linux-kernel - Re: [PATCH 0/3] Suppress undesirable hung task warnings.

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Message-ID: <4ntd7nnkoidyakbfm3caieku5tvpmzklhm27vgr3fu746hsrov@wqiqxasc4s7p>
Date: Mon, 22 Sep 2025 17:27:35 +0200
From: Jan Kara <jack@...e.cz>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Julian Sun <sunjunchao@...edance.com>, cgroups@...r.kernel.org, 
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, viro@...iv.linux.org.uk, 
	brauner@...nel.org, jack@...e.cz, mingo@...hat.com, juri.lelli@...hat.com, 
	vincent.guittot@...aro.org, dietmar.eggemann@....com, rostedt@...dmis.org, 
	bsegall@...gle.com, mgorman@...e.de, vschneid@...hat.com, akpm@...ux-foundation.org, 
	lance.yang@...ux.dev, mhiramat@...nel.org, agruenba@...hat.com, hannes@...xchg.org, 
	mhocko@...nel.org, roman.gushchin@...ux.dev, shakeel.butt@...ux.dev, 
	muchun.song@...ux.dev
Subject: Re: [PATCH 0/3] Suppress undesirable hung task warnings.

On Mon 22-09-25 15:27:18, Peter Zijlstra wrote:
> On Mon, Sep 22, 2025 at 05:41:43PM +0800, Julian Sun wrote:
> > As suggested by Andrew Morton in [1], we need a general mechanism 
> > that allows the hung task detector to ignore unnecessary hung 
> > tasks. This patch set implements this functionality.
> > 
> > Patch 1 introduces a PF_DONT_HUNG flag. The hung task detector will 
> > ignores all tasks that have the PF_DONT_HUNG flag set.
> > 
> > Patch 2 introduces wait_event_no_hung() and wb_wait_for_completion_no_hung(), 
> > which enable the hung task detector to ignore hung tasks caused by these
> > wait events.
> > 
> > Patch 3 uses wb_wait_for_completion_no_hung() in the final phase of memcg 
> > teardown to eliminate the hung task warning.
> > 
> > Julian Sun (3):
> >   sched: Introduce a new flag PF_DONT_HUNG.
> >   writeback: Introduce wb_wait_for_completion_no_hung().
> >   memcg: Don't trigger hung task when memcg is releasing.
> 
> This is all quite terrible. I'm not at all sure why a task that is
> genuinely not making progress and isn't killable should not be reported.

In principle it is a variation of the old problem where hung task detector
was reporting tasks that were waiting for IO to complete for too long (e.g.
if sync(2) took longer than 2 minutes or whatever the limit is set). In
this case we are waiting for IO in a cgroup to complete which has the
additional dimension that cgroup IO may be throttled so it progresses extra
slowly. But the reports of hang check firing for long running IO
disappeared quite some time ago - I have a vague recollection there were
some tweaks to it for this case but maybe I'm wrong and people just learned
to tune the hang check timer :).

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR