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

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Message-ID: <20250716183637.GJ4105545@noisy.programming.kicks-ass.net>
Date: Wed, 16 Jul 2025 20:36:37 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Oleg Nesterov <oleg@...hat.com>
Cc: Zihuan Zhang <zhangzihuan@...inos.cn>,
	"rafael J . wysocki" <rafael@...nel.org>,
	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] PM / Freezer: Skip zombie/dead processes to reduce
 freeze latency

On Wed, Jul 16, 2025 at 06:38:55PM +0200, Oleg Nesterov wrote:
> On 07/16, Zihuan Zhang wrote:
> >
> > @@ -51,7 +51,15 @@ static int try_to_freeze_tasks(bool user_only)
> >  		todo = 0;
> >  		read_lock(&tasklist_lock);
> >  		for_each_process_thread(g, p) {
> > -			if (p == current || !freeze_task(p))
> > +			/*
> > +			 * Zombie and dead tasks are not running anymore and cannot enter
> > +			 * the __refrigerator(). Skipping them avoids unnecessary freeze attempts.
> > +			 *
> > +			 * TODO: Consider using PF_NOFREEZE instead, which may provide
> > +			 * a more generic exclusion mechanism for other non-freezable tasks.
> > +			 * However, for now, exit_state is sufficient to skip user processes.
> 
> I don't really understand the comment... The freeze_task() paths already
> consider PF_NOFREEZE, although we can check it earlier as Peter suggests.

Right; I really don't understand why we should special case
->exit_state. Why not DTRT and optimize NOFREEZE if all this really
matters (smalls gains from what ISTR from the previous discussion).