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

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Message-ID: <20250811112119.GC5250@redhat.com>
Date: Mon, 11 Aug 2025 13:21:20 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: "Luis Claudio R. Goncalves" <lgoncalv@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	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 08/11, Sebastian Andrzej Siewior wrote:
>
> On 2025-08-11 12:40:34 [+0200], Oleg Nesterov wrote:
> >
> > What exactly do you think we could do regardless of PREEMPT_RT?
>
> Do what we do now and have one free path for task_struct regardless if
> PREEMPT_RT is enabled or not. The one via RCU which delays the freeing
> until after the grace period.

Ah, got it. I won't really argue, but...

call_rcu() is not free, it obviously delays free_task/etc. To me this
!PREEMPT_RT optimization makes sense.

But lets forget it for the moment. This patch

	https://lore.kernel.org/all/aGvTz5VaPFyj0pBV@uudg.org
	[PATCH v6] sched: do not call __put_task_struct() on rt if pi_blocked_on is set

removed that optimization by mistake, this doesn't even match the changelog.
I think it should be restored, and this is what the new patch from Luis does.

Then we can discuss this all again and possibly remove it, but this
should be explicitly documented in the changelog.

Oleg.