linux-kernel - Re: [RFC PATCH v15 4/7] sched: Fix runtime accounting w/ split exec & sched contexts

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Message-ID: <20250313062607.5fd9052e@batman.local.home>
Date: Thu, 13 Mar 2025 06:26:07 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: John Stultz <jstultz@...gle.com>
Cc: LKML <linux-kernel@...r.kernel.org>, Joel Fernandes
 <joelagnelf@...dia.com>, Qais Yousef <qyousef@...alina.io>, Ingo Molnar
 <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>, Juri Lelli
 <juri.lelli@...hat.com>, Vincent Guittot <vincent.guittot@...aro.org>,
 Dietmar Eggemann <dietmar.eggemann@....com>, Valentin Schneider
 <vschneid@...hat.com>, Ben Segall <bsegall@...gle.com>, Zimuzo Ezeozue
 <zezeozue@...gle.com>, Mel Gorman <mgorman@...e.de>, Will Deacon
 <will@...nel.org>, Waiman Long <longman@...hat.com>, Boqun Feng
 <boqun.feng@...il.com>, "Paul E. McKenney" <paulmck@...nel.org>, Metin Kaya
 <Metin.Kaya@....com>, Xuewen Yan <xuewen.yan94@...il.com>, K Prateek Nayak
 <kprateek.nayak@....com>, Thomas Gleixner <tglx@...utronix.de>, Daniel
 Lezcano <daniel.lezcano@...aro.org>, Suleiman Souhlal
 <suleiman@...gle.com>, kernel-team@...roid.com
Subject: Re: [RFC PATCH v15 4/7] sched: Fix runtime accounting w/ split exec
 & sched contexts

On Wed, 12 Mar 2025 15:11:34 -0700
John Stultz <jstultz@...gle.com> wrote:

> The idea here is we want to charge the scheduler-context task's
> vruntime but charge the execution-context task's sum_exec_runtime.

The "but" is confusing me. Do you mean, "and also"? The sentence
doesn't make sense with "but" unless it was:

  "The idea here is we DON'T want to charge the scheduler-context
  task's vruntime but charge the execution-context task's
  sum_exec_runtime INSTEAD."

> 
> This way cputime accounting goes against the task actually running
> but vruntime accounting goes against the rq->donor task so we get
> proper fairness.

But this shows that you want to do both, although, I would remove the
"but" here too. Replace it with "while".

Or maybe I'm just confused.

> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index c798d27952431..f8ad3a44b3771 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -1129,22 +1129,33 @@ static void update_tg_load_avg(struct cfs_rq *cfs_rq)
>  }
>  #endif /* CONFIG_SMP */
>  
> -static s64 update_curr_se(struct rq *rq, struct sched_entity *curr)
> +static s64 update_curr_se(struct rq *rq, struct sched_entity *se)

Should this be renamed to "update_se()" as it no longer appears to be
updating "curr_se".

>  {
>  	u64 now = rq_clock_task(rq);
>  	s64 delta_exec;
>  
> -	delta_exec = now - curr->exec_start;
> +	delta_exec = now - se->exec_start;
>  	if (unlikely(delta_exec <= 0))
>  		return delta_exec;
>  
> -	curr->exec_start = now;
> -	curr->sum_exec_runtime += delta_exec;
> +	se->exec_start = now;
> +	if (entity_is_task(se)) {
> +		struct task_struct *running = rq->curr;
> +		/*
> +		 * If se is a task, we account the time against the running
> +		 * task, as w/ proxy-exec they may not be the same.
> +		 */
> +		running->se.exec_start = now;
> +		running->se.sum_exec_runtime += delta_exec;
> +	} else {
> +		/* If not task, account the time against se */
> +		se->sum_exec_runtime += delta_exec;
> +	}

Or maybe: update_proxy_se() ?

-- Steve

>  
>  	if (schedstat_enabled()) {
>  		struct sched_statistics *stats;
>  
> -		stats = __schedstats_from_se(curr);
> +		stats = __schedstats_from_se(se);
>  		__schedstat_set(stats->exec_max,
>  				max(delta_exec, stats->exec_max));
>  	}