linux-kernel - Re: [RFC PATCH v1 5/9] freezer: set default freeze priority for PF_SUSPEND

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Message-ID: <20250811094651.GD11928@redhat.com>
Date: Mon, 11 Aug 2025 11:46:52 +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>,
	David Hildenbrand <david@...hat.com>,
	Michal Hocko <mhocko@...e.com>, Jonathan Corbet <corbet@....net>,
	Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>,
	len brown <len.brown@...el.com>, pavel machek <pavel@...nel.org>,
	Kees Cook <kees@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	"Liam R . Howlett" <Liam.Howlett@...cle.com>,
	Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
	Suren Baghdasaryan <surenb@...gle.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Nico Pache <npache@...hat.com>, xu xin <xu.xin16@....com.cn>,
	wangfushuai <wangfushuai@...du.com>,
	Andrii Nakryiko <andrii@...nel.org>,
	Christian Brauner <brauner@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jeff Layton <jlayton@...nel.org>, Al Viro <viro@...iv.linux.org.uk>,
	Adrian Ratiu <adrian.ratiu@...labora.com>, linux-pm@...r.kernel.org,
	linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v1 5/9] freezer: set default freeze priority for
 PF_SUSPEND_TASK processes

On 08/11, Zihuan Zhang wrote:
> 
> 在 2025/8/11 17:32, Oleg Nesterov 写道:
> >On 08/11, Zihuan Zhang wrote:
> >>在 2025/8/8 22:39, Oleg Nesterov 写道:
> >>>On 08/07, Zihuan Zhang wrote:
> >>>>--- a/kernel/power/process.c
> >>>>+++ b/kernel/power/process.c
> >>>>@@ -147,6 +147,7 @@ int freeze_processes(void)
> >>>>
> >>>>  	pm_wakeup_clear(0);
> >>>>  	pm_freezing = true;
> >>>>+	freeze_set_default_priority(current, FREEZE_PRIORITY_NEVER);
> >>>But why?
> >>>
> >>>Again, freeze_task() will return false anyway, this process is
> >>>PF_SUSPEND_TASK.
> >>I  think there is resaon put it here. For example, systemd-sleep is a
> >>user-space process that executes the suspend flow.
> >>
> >>  If we don’t set its freeze priority explicitly, our current code may end up
> >>with this user process being the last one that cannot freeze.
> >How so? sorry I don't follow.
>
> The problem is in this part:
>
> +            if (user_only && !(p->flags & PF_KTHREAD) && round <
> p->freeze_priority)
> +                continue;
>
> PF_SUSPEND_TASK is a user process, so it meets the “needs freezing”
> condition and todo gets incremented.
            ^^^^^^^^^^^^^^^^^^^^^^^^^

No.
	if (p == current || !freeze_task(p))
		continue;

	todo++;

Again, again, freeze_task(p) returns false.

> But it actually doesn’t need to freeze,
> so resulting in an infinite loop

I don't think so.

Oleg.