint32_t aFeature)
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{
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uint32_t af = sActiveAndFeatures; // copy it first
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return (af & Active) && (af & aFeature);
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}
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static bool IsActiveWithoutPrivacy()
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{
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uint32_t af = sActiveAndFeatures; // copy it first
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return (af & Active) && !(af & ProfilerFeature::Privacy);
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}
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private:
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static const uint32_t Active = 1u << 31;
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// Ensure Active doesn't overlap with any of the feature bits.
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#define NO_OVERLAP(n_, str_, Name_) \
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static_assert(ProfilerFeature::Name_ != Active, "bad Active value");
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PROFILER_FOR_EACH_FEATURE(NO_OVERLAP);
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#undef NO_OVERLAP
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// We combine the active bit with the feature bits so they can be read or
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// written in a single atomic operation.
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static Atomic<uint32_t> sActiveAndFeatures;
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};
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Atomic<uint32_t> RacyFeatures::sActiveAndFeatures(0);
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// Each live thread has a ThreadInfo, and we store a reference to it in TLS.
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// This class encapsulates that TLS.
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class TLSInfo
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@ -2411,6 +2457,9 @@ locked_profiler_start(PSLockRef aLock, int aEntries, double aInterval,
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mozilla::java::GeckoJavaSampler::Start(javaInterval, 1000);
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}
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#endif
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// At the very end, set up RacyFeatures.
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RacyFeatures::SetActive(ActivePS::Features(aLock));
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}
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void
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@ -2457,6 +2506,9 @@ locked_profiler_stop(PSLockRef aLock)
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MOZ_RELEASE_ASSERT(NS_IsMainThread());
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MOZ_RELEASE_ASSERT(CorePS::Exists() && ActivePS::Exists(aLock));
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// At the very start, clear RacyFeatures.
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RacyFeatures::SetInactive();
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#ifdef MOZ_TASK_TRACER
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if (ActivePS::FeatureTaskTracer(aLock)) {
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mozilla::tasktracer::StopLogging();
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@ -2599,13 +2651,8 @@ profiler_feature_active(uint32_t aFeature)
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MOZ_RELEASE_ASSERT(CorePS::Exists());
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PSAutoLock lock(gPSMutex);
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if (!ActivePS::Exists(lock)) {
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return false;
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}
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return !!(ActivePS::Features(lock) & aFeature);
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// This function is hot enough that we use RacyFeatures, not ActivePS.
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return RacyFeatures::IsActiveWithFeature(aFeature);
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}
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bool
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@ -2615,9 +2662,8 @@ profiler_is_active()
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MOZ_RELEASE_ASSERT(CorePS::Exists());
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PSAutoLock lock(gPSMutex);
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return ActivePS::Exists(lock);
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// This function is hot enough that we use RacyFeatures, notActivePS.
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return RacyFeatures::IsActive();
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}
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void
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@ -2857,14 +2903,19 @@ profiler_get_backtrace_noalloc(char *output, size_t outputSize)
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}
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static void
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locked_profiler_add_marker(PSLockRef aLock, const char* aMarkerName,
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ProfilerMarkerPayload* aPayload)
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racy_profiler_add_marker(const char* aMarkerName,
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ProfilerMarkerPayload* aPayload)
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{
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// This function runs both on and off the main thread.
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MOZ_RELEASE_ASSERT(CorePS::Exists());
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MOZ_RELEASE_ASSERT(ActivePS::Exists(aLock) &&
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!ActivePS::FeaturePrivacy(aLock));
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// We don't assert that RacyFeatures::IsActiveWithoutPrivacy() is true here,
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// because it's possible that the result has changed since we tested it in
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// the caller.
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//
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// Because of this imprecision it's possible to miss a marker or record one
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// we shouldn't. Either way is not a big deal.
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// aPayload must be freed if we return early.
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mozilla::UniquePtr<ProfilerMarkerPayload> payload(aPayload);
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@ -2889,16 +2940,15 @@ profiler_add_marker(const char* aMarkerName, ProfilerMarkerPayload* aPayload)
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MOZ_RELEASE_ASSERT(CorePS::Exists());
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PSAutoLock lock(gPSMutex);
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// aPayload must be freed if we return early.
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mozilla::UniquePtr<ProfilerMarkerPayload> payload(aPayload);
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if (!ActivePS::Exists(lock) || ActivePS::FeaturePrivacy(lock)) {
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// This function is hot enough that we use RacyFeatures, notActivePS.
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if (RacyFeatures::IsActiveWithoutPrivacy()) {
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return;
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}
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locked_profiler_add_marker(lock, aMarkerName, payload.release());
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racy_profiler_add_marker(aMarkerName, payload.release());
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}
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void
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@ -2909,14 +2959,13 @@ profiler_tracing(const char* aCategory, const char* aMarkerName,
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MOZ_RELEASE_ASSERT(CorePS::Exists());
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PSAutoLock lock(gPSMutex);
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if (!ActivePS::Exists(lock) || ActivePS::FeaturePrivacy(lock)) {
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// This function is hot enough that we use RacyFeatures, notActivePS.
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if (RacyFeatures::IsActiveWithoutPrivacy()) {
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return;
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}
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auto payload = new ProfilerMarkerTracing(aCategory, aKind);
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locked_profiler_add_marker(lock, aMarkerName, payload);
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racy_profiler_add_marker(aMarkerName, payload);
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}
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void
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@ -2927,15 +2976,14 @@ profiler_tracing(const char* aCategory, const char* aMarkerName,
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MOZ_RELEASE_ASSERT(CorePS::Exists());
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PSAutoLock lock(gPSMutex);
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if (!ActivePS::Exists(lock) || ActivePS::FeaturePrivacy(lock)) {
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// This function is hot enough that we use RacyFeatures, notActivePS.
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if (RacyFeatures::IsActiveWithoutPrivacy()) {
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return;
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}
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auto payload =
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new ProfilerMarkerTracing(aCategory, aKind, mozilla::Move(aCause));
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locked_profiler_add_marker(lock, aMarkerName, payload);
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racy_profiler_add_marker(aMarkerName, payload);
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}
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void
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@ -290,7 +290,9 @@ ProfilerBacktraceDestructor::operator()(ProfilerBacktrace* aBacktrace) {}
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PROFILER_FUNC(bool profiler_is_active(), false)
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// Check if a profiler feature (specified via the ProfilerFeature type) is
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// active. Returns false if the profiler is inactive.
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// active. Returns false if the profiler is inactive. Note: the return value
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// can become immediately out-of-date, much like the return value of
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// profiler_is_active().
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PROFILER_FUNC(bool profiler_feature_active(uint32_t aFeature), false)
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// Get the profile encoded as a JSON string. A no-op (returning nullptr) if the
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