[PATCH v5 1/2] sched/core: Minor optimize ttwu_runnable()

[PATCH v5 1/2] sched/core: Minor optimize ttwu_runnable()

[Chengming Zhou]

ttwu_runnable() is used as a fast wakeup path when the wakee task
is running on CPU or runnable on RQ, in both cases we can just
set its state to TASK_RUNNING to prevent a sleep.

If the wakee task is on_cpu running, we don't need to update_rq_clock()
or check_preempt_curr().

But if the wakee task is on_rq && !on_cpu (e.g. an IRQ hit before
the task got to schedule() and the task been preempted), we should
check_preempt_curr() to see if it can preempt the current running.

This also removes the class->task_woken() callback from ttwu_runnable(),
which wasn't required per the RT/DL implementations: any required push
operation would have been queued during class->set_next_task() when p
got preempted.

ttwu_runnable() also loses the update to rq->idle_stamp, as by definition
the rq cannot be idle in this scenario.

Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
Suggested-by: Valentin Schneider <vschneid@redhat.com>
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Reviewed-by: Valentin Schneider <vschneid@redhat.com>
---
v5:
 - Split the reorganization and optimization into two patches
   per Chen Yu. Thanks!

v4:
 - s/This patch reorg/Reorganize/ per Bagas Sanjaya. Thanks!

v3:
 - Improve the changelog per Valentin Schneider. Thanks!

v2:
 - keep check_preempt_curr() for on_rq && !on_cpu case in ttwu_runnable(),
   per Valentin Schneider.
 - reorg ttwu_do_wakeup() and ttwu_do_activate() code, so ttwu_do_wakeup()
   can be reused in ttwu_runnable(), per Peter Zijlstra.
 - reuse ttwu_do_wakeup() in try_to_wake_up() (p == current) fast path too,
   so ttwu_do_wakeup() become the only place we mark task runnable.
---
 kernel/sched/core.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 314c2c0219d9..60aa7774e5f4 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3718,9 +3718,16 @@ 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);
+		if (!task_on_cpu(rq, p)) {
+			/*
+			 * When on_rq && !on_cpu the task is preempted, see if
+			 * it should preempt whatever is current there now.
+			 */
+			update_rq_clock(rq);
+			check_preempt_curr(rq, p, wake_flags);
+		}
+		WRITE_ONCE(p->__state, TASK_RUNNING);
+		trace_sched_wakeup(p);
 		ret = 1;
 	}
 	__task_rq_unlock(rq, &rf);
-- 
2.37.2

[PATCH v5 2/2] sched/core: Reorganize ttwu_do_wakeup() and ttwu_do_activate()

[Chengming Zhou]

ttwu_do_activate() is used for a complete wakeup, in which we will
activate_task() and use ttwu_do_wakeup() to mark the task runnable
and perform wakeup-preemption, also call class->task_woken() callback
and update the rq->idle_stamp.

Since ttwu_runnable() is not a complete wakeup, don't need all those
done in ttwu_do_wakeup(), so we can move those to ttwu_do_activate()
to simplify ttwu_do_wakeup(), making it only mark the task runnable
to be reused in ttwu_runnable() and try_to_wake_up().

This patch should not have any functional changes.

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
---
 kernel/sched/core.c | 64 ++++++++++++++++++++++-----------------------
 1 file changed, 31 insertions(+), 33 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 60aa7774e5f4..d8216485b0ad 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3623,14 +3623,39 @@ ttwu_stat(struct task_struct *p, int cpu, int wake_flags)
 }
 
 /*
- * 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) {
@@ -3660,31 +3685,6 @@ 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:
  *
@@ -3726,8 +3726,7 @@ static int ttwu_runnable(struct task_struct *p, int wake_flags)
 			update_rq_clock(rq);
 			check_preempt_curr(rq, p, wake_flags);
 		}
-		WRITE_ONCE(p->__state, TASK_RUNNING);
-		trace_sched_wakeup(p);
+		ttwu_do_wakeup(p);
 		ret = 1;
 	}
 	__task_rq_unlock(rq, &rf);
@@ -4093,8 +4092,7 @@ 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;
 	}
 
-- 
2.37.2

Re: [PATCH v5 1/2] sched/core: Minor optimize ttwu_runnable()

[Chengming Zhou]

Hello, ping :-)


On 2022/12/8 11:20, Chengming Zhou wrote:
> ttwu_runnable() is used as a fast wakeup path when the wakee task
> is running on CPU or runnable on RQ, in both cases we can just
> set its state to TASK_RUNNING to prevent a sleep.
> 
> If the wakee task is on_cpu running, we don't need to update_rq_clock()
> or check_preempt_curr().
> 
> But if the wakee task is on_rq && !on_cpu (e.g. an IRQ hit before
> the task got to schedule() and the task been preempted), we should
> check_preempt_curr() to see if it can preempt the current running.
> 
> This also removes the class->task_woken() callback from ttwu_runnable(),
> which wasn't required per the RT/DL implementations: any required push
> operation would have been queued during class->set_next_task() when p
> got preempted.
> 
> ttwu_runnable() also loses the update to rq->idle_stamp, as by definition
> the rq cannot be idle in this scenario.
> 
> Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
> Suggested-by: Valentin Schneider <vschneid@redhat.com>
> Suggested-by: Peter Zijlstra <peterz@infradead.org>
> Reviewed-by: Valentin Schneider <vschneid@redhat.com>
> ---
> v5:
>  - Split the reorganization and optimization into two patches
>    per Chen Yu. Thanks!
> 
> v4:
>  - s/This patch reorg/Reorganize/ per Bagas Sanjaya. Thanks!
> 
> v3:
>  - Improve the changelog per Valentin Schneider. Thanks!
> 
> v2:
>  - keep check_preempt_curr() for on_rq && !on_cpu case in ttwu_runnable(),
>    per Valentin Schneider.
>  - reorg ttwu_do_wakeup() and ttwu_do_activate() code, so ttwu_do_wakeup()
>    can be reused in ttwu_runnable(), per Peter Zijlstra.
>  - reuse ttwu_do_wakeup() in try_to_wake_up() (p == current) fast path too,
>    so ttwu_do_wakeup() become the only place we mark task runnable.
> ---
>  kernel/sched/core.c | 13 ++++++++++---
>  1 file changed, 10 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 314c2c0219d9..60aa7774e5f4 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3718,9 +3718,16 @@ 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);
> +		if (!task_on_cpu(rq, p)) {
> +			/*
> +			 * When on_rq && !on_cpu the task is preempted, see if
> +			 * it should preempt whatever is current there now.
> +			 */
> +			update_rq_clock(rq);
> +			check_preempt_curr(rq, p, wake_flags);
> +		}
> +		WRITE_ONCE(p->__state, TASK_RUNNING);
> +		trace_sched_wakeup(p);
>  		ret = 1;
>  	}
>  	__task_rq_unlock(rq, &rf);

Re: [PATCH v5 1/2] sched/core: Minor optimize ttwu_runnable()

[Steven Rostedt]

On Thu,  8 Dec 2022 11:20:06 +0800
Chengming Zhou <zhouchengming@bytedance.com> wrote:

One nit.

> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3718,9 +3718,16 @@ 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);
> +		if (!task_on_cpu(rq, p)) {
> +			/*
> +			 * When on_rq && !on_cpu the task is preempted, see if
> +			 * it should preempt whatever is current there now.

			* it should preempt the task that is current now.

Other than that.

Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>

-- Steve

> +			 */
> +			update_rq_clock(rq);
> +			check_preempt_curr(rq, p, wake_flags);
> +		}
> +		WRITE_ONCE(p->__state, TASK_RUNNING);
> +		trace_sched_wakeup(p);
>  		ret = 1;
>  	}
>  	__task_rq_unlock(rq, &rf);