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

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Message-ID: <20250915123539.GC23082@redhat.com>
Date: Mon, 15 Sep 2025 14:35:40 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: "Luis Claudio R. Goncalves" <lgoncalv@...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 09/15, Peter Zijlstra wrote:
>
> 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?

Well, this was discussed several times, in particular see
https://lore.kernel.org/all/CAHk-=whtj+aSYftniMRG4xvFE8dmmYyrqcJyPmzStsfj5w9r=w@mail.gmail.com/

And task_struct->rcu_users was introduced to avoid RCU call in put_task_struct() ...

But I won't really argue if you decide to remove this !RT optimization, although
I think it would be better to do this in a separate patch.

Oleg.