linux-kernel - Re: [PATCH v2 4/5] sched: Add task enqueue/dequeue trace points

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Message-ID: <20250815134016.GB3289052@noisy.programming.kicks-ass.net>
Date: Fri, 15 Aug 2025 15:40:16 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Nam Cao <namcao@...utronix.de>
Cc: Steven Rostedt <rostedt@...dmis.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Gabriele Monaco <gmonaco@...hat.com>,
	linux-trace-kernel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>,
	K Prateek Nayak <kprateek.nayak@....com>
Subject: Re: [PATCH v2 4/5] sched: Add task enqueue/dequeue trace points

On Wed, Aug 06, 2025 at 10:01:20AM +0200, Nam Cao wrote:

> +/*
> + * The two trace points below may not work as expected for fair tasks due
> + * to delayed dequeue. See:
> + * https://lore.kernel.org/lkml/179674c6-f82a-4718-ace2-67b5e672fdee@amd.com/
> + */

> +DECLARE_TRACE(dequeue_task,
> +	TP_PROTO(int cpu, struct task_struct *task),
> +	TP_ARGS(cpu, task));
> +

> @@ -2119,7 +2121,11 @@ inline bool dequeue_task(struct rq *rq, struct task_struct *p, int flags)
>  	 * and mark the task ->sched_delayed.
>  	 */
>  	uclamp_rq_dec(rq, p);
> -	return p->sched_class->dequeue_task(rq, p, flags);
> +	if (p->sched_class->dequeue_task(rq, p, flags)) {
> +		trace_dequeue_task_tp(rq->cpu, p);
> +		return true;
> +	}
> +	return false;
>  }

Hurmpff.. that's not very nice.

How about something like:

dequeue_task():
	...
	ret = p->sched_class->dequeue_task(rq, p, flags);
	if (trace_dequeue_task_p_enabled() && !(flags & DEQUEUE_SLEEP))
		__trace_dequeue_task_tp(rq->cpu, p);
	return ret;


__block_task():
	trace_dequeue_task_tp(rq->cpu, p);
	...


Specifically, only DEQUEUE_SLEEP is allowed to fail, and DEQUEUE_SLEEP
will eventually cause __block_task() to be called, either directly, or
delayed.