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

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Message-ID: <20250811121536.taFIcCHt@linutronix.de>
Date: Mon, 11 Aug 2025 14:15:36 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Oleg Nesterov <oleg@...hat.com>
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 2025-08-11 13:21:20 [+0200], Oleg Nesterov wrote:
> 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.

Certainly. I am not saying we should keep it as is. The added comment
appears wrong but I am all for getting this merged and then sorting out
later.
Thank you for spotting this ;)

> Oleg.

Sebastian