freezer,sched: Use saved_state to reduce some spurious wakeups

After commit f5d39b020809 ("freezer,sched: Rewrite core freezer logic"),
tasks that transition directly from TASK_FREEZABLE to TASK_FROZEN  are
always woken up on the thaw path. Prior to that commit, tasks could ask
freezer to consider them "frozen enough" via freezer_do_not_count(). The
commit replaced freezer_do_not_count() with a TASK_FREEZABLE state which
allows freezer to immediately mark the task as TASK_FROZEN without
waking up the task.  This is efficient for the suspend path, but on the
thaw path, the task is always woken up even if the task didn't need to
wake up and goes back to its TASK_(UN)INTERRUPTIBLE state. Although
these tasks are capable of handling of the wakeup, we can observe a
power/perf impact from the extra wakeup.

We observed on Android many tasks wait in the TASK_FREEZABLE state
(particularly due to many of them being binder clients). We observed
nearly 4x the number of tasks and a corresponding linear increase in
latency and power consumption when thawing the system. The latency
increased from ~15ms to ~50ms.

Avoid the spurious wakeups by saving the state of TASK_FREEZABLE tasks.
If the task was running before entering TASK_FROZEN state
(__refrigerator()) or if the task received a wake up for the saved
state, then the task is woken on thaw. saved_state from PREEMPT_RT locks
can be re-used because freezer would not stomp on the rtlock wait flow:
TASK_RTLOCK_WAIT isn't considered freezable.

Reported-by: Prakash Viswalingam <quic_prakashv@quicinc.com>
Signed-off-by: Elliot Berman <quic_eberman@quicinc.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 19 insertions, 22 deletions

diff --git a/kernel/freezer.c b/kernel/freezer.c
index 4fad0e6fca64..c450fa8b8b5e 100644
--- a/kernel/freezer.c
+++ b/kernel/freezer.c
@@ -71,7 +71,11 @@ bool __refrigerator(bool check_kthr_stop)
 	for (;;) {
 		bool freeze;
 
+		raw_spin_lock_irq(&current->pi_lock);
 		set_current_state(TASK_FROZEN);
+		/* unstale saved_state so that __thaw_task() will wake us up */
+		current->saved_state = TASK_RUNNING;
+		raw_spin_unlock_irq(&current->pi_lock);
 
 		spin_lock_irq(&freezer_lock);
 		freeze = freezing(current) && !(check_kthr_stop && kthread_should_stop());
@@ -129,6 +133,7 @@ static int __set_task_frozen(struct task_struct *p, void *arg)
 		WARN_ON_ONCE(debug_locks && p->lockdep_depth);
 #endif
 
+	p->saved_state = p->__state;
 	WRITE_ONCE(p->__state, TASK_FROZEN);
 	return TASK_FROZEN;
 }
@@ -170,42 +175,34 @@ bool freeze_task(struct task_struct *p)
 }
 
 /*
- * The special task states (TASK_STOPPED, TASK_TRACED) keep their canonical
- * state in p->jobctl. If either of them got a wakeup that was missed because
- * TASK_FROZEN, then their canonical state reflects that and the below will
- * refuse to restore the special state and instead issue the wakeup.
+ * Restore the saved_state before the task entered freezer. For typical task
+ * in the __refrigerator(), saved_state == TASK_RUNNING so nothing happens
+ * here. For tasks which were TASK_NORMAL | TASK_FREEZABLE, their initial state
+ * is restored unless they got an expected wakeup (see ttwu_state_match()).
+ * Returns 1 if the task state was restored.
  */
-static int __set_task_special(struct task_struct *p, void *arg)
+static int __restore_freezer_state(struct task_struct *p, void *arg)
 {
-	unsigned int state = 0;
+	unsigned int state = p->saved_state;
 
-	if (p->jobctl & JOBCTL_TRACED)
-		state = TASK_TRACED;
-
-	else if (p->jobctl & JOBCTL_STOPPED)
-		state = TASK_STOPPED;
-
-	if (state)
+	if (state != TASK_RUNNING) {
 		WRITE_ONCE(p->__state, state);
+		return 1;
+	}
 
-	return state;
+	return 0;
 }
 
 void __thaw_task(struct task_struct *p)
 {
-	unsigned long flags, flags2;
+	unsigned long flags;
 
 	spin_lock_irqsave(&freezer_lock, flags);
 	if (WARN_ON_ONCE(freezing(p)))
 		goto unlock;
 
-	if (lock_task_sighand(p, &flags2)) {
-		/* TASK_FROZEN -> TASK_{STOPPED,TRACED} */
-		bool ret = task_call_func(p, __set_task_special, NULL);
-		unlock_task_sighand(p, &flags2);
-		if (ret)
-			goto unlock;
-	}
+	if (task_call_func(p, __restore_freezer_state, NULL))
+		goto unlock;
 
 	wake_up_state(p, TASK_FROZEN);
 unlock: