linux-kernel - Re: [PATCH] smp: Suppress false DEBUG_PREEMPT warning in smp_call_on

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Message-ID: <20251105084808.7-nsQ6lT@linutronix.de>
Date: Wed, 5 Nov 2025 09:48:08 +0100
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Nam Cao <namcao@...utronix.de>
Cc: Ingo Molnar <mingo@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Mel Gorman <mgorman@...e.de>,
	Shrikanth Hegde <sshegde@...ux.ibm.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Valentin Schneider <vschneid@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] smp: Suppress false DEBUG_PREEMPT warning in
 smp_call_on_cpu()

On 2025-11-05 09:42:42 [+0100], Nam Cao wrote:
> >> +
> >> +	/* suppress warnings from debug_smp_processor_id() */
> >
> > If you want to add a comment, what about something like
> >
> >   /* Preserve not being migratable such as SMP variant does */ 
> 
> Wouldn't it be a bit misleading? It's true that technically SMP variant
> is not migratable, but that's because it is a per-cpu thread, not
> because of migrate_disable(). To me, the comment sounds like that there
> is also a migrate_disable() in SMP variant.

A per-cpu thread ensures that the control flow can not migrate to
another CPU. This is also what stops debug_smp_processor_id() warnings
from work invoked from queue_work_on() vs queue_work().

> Nam

Sebastian