linux-kernel - Re: [PATCH v3 1/1] PM / Freezer: Skip zombie/dead processes to

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Message-ID: <20250704081941.GC2001818@noisy.programming.kicks-ass.net>
Date: Fri, 4 Jul 2025 10:19:41 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: "Rafael J. Wysocki" <rafael@...nel.org>
Cc: Zihuan Zhang <zhangzihuan@...inos.cn>, pavel@...nel.org,
	len.brown@...el.com, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org, Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH v3 1/1] PM / Freezer: Skip zombie/dead processes to

On Thu, Jul 03, 2025 at 07:15:23PM +0200, Rafael J. Wysocki wrote:

> > How realistic is it to have a significant amount of zombies when
> > freezing? This seems like an artificial corner case at best.
> >
> > Zombie tasks are stuck waiting on their parent to consume their exit
> > state or something, right? And those parents being frozen, they pretty
> > much stay there.
> >
> > So I suppose the logic holds, but urgh, do we really need this?
> 
> Unlikely in practice, but the code change is small and it would be
> prudent to get this addressed IMV (at least so we don't need to
> revisit it).
> 
> But I would ask for a comment above this check to explain that zombies
> need not be frozen.

Depending on where they wait (I can't seem to find in a hurry) it might
make sense to make that wait FREEZABLE anyway.

For example, AFAICT it wouldn't hurt, and might even help some, to make
kernel/exit.c:do_wait() TASK_FREEZABLE.

So where do ZOMBIEs sleep? Don't they simply pass through do_task_dead()
and never get scheduled again? Notably, do_task_dead() already marks the
tasks as PF_NOFREEZE.

Anyway, yes, the condition it adds is relatively simple, but I really
don't see why we should complicate things *at*all*.