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

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Message-ID: <20250717014356.GG16401@redhat.com>
Date: Thu, 17 Jul 2025 03:43:57 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Zihuan Zhang <zhangzihuan@...inos.cn>
Cc: Peter Zijlstra <peterz@...radead.org>,
	"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 07/17, Zihuan Zhang wrote:
>
> The main reason we didn’t rely directly on PF_NOFREEZE is that it’s a
> mutable flag — in some cases, it can be cleared later, which makes early
> skipping potentially unsafe.

Afaics userspace tasks can only set PF_NOFREEZE in do_task_dead() and never
clear it.

Apart from lock_system_sleep(). That is why (I think) Peter rightly suggests
to take system_transition_mutex in this function earlier.

> In contrast, exit_state is stable and skipping tasks based on it is safe.

I don't think it is really safe...

Oleg.