Re: [PATCH] sched/core: Minor optimize ttwu_runnable()

[Peter Zijlstra]

On Wed, Nov 02, 2022 at 06:23:43PM +0800, Chengming Zhou wrote:
> ttwu_runnable() is used as a fast wakeup path when the wakee task
> is between set_current_state() and schedule(), in which case all
> we need to do is change p->state back to TASK_RUNNING. So we don't
> need to update_rq_clock() and check_preempt_curr() in this case.
> 
> Some performance numbers using mmtests/perfpipe on Intel Xeon server:
> 
>                            linux-next                patched
> Min       Time        8.67 (   0.00%)        8.66 (   0.13%)
> 1st-qrtle Time        8.83 (   0.00%)        8.72 (   1.19%)
> 2nd-qrtle Time        8.90 (   0.00%)        8.76 (   1.57%)
> 3rd-qrtle Time        8.98 (   0.00%)        8.82 (   1.82%)
> Max-1     Time        8.67 (   0.00%)        8.66 (   0.13%)
> Max-5     Time        8.67 (   0.00%)        8.66 (   0.13%)
> Max-10    Time        8.79 (   0.00%)        8.69 (   1.09%)
> Max-90    Time        9.01 (   0.00%)        8.84 (   1.94%)
> Max-95    Time        9.02 (   0.00%)        8.85 (   1.86%)
> Max-99    Time        9.02 (   0.00%)        8.88 (   1.56%)
> Max       Time        9.59 (   0.00%)        8.89 (   7.29%)
> Amean     Time        8.92 (   0.00%)        8.77 *   1.65%*
> 
> Signed-off-by: Chengming Zhou <zhouchengming@xxxxxxxxxxxxx>
> ---
>  kernel/sched/core.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 87c9cdf37a26..3785418de127 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3718,9 +3718,8 @@ static int ttwu_runnable(struct task_struct *p, int wake_flags)
>  
>  	rq = __task_rq_lock(p, &rf);
>  	if (task_on_rq_queued(p)) {
> -		/* check_preempt_curr() may use rq clock */
> -		update_rq_clock(rq);
> -		ttwu_do_wakeup(rq, p, wake_flags, &rf);
> +		WRITE_ONCE(p->__state, TASK_RUNNING);
> +		trace_sched_wakeup(p);
>  		ret = 1;
>  	}
>  	__task_rq_unlock(rq, &rf);

Yes, I think this is correct; however I would re-organize code a little.

How's this?


diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index cb2aa2b54c7a..43d9a1551a5d 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3602,15 +3602,40 @@ ttwu_stat(struct task_struct *p, int cpu, int wake_flags)
 		__schedstat_inc(p->stats.nr_wakeups_sync);
 }
 
 /*
- * Mark the task runnable and perform wakeup-preemption.
+ * Mark the task runnable...
  */
-static void ttwu_do_wakeup(struct rq *rq, struct task_struct *p, int wake_flags,
-			   struct rq_flags *rf)
+static inline void ttwu_do_wakeup(struct task_struct *p)
 {
-	check_preempt_curr(rq, p, wake_flags);
 	WRITE_ONCE(p->__state, TASK_RUNNING);
 	trace_sched_wakeup(p);
+}
+
+static void
+ttwu_do_activate(struct rq *rq, struct task_struct *p, int wake_flags,
+		 struct rq_flags *rf)
+{
+	int en_flags = ENQUEUE_WAKEUP | ENQUEUE_NOCLOCK;
+
+	lockdep_assert_rq_held(rq);
+
+	if (p->sched_contributes_to_load)
+		rq->nr_uninterruptible--;
+
+#ifdef CONFIG_SMP
+	if (wake_flags & WF_MIGRATED)
+		en_flags |= ENQUEUE_MIGRATED;
+	else
+#endif
+	if (p->in_iowait) {
+		delayacct_blkio_end(p);
+		atomic_dec(&task_rq(p)->nr_iowait);
+	}
+
+	activate_task(rq, p, en_flags);
+	check_preempt_curr(rq, p, wake_flags);
+
+	ttwu_do_wakeup(p);
 
 #ifdef CONFIG_SMP
 	if (p->sched_class->task_woken) {
@@ -3640,31 +3666,6 @@ static void ttwu_do_wakeup(struct rq *rq, struct task_struct *p, int wake_flags,
 #endif
 }
 
-static void
-ttwu_do_activate(struct rq *rq, struct task_struct *p, int wake_flags,
-		 struct rq_flags *rf)
-{
-	int en_flags = ENQUEUE_WAKEUP | ENQUEUE_NOCLOCK;
-
-	lockdep_assert_rq_held(rq);
-
-	if (p->sched_contributes_to_load)
-		rq->nr_uninterruptible--;
-
-#ifdef CONFIG_SMP
-	if (wake_flags & WF_MIGRATED)
-		en_flags |= ENQUEUE_MIGRATED;
-	else
-#endif
-	if (p->in_iowait) {
-		delayacct_blkio_end(p);
-		atomic_dec(&task_rq(p)->nr_iowait);
-	}
-
-	activate_task(rq, p, en_flags);
-	ttwu_do_wakeup(rq, p, wake_flags, rf);
-}
-
 /*
  * Consider @p being inside a wait loop:
  *
@@ -3698,9 +3699,7 @@ static int ttwu_runnable(struct task_struct *p, int wake_flags)
 
 	rq = __task_rq_lock(p, &rf);
 	if (task_on_rq_queued(p)) {
-		/* check_preempt_curr() may use rq clock */
-		update_rq_clock(rq);
-		ttwu_do_wakeup(rq, p, wake_flags, &rf);
+		ttwu_do_wakeup(p);
 		ret = 1;
 	}
 	__task_rq_unlock(rq, &rf);
@@ -4062,8 +4061,7 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
 			goto out;
 
 		trace_sched_waking(p);
-		WRITE_ONCE(p->__state, TASK_RUNNING);
-		trace_sched_wakeup(p);
+		ttwu_do_wakeup(p);
 		goto out;
 	}