Re: [RFC PATCH v2 2/7] sched/fair: Handle throttle path for task based throttle

[Aaron Lu <ziqianlu@bytedance.com>]

On Wed, Apr 09, 2025 at 08:07:41PM +0800, Aaron Lu wrote:
... ...
> @@ -8888,6 +8884,9 @@ pick_next_task_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf
>  		goto idle;
>  	se = &p->se;
>  
> +	if (throttled_hierarchy(cfs_rq_of(se)))
> +		task_throttle_setup_work(p);
> +

Looks like this will miss core scheduling case, where the task pick is
done in pick_task_fair().

I plan to do something below on top to fix core scheduling case:

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 70f7de82d1d9d..500b41f9aea72 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8858,6 +8858,7 @@ static struct task_struct *pick_task_fair(struct rq *rq)
 {
 	struct sched_entity *se;
 	struct cfs_rq *cfs_rq;
+	struct task_struct *p;
 
 again:
 	cfs_rq = &rq->cfs;
@@ -8877,7 +8878,11 @@ static struct task_struct *pick_task_fair(struct rq *rq)
 		cfs_rq = group_cfs_rq(se);
 	} while (cfs_rq);
 
-	return task_of(se);
+	p = task_of(se);
+	if (throttled_hierarchy(cfs_rq_of(se)))
+		task_throttle_setup_work(p);
+
+	return p;
 }
 
 static void __set_next_task_fair(struct rq *rq, struct task_struct *p, bool first);
@@ -8896,9 +8901,6 @@ pick_next_task_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf
 		goto idle;
 	se = &p->se;
 
-	if (throttled_hierarchy(cfs_rq_of(se)))
-		task_throttle_setup_work(p);
-
 #ifdef CONFIG_FAIR_GROUP_SCHED
 	if (prev->sched_class != &fair_sched_class)
 		goto simple;

For non-core-scheduling, this has the same effect as current and for
core-scheduling, this will make sure task picked will also get throttle
task work added. It could add throttle task work to a task unnecessarily
because in core scheduling case, a task picked may not be able to run
due to cookie and priority reasons but at least, it will not miss the
throttle work this way.

Alternatively, I can add a task_throttle_setup_work(p) somewhere in
set_next_task_fair() but that would add one more callsite of
throttle_setup_work() and is not as clean and simple as the above diff.

Feel free to let me know your thoughts, thanks!

>  #ifdef CONFIG_FAIR_GROUP_SCHED
>  	if (prev->sched_class != &fair_sched_class)
>  		goto simple;