[PATCH v1] sched/rt: Fix rt task's sched latency statistics in sched_stat_wait trace_point

[PATCH v1] sched/rt: Fix rt task's sched latency statistics in sched_stat_wait trace_point

[Junwen Wu]

When enable sched_stat_wait trace_point, some rt tasks sched latency
so long, like this:
sched_stat_wait: comm=rcu_preempt pid=14 delay=4936139545261 [ns]
Rt task has low latency, it must have a bug. I found the reason is 
When rt task balance off source cpu, dequeue operation not update 
the sched_statistics, so follow update_stats_wait_end_fair
update method.

Signed-off-by: Junwen Wu <wudaemon@163.com>
---
 kernel/sched/rt.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 6aaf0a3d6081..c75215947c20 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1360,12 +1360,15 @@ update_stats_dequeue_rt(struct rt_rq *rt_rq, struct sched_rt_entity *rt_se,
 			int flags)
 {
 	struct task_struct *p = NULL;
+	struct rq *rq = rq_of_rt_se(rt_se);
 
 	if (!schedstat_enabled())
 		return;
 
 	if (rt_entity_is_task(rt_se))
 		p = rt_task_of(rt_se);
+	if (p != rq->curr)
+		update_stats_wait_end_rt(rt_rq, rt_se);
 
 	if ((flags & DEQUEUE_SLEEP) && p) {
 		unsigned int state;
-- 
2.34.1

Re: [PATCH v1] sched/rt: Fix rt task's sched latency statistics in sched_stat_wait trace_point

[Yafang Shao]

On Mon, Dec 18, 2023 at 11:05 PM Junwen Wu <wudaemon@163.com> wrote:
>
> When enable sched_stat_wait trace_point, some rt tasks sched latency
> so long, like this:
> sched_stat_wait: comm=rcu_preempt pid=14 delay=4936139545261 [ns]
> Rt task has low latency, it must have a bug. I found the reason is
> When rt task balance off source cpu, dequeue operation not update
> the sched_statistics, so follow update_stats_wait_end_fair
> update method.

Thanks for your report!

>
> Signed-off-by: Junwen Wu <wudaemon@163.com>
> ---
>  kernel/sched/rt.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index 6aaf0a3d6081..c75215947c20 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -1360,12 +1360,15 @@ update_stats_dequeue_rt(struct rt_rq *rt_rq, struct sched_rt_entity *rt_se,
>                         int flags)
>  {
>         struct task_struct *p = NULL;
> +       struct rq *rq = rq_of_rt_se(rt_se);
>
>         if (!schedstat_enabled())
>                 return;
>
>         if (rt_entity_is_task(rt_se))
>                 p = rt_task_of(rt_se);
> +       if (p != rq->curr)

This seems wrong, the rt_se might not be a task.

I think we'd better use if (!on_rt_rq(rt_se))

> +               update_stats_wait_end_rt(rt_rq, rt_se);
>
>         if ((flags & DEQUEUE_SLEEP) && p) {
>                 unsigned int state;
> --
> 2.34.1
>


--
Regards
Yafang

Re: [PATCH v1] sched/rt: Fix rt task's sched latency statistics in sched_stat_wait trace_point

[Junwen Wu]

ok, I will take your advise, and test my platform.

--
Regards
Junwen

Re: [PATCH v1] sched/rt: Fix rt task's sched latency statistics in sched_stat_wait trace_point

[Junwen Wu]

Re: [PATCH v1] sched/rt: Fix rt task's sched latency statistics error in sched_stat_wait trace_point

[Junwen Wu]

>>I think we'd better use if (!on_rt_rq(rt_se))
>>
>> +               update_stats_wait_end_rt(rt_rq, rt_se);
>>
hi, Yafang.when execute update_stats_dequeue_rt, rt_se->on_rq is still 0, util dequeue_rt_stack,
the method is not effect.
I this we can use if (p && p != rq->curr) /*Mark the end of the wait period if dequeueing task*/
because schedstats is not supported for rt group,we only need to update rt_se that is realy task.
--
Best regards

Re: [PATCH v1] sched/rt: Fix rt task's sched latency statistics error in sched_stat_wait trace_point

[Junwen Wu]

sorry, type error.  rt_se->on_rq is still 1
--
Best regards

Re: [PATCH v1] sched/rt: Fix rt task's sched latency statistics error in sched_stat_wait trace_point

[Yafang Shao]

On Wed, Dec 20, 2023 at 10:33 PM Junwen Wu <wudaemon@163.com> wrote:
>
> >>I think we'd better use if (!on_rt_rq(rt_se))
> >>
> >> +               update_stats_wait_end_rt(rt_rq, rt_se);
> >>
> hi, Yafang.when execute update_stats_dequeue_rt, rt_se->on_rq is still 0, util dequeue_rt_stack,
> the method is not effect.

Ah, you are right.

> I this we can use if (p && p != rq->curr) /*Mark the end of the wait period if dequeueing task*/

Agreed.

> because schedstats is not supported for rt group,we only need to update rt_se that is realy task.
> --
> Best regards
>


-- 
Regards
Yafang

Re: [PATCH v1] sched/rt: Fix rt task's sched latency statistics error in sched_stat_wait trace_point

[Junwen Wu]

>Ah, you are right.
>
>> I this we can use if (p && p != rq->curr) /*Mark the end of the wait period if dequeueing task*/
>
>Agreed.
>
>> because schedstats is not supported for rt group,we only need to update rt_se that is realy task.
>> --
>> Best regards
>>
ok, I send PATCH v2 later
--
Best regards