linux-kernel - Re: [PATCH v4 2/3] mm/oom_kill: Only delay OOM reaper for processes using robust futexes

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Message-ID: <20250818120819.26709-1-zhongjinji@honor.com>
Date: Mon, 18 Aug 2025 20:08:19 +0800
From: zhongjinji <zhongjinji@...or.com>
To: <mhocko@...e.com>
CC: <akpm@...ux-foundation.org>, <andrealmeid@...lia.com>,
	<dave@...olabs.net>, <dvhart@...radead.org>, <feng.han@...or.com>,
	<liam.howlett@...cle.com>, <linux-kernel@...r.kernel.org>,
	<linux-mm@...ck.org>, <liulu.liu@...or.com>, <mingo@...hat.com>,
	<npache@...hat.com>, <peterz@...radead.org>, <rientjes@...gle.com>,
	<shakeel.butt@...ux.dev>, <tglx@...utronix.de>, <zhongjinji@...or.com>
Subject: Re: [PATCH v4 2/3] mm/oom_kill: Only delay OOM reaper for processes using robust futexes

> On Thu 14-08-25 21:55:54, zhongjinji@...or.com wrote:
> > From: zhongjinji <zhongjinji@...or.com>
> > 
> > The OOM reaper can quickly reap a process's memory when the system encounters
> > OOM, helping the system recover. Without the OOM reaper, if a process frozen
> > by cgroup v1 is OOM killed, the victims' memory cannot be freed, and the
> > system stays in a poor state. Even if the process is not frozen by cgroup v1,
> > reaping victims' memory is still meaningful, because having one more process
> > working speeds up memory release.
> > 
> > When processes holding robust futexes are OOM killed but waiters on those
> > futexes remain alive, the robust futexes might be reaped before
> > futex_cleanup() runs. It would cause the waiters to block indefinitely.
> > To prevent this issue, the OOM reaper's work is delayed by 2 seconds [1].
> > The OOM reaper now rarely runs since many killed processes exit within 2
> > seconds.
> > 
> > Because robust futex users are few, it is unreasonable to delay OOM reap for
> > all victims. For processes that do not hold robust futexes, the OOM reaper
> > should not be delayed and for processes holding robust futexes, the OOM
> > reaper must still be delayed to prevent the waiters to block indefinitely [1].
> > 
> > Link: https://lore.kernel.org/all/20220414144042.677008-1-npache@redhat.com/T/#u [1]
> 
> What has happened to
> https://lore.kernel.org/all/aJGiHyTXS_BqxoK2@tiehlicka/T/#u ?

If a process holding robust futexes gets frozen, robust futexes might be reaped before
futex_cleanup() runs when an OOM occurs. I am not sure if this will actually happen.

> 
> Generally speaking it would be great to provide a link to previous
> versions of the patchset. I do not see v3 in my inbox (which is quite
> messy ATM so I might have easily missed it).

This is version v3, where I mainly fixed the error in the Subject prefix,
changing it from futex to mm/oom_kill.

https://lore.kernel.org/all/20250804030341.18619-1-zhongjinji@honor.com/
https://lore.kernel.org/all/20250804030341.18619-2-zhongjinji@honor.com/

> -- 
> Michal Hocko
> SUSE Labs