linux-kernel - Re: [RESEND PATCH] sched: restore the behavior of put_task

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Message-ID: <20250915113812.GB3245006@noisy.programming.kicks-ass.net>
Date: Mon, 15 Sep 2025 13:38:12 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: "Luis Claudio R. Goncalves" <lgoncalv@...hat.com>
Cc: Oleg Nesterov <oleg@...hat.com>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Clark Williams <clrkwllms@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>, Tejun Heo <tj@...nel.org>,
	David Vernet <dvernet@...a.com>, Barret Rhoden <brho@...gle.com>,
	Josh Don <joshdon@...gle.com>, Crystal Wood <crwood@...hat.com>,
	linux-kernel@...r.kernel.org, linux-rt-devel@...ts.linux.dev,
	Juri Lelli <juri.lelli@...hat.com>, Ben Segall <bsegall@...gle.com>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Ingo Molnar <mingo@...hat.com>, Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Wander Lairson Costa <wander@...hat.com>
Subject: Re: [RESEND PATCH] sched: restore the behavior of put_task_struct()
 for non-rt

On Mon, Sep 15, 2025 at 08:15:19AM -0300, Luis Claudio R. Goncalves wrote:
> Commit 8671bad873eb ("sched: Do not call __put_task_struct() on rt
> if pi_blocked_on is set") changed the behavior of put_task_struct()
> unconditionally, even when PREEMPT_RT was not enabled, in clear mismatch
> with the commit description.
> 
> Restore the previous behavior of put_task_struct() for the PREEMPT_RT
> disabled case.
> 
> Fixes: 8671bad873eb ("sched: Do not call __put_task_struct() on rt if pi_blocked_on is set")
> Acked-by: Oleg Nesterov <oleg@...hat.com>
> Signed-off-by: Luis Claudio R. Goncalves <lgoncalv@...hat.com>
> ---
> 
>   Note: This patch is a fix motivated by Oleg Nesterov's question at
>   https://lore.kernel.org/linux-rt-devel/20250728201441.GA4690@redhat.com/

Right, but I thought we did want to make this behaviour consistent.

Why have !RT behave differently? That is, why isn't this simply a
'buggy' comment/changelog issue?

>   Also, the kernel test robot reported a problem found in a x86 (32 bit) VM
>   test that was bisected to the original fix being amended here:
> 
>     https://lkml.org/lkml/2025/9/5/147
> 
>   Though I was not able the reproduce the reported problem, this patch here
>   would minimize the problem by isolating the behavior to PREEMPT_RT-enabled
>   kernels.

Yeah, robot also had trouble reproducing that if I read that
%reproduction thing right.

And IIRC RT has been running with this for ages, and never seen a
problem.