linux-kernel - Re: [PATCH] sched: Use printk_deferred_once() instead of WARN

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Message-ID: <20250113131654.GB8385@noisy.programming.kicks-ass.net>
Date: Mon, 13 Jan 2025 14:16:54 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Maria Yu <quic_aiquny@...cinc.com>
Cc: 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>,
	Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
	kernel@...cinc.com
Subject: Re: [PATCH] sched: Use printk_deferred_once() instead of WARN_ONCE()

On Mon, Jan 13, 2025 at 02:12:43PM +0100, Peter Zijlstra wrote:
> On Fri, Dec 27, 2024 at 05:27:10PM +0800, Maria Yu wrote:
> > A deadlock is observed when WARN_ONCE() uses printk() inside the scheduler
> > logic. printk_deferred_once() is a WARN_ONCE() similar special printk
> > facility for the scheduler to avoid unnecessary deadlocks.
> 
> No, problem is with printk. Using delayed will make it so that you'll
> never see the output if it dies.
> 
> Only use delayed for printk()s that are expected and non fatal.

Also, if you trip WARN in scheduler, correct thing to do is fix WARN,
not make WARN run 'better'. No WARN, no problem.