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

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Message-ID: <775aaf10-3d19-4d5a-bf2b-703211166be4@redhat.com>
Date: Mon, 16 Jun 2025 09:45:59 +0200
From: David Hildenbrand <david@...hat.com>
To: Zihuan Zhang <zhangzihuan@...inos.cn>, Michal Hocko <mhocko@...e.com>
Cc: 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


>> [...]
> 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?

I would imagine that try_to_freeze_tasks() should force any new 
processes (forked children) to start in the frozen state directly and 
not get scheduled in the first place.

-- 
Cheers,

David / dhildenb