linux-kernel - Re: [PATCH v3] sched/stats: Fix rt/dl task's sched latency statistics error in sched_stat_wait trace

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Message-ID: <87d33909-ef8c-43a6-a556-d01fe692d5cd@redhat.com>
Date: Thu, 4 Jan 2024 12:02:20 +0100
From: Daniel Bristot de Oliveira <bristot@...hat.com>
To: Junwen Wu <wudaemon@....com>, mingo@...hat.com, laoar.shao@...il.com,
 peterz@...radead.org, juri.lelli@...hat.com, vincent.guittot@...aro.org,
 dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com
Cc: mgorman@...e.de, vschneid@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] sched/stats: Fix rt/dl task's sched latency statistics
 error in sched_stat_wait trace_point

Hi Junwen

s/trace_point/tracepoint/g

On 1/1/24 04:17, Junwen Wu 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. When rt task balance off source cpu,
> dequeue operation not update the sched_statistics, so follow update_stats_wait_end_fair
> update method, so do dl tasks.

This is a little bit hard to read, how about something like this?

The sched_stat_wait tracepoint is showing unreasonably long
latencies for real-time tasks. For example:

sched_stat_wait: comm=rcu_preempt pid=14 delay=4936139545261 [ns]

This error happens when the rt task balances off the source CPU because
the dequeue operation is not updating the sched_statistics. So, follow
update_stats_wait_end_fair() and update the stats. Do the same for
SCHED_DEADLINE.

[ feel free to change ]

> 
> Fixes: 57a5c2dafca8 ("sched/rt: Support schedstats for RT sched class")
> Fixes: b5eb4a5f6521 ("sched/dl: Support schedstats for deadline sched class")
> Signed-off-by: Junwen Wu <wudaemon@....com>
> ---
>  kernel/sched/deadline.c | 8 +++++++-
>  kernel/sched/rt.c       | 7 +++++++
>  2 files changed, 14 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index b28114478b82..29223163ee22 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -1558,10 +1558,16 @@ update_stats_dequeue_dl(struct dl_rq *dl_rq, struct sched_dl_entity *dl_se,
>  			int flags)
>  {
>  	struct task_struct *p = dl_task_of(dl_se);
> +	struct rq *rq = rq_of_dl_rq(dl_rq);
>  
>  	if (!schedstat_enabled())
>  		return;
> -
> +	/*
> +	 * Mark the end of the wait period
> +	 * if dequeueing a waiting task.
> +	 */
make this a single line comment.

> +	if (p && (p != rq->curr))
> +		 update_stats_wait_end_dl(dl_rq, dl_se);
>  	if ((flags & DEQUEUE_SLEEP)) {
>  		unsigned int state;
>  
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index 6aaf0a3d6081..6a2600213991 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -1360,12 +1360,19 @@ 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);
> +	 /*
> +	  * Mark the end of the wait period
> +	  * if dequeueing a waiting task.
> +	  */

ditto

-- Daniel