linux-kernel - Re: [PATCH v4 2/3] sched_ext: Fix cpu_released while RT task and SCX task are scheduled concurrently

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Message-ID: <aKX9XPDt0yOzPQWv@slm.duckdns.org>
Date: Wed, 20 Aug 2025 06:52:44 -1000
From: 'Tejun Heo' <tj@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: liuwenfang <liuwenfang@...or.com>, 'David Vernet' <void@...ifault.com>,
	'Andrea Righi' <arighi@...dia.com>,
	'Changwoo Min' <changwoo@...lia.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'" <linux-kernel@...r.kernel.org>,
	Joel Fernandes <joelagnelf@...dia.com>
Subject: Re: [PATCH v4 2/3] sched_ext: Fix cpu_released while RT task and SCX
 task are scheduled concurrently

Hello,

On Wed, Aug 20, 2025 at 11:18:10AM +0200, Peter Zijlstra wrote:
> >   This guarantee changes if we add @rf to pick_task() and let it unlock and
> >   relock. A higher priority task may get queued while the rq lock is
> >   released and then the lower priority pick_task() may still return a task
> >   of its own.
> 
> No, this would be broken. This guarantee must not change.
> 
> What you can do however is something like:
> 
> again:
>    p = pick_local_task();
>    if (!p) {
>      unlock(rq, rf);
>      // get yourself a local task
>      lock(rq, rf);
>      if (higher-class-task-available(rq)) {
>        // roll back whatever state
>        return RETRY_TASK;
>      }
>      goto again;
>    }
> 
>    return p;

Isn't that kinda nasty as we'd have to do that in every pick_task(). What'd
be equivalent but in a central location would be making sure that
wakeup_preempt() asserts a resched event if it hits a race window like that.
I haven't thought too much about how that should be done exactly but the
races should be pretty rare, so it'd be surprising if the behavior
difference is noticeable.

Thanks.

-- 
tejun