linux-kernel - Re: [PATCH v2] sched_ext: Trigger ops.update_idle() from pick_task

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Message-ID: <20241015074526.GO16066@noisy.programming.kicks-ass.net>
Date: Tue, 15 Oct 2024 09:45:26 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Andrea Righi <andrea.righi@...ux.dev>
Cc: Tejun Heo <tj@...nel.org>, David Vernet <void@...ifault.com>,
	Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>,
	linux-kernel@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [PATCH v2] sched_ext: Trigger ops.update_idle() from
 pick_task_idle()

On Tue, Oct 15, 2024 at 12:06:03AM +0200, Andrea Righi wrote:

> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> index d2f096bb274c..5a10cbc7e9df 100644
> --- a/kernel/sched/idle.c
> +++ b/kernel/sched/idle.c
> @@ -459,13 +459,13 @@ static void put_prev_task_idle(struct rq *rq, struct task_struct *prev, struct t
>  static void set_next_task_idle(struct rq *rq, struct task_struct *next, bool first)
>  {
>  	update_idle_core(rq);
> -	scx_update_idle(rq, true);
>  	schedstat_inc(rq->sched_goidle);
>  	next->se.exec_start = rq_clock_task(rq);
>  }
>  
>  struct task_struct *pick_task_idle(struct rq *rq)
>  {
> +	scx_update_idle(rq, true);
>  	return rq->idle;
>  }

Does this do the right thing in the case of core-scheduling doing
pick_task() for force-idle on a remote cpu?

The core-sched case is somewhat special in that the pick can be ignored
-- in which case you're doing a spurious scx_update_idle() call.