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

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Message-ID: <20250716163854.GE16401@redhat.com>
Date: Wed, 16 Jul 2025 18:38:55 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Zihuan Zhang <zhangzihuan@...inos.cn>
Cc: "rafael J . wysocki" <rafael@...nel.org>,
	Peter Zijlstra <peterz@...radead.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 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.

> +			 */
> +			if (p == current || p->exit_state || !freeze_task(p))
>  				continue;

I leave this to you and Rafael, but this change doesn't look safe to me.
What if the exiting task does some IO after exit_notify() ?

Oleg.