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Date: Mon, 17 Jul 2017 17:15:58 +0200
From: Michal Hocko <mhocko@kernel.org>
Subject: Re: [PATCH v2] mm/page_alloc: Wait for oom_lock before retrying.
Message-ID: <20170717151558.GL12888@dhcp22.suse.cz>
References: <1500202791-5427-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp>
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To: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: linux-mm@kvack.org, hannes@cmpxchg.org, rientjes@google.com, linux-kernel@vger.kernel.org

On Mon 17-07-17 22:50:47, Tetsuo Handa wrote:
> Michal Hocko wrote:
> > 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?
> 
> Not 1s. It sleeps for only 1 jiffies, which is 1ms if CONFIG_HZ=1000.

Right, for some reason I have seen HZ. My bad!
-- 
Michal Hocko
SUSE Labs

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