linux-kernel - Re: [PATCH v2] mm/page_alloc: Wait for oom

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Message-ID: <20170717085605.GE12888@dhcp22.suse.cz>
Date:   Mon, 17 Jul 2017 10:56:05 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:     linux-mm@...ck.org, hannes@...xchg.org, rientjes@...gle.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm/page_alloc: Wait for oom_lock before retrying.

On Sun 16-07-17 19:59:51, Tetsuo Handa wrote:
> Since the whole memory reclaim path has never been designed to handle the
> scheduling priority inversions, those locations which are assuming that
> execution of some code path shall eventually complete without using
> synchronization mechanisms can get stuck (livelock) due to scheduling
> priority inversions, for CPU time is not guaranteed to be yielded to some
> thread doing such code path.
> 
> mutex_trylock() in __alloc_pages_may_oom() (waiting for oom_lock) and
> schedule_timeout_killable(1) in out_of_memory() (already held oom_lock) is
> one of such locations, and it was demonstrated using artificial stressing
> that the system gets stuck effectively forever because SCHED_IDLE priority
> thread is unable to resume execution at schedule_timeout_killable(1) if
> a lot of !SCHED_IDLE priority threads are wasting CPU time [1].

I do not understand this. All the contending tasks will go and sleep for
1s. How can they preempt the lock holder?
-- 
Michal Hocko
SUSE Labs