linux-kernel - Re: [PATCH] sched/ext: Suppress warning in __this_cpu

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Message-ID: <20250716125128.GX905792@noisy.programming.kicks-ass.net>
Date: Wed, 16 Jul 2025 14:51:28 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Breno Leitao <leitao@...ian.org>
Cc: Tejun Heo <tj@...nel.org>, 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>, sched-ext@...ts.linux.dev,
	linux-kernel@...r.kernel.org, kernel-team@...a.com,
	jake@...lion.co.uk
Subject: Re: [PATCH] sched/ext: Suppress warning in __this_cpu_write() by
 disabling preemption

On Wed, Jul 16, 2025 at 05:46:15AM -0700, Breno Leitao wrote:
> __this_cpu_write() emits a warning if used with preemption enabled.
> 
> Function update_locked_rq() might be called with preemption enabled,
> which causes the following warning:
> 
> 	BUG: using __this_cpu_write() in preemptible [00000000] code: scx_layered_6-9/68770
> 
> Disable preemption around the __this_cpu_write() call in
> update_locked_rq() to suppress the warning, without affecting behavior.
> 
> If preemption triggers a  jump to another CPU during the callback it's
> fine, since we would track the rq state on the other CPU with its own
> local variable.
> 
> Suggested-by: Andrea Righi <arighi@...dia.com>
> Signed-off-by: Breno Leitao <leitao@...ian.org>
> Fixes: 18853ba782bef ("sched_ext: Track currently locked rq")
> Acked-by: Andrea Righi <arighi@...dia.com>
> ---
>  kernel/sched/ext.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> index b498d867ba210..24fcbd7331f73 100644
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -1258,7 +1258,14 @@ static inline void update_locked_rq(struct rq *rq)
>  	 */
>  	if (rq)
>  		lockdep_assert_rq_held(rq);
> +	/*
> +	 * __this_cpu_write() emits a warning when used with preemption enabled.
> +	 * While there's no functional issue if the callback runs on another
> +	 * CPU, we disable preemption here solely to suppress that warning.
> +	 */
> +	preempt_disable();
>  	__this_cpu_write(locked_rq, rq);
> +	preempt_enable();
>  }

This looks dodgy as heck. Why can't we do this update_locked_rq(NULL)
call while still holding rq? Also, I don't seem to have this scx_layered
thing.