linux-kernel - Re: [PATCH v4 0/1] PM / Freezer: Skip zombie/dead processes to reduce

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Message-ID: <CAJZ5v0jvzTupABXQLmTsu7+jEUp14u5XEN4=W7opGi8X2OWorQ@mail.gmail.com>
Date: Thu, 17 Jul 2025 11:50:48 +0200
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Zihuan Zhang <zhangzihuan@...inos.cn>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>, Peter Zijlstra <peterz@...radead.org>, 
	Oleg Nesterov <oleg@...hat.com>, len brown <len.brown@...el.com>, pavel machek <pavel@...nel.org>, 
	linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 0/1] PM / Freezer: Skip zombie/dead processes to reduce

On Thu, Jul 17, 2025 at 3:02 AM Zihuan Zhang <zhangzihuan@...inos.cn> wrote:
>
> HI Rafael,
>
> 在 2025/7/16 20:26, Rafael J. Wysocki 写道:
> > Hi,
> >
> > On Wed, Jul 16, 2025 at 8:26 AM Zihuan Zhang <zhangzihuan@...inos.cn> wrote:
> >> Hi all,
> >>
> >> This patch series improves the performance of the process freezer by
> >> skipping zombie tasks during freezing.
> >>
> >> In the suspend and hibernation paths, the freezer traverses all tasks
> >> and attempts to freeze them. However, zombie tasks (EXIT_ZOMBIE with
> >> PF_EXITING) are already dead — they are not schedulable and cannot enter
> >> the refrigerator. Attempting to freeze such tasks is redundant and
> >> unnecessarily increases freezing time.
> >>
> >> In particular, on systems under fork storm conditions (e.g., many
> >> short-lived processes quickly becoming zombies), the number of zombie tasks
> >> can spike into the thousands or more. We observed that this causes the
> >> freezer loop to waste significant time processing tasks that are guaranteed
> >> to not need freezing.
> > I think that the discussion with Peter regarding this has not been concluded.
> >
> > I thought that there was an alternative patch proposed during that
> > discussion.  If I'm not mistaken about this, what happened to that
> > patch?
> >
> > Thanks!
> >
>
> Currently, the general consensus from the discussion is that skipping
> zombie or dead tasks can help reduce locking overhead during freezing.

Peter doesn't seem to be convinced that this is the case.

> The remaining question is how best to implement that.
>
> Peter suggested skipping all tasks with PF_NOFREEZE, which would make
> the logic more general and cover all cases. However, as Oleg pointed
> out, the current implementation based on PF_NOFREEZE might be problematic.
>
> My current thought is that exit_state already reliably covers all
> exiting user processes, and it’s a good fit for skipping user-space
> tasks. For the kernel side, we may safely skip a few kernel threads like
> kthreadd that set PF_NOFREEZE and never change it — we can consider
> refining this further in the future.

There is the counter argument of special-casing of p->exit_state and
the relatively weak justification for it.

You have created a synthetic workload where it matters, but how likely
is it to be the case in practice?