linux-kernel - Re: [PATCH] mm,oom: Try last second allocation before and after selecting an OOM victim.

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Message-ID: <20171031132259.irkladqbucz2qa3g@dhcp22.suse.cz>
Date:   Tue, 31 Oct 2017 14:22:59 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:     aarcange@...hat.com, akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, rientjes@...gle.com,
        hannes@...xchg.org, mjaggi@...iumnetworks.com, mgorman@...e.de,
        oleg@...hat.com, vdavydov.dev@...il.com, vbabka@...e.cz
Subject: Re: [PATCH] mm,oom: Try last second allocation before and after
 selecting an OOM victim.

On Tue 31-10-17 22:13:05, Tetsuo Handa wrote:
> Michal Hocko wrote:
> > On Tue 31-10-17 21:42:23, Tetsuo Handa wrote:
> > > > While both have some merit, the first reason is mostly historical
> > > > because we have the explicit locking now and it is really unlikely that
> > > > the memory would be available right after we have given up trying.
> > > > Last attempt allocation makes some sense of course but considering that
> > > > the oom victim selection is quite an expensive operation which can take
> > > > a considerable amount of time it makes much more sense to retry the
> > > > allocation after the most expensive part rather than before. Therefore
> > > > move the last attempt right before we are trying to kill an oom victim
> > > > to rule potential races when somebody could have freed a lot of memory
> > > > in the meantime. This will reduce the time window for potentially
> > > > pre-mature OOM killing considerably.
> > > 
> > > But this is about "doing last second allocation attempt after selecting
> > > an OOM victim". This is not about "allowing OOM victims to try ALLOC_OOM
> > > before selecting next OOM victim" which is the actual problem I'm trying
> > > to deal with.
> > 
> > then split it into two. First make the general case and then add a more
> > sophisticated on top. Dealing with multiple issues at once is what makes
> > all those brain cells suffer.
> 
> I'm failing to understand. I was dealing with single issue at once.
> The single issue is "MMF_OOM_SKIP prematurely prevents OOM victims from trying
> ALLOC_OOM before selecting next OOM victims". Then, what are the general case and
> a more sophisticated? I wonder what other than "MMF_OOM_SKIP should allow OOM
> victims to try ALLOC_OOM for once before selecting next OOM victims" can exist...

Try to think little bit out of your very specific and borderline usecase
and it will become obvious. ALLOC_OOM is a trivial update on top of
moving get_page_from_freelist to oom_kill_process which is a more
generic race window reducer.

-- 
Michal Hocko
SUSE Labs