Optimizing OneShotTimer::reset() function
When running simpleperf, reset() function caused every once
in a while locks, which we suspect cause occasional janking.
- Moving the state to atomic, so there is no time spent
acquiring the lock.
- When using condition_variable, atomic still needs to be
changed under mutex, so moving to semaphores to guard
and signal.
See b/170665374 for analysis report.
Bug: 170665374
Test: atest DisplayMicrobenchTests while running simple perf
Test: atest OneShotTimerTest
Change-Id: Ic450ea074bc07175c3fc681b69a8009be84d30a0
diff --git a/services/surfaceflinger/Scheduler/OneShotTimer.h b/services/surfaceflinger/Scheduler/OneShotTimer.h
index b005754..8bbd4f5 100644
--- a/services/surfaceflinger/Scheduler/OneShotTimer.h
+++ b/services/surfaceflinger/Scheduler/OneShotTimer.h
@@ -16,6 +16,7 @@
#pragma once
+#include <semaphore.h>
#include <chrono>
#include <condition_variable>
#include <thread>
@@ -70,17 +71,15 @@
// Function that loops until the condition for stopping is met.
void loop();
+ // Checks whether mResetTriggered and mStopTriggered were set and updates
+ // mState if so.
+ TimerState checkForResetAndStop(TimerState state);
+
// Thread waiting for timer to expire.
std::thread mThread;
- // Condition used to notify mThread.
- std::condition_variable_any mCondition;
-
- // Lock used for synchronizing the waiting thread with the application thread.
- std::mutex mMutex;
-
- // Current timer state
- TimerState mState GUARDED_BY(mMutex) = TimerState::RESET;
+ // Semaphore to keep mThread synchronized.
+ sem_t mSemaphore;
// Interval after which timer expires.
const Interval mInterval;
@@ -90,6 +89,12 @@
// Callback that happens when timer expires.
const TimeoutCallback mTimeoutCallback;
+
+ // After removing lock guarding mState, the state can be now accessed at
+ // any time. Keep a bool if the reset or stop were requested, and occasionally
+ // check in the main loop if they were.
+ std::atomic<bool> mResetTriggered = false;
+ std::atomic<bool> mStopTriggered = false;
};
} // namespace scheduler