linux-kernel - Re: [RFC PATCH] PM: Optionally block user fork during freeze to improve performance

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Message-ID: <aE_-4nMTvx3m9lmY@tiehlicka>
Date: Mon, 16 Jun 2025 13:24:18 +0200
From: Michal Hocko <mhocko@...e.com>
To: David Hildenbrand <david@...hat.com>
Cc: Zihuan Zhang <zhangzihuan@...inos.cn>,
	Peter Zijlstra <peterz@...radead.org>, rafael@...nel.org,
	len.brown@...el.com, pavel@...nel.org, kees@...nel.org,
	mingo@...hat.com, juri.lelli@...hat.com, vincent.guittot@...aro.org,
	dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
	mgorman@...e.de, vschneid@...hat.com, akpm@...ux-foundation.org,
	lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com, vbabka@...e.cz,
	rppt@...nel.org, surenb@...gle.com, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC PATCH] PM: Optionally block user fork during freeze to
 improve performance

On Mon 16-06-25 09:45:59, David Hildenbrand wrote:
> 
> > > [...]
> > In our test scenario, although new processes can indeed be created
> > during the usleep_range() intervals between freeze iterations, it’s
> > actually difficult to make the freezer fail outright. This is because
> > user processes are forcibly frozen: when they return to user space and
> > check for pending signals, they enter try_to_freeze() and transition
> > into the refrigerator.
> > 
> > However, since the scheduler is fair by design, it gives both newly
> > forked tasks and yet-to-be-frozen tasks a chance to run. This
> > competition for CPU time can slightly delay the overall freeze process.
> > While this typically doesn’t lead to failure, it does cause more retries
> > than necessary, especially under CPU pressure.
> 
> I think that goes back to my original comment: why are we even allowing fork
> children to run at all when we are currently freezing all tasks?

The same should be the case for cgroup freezer as well.
-- 
Michal Hocko
SUSE Labs