From: Michal Hocko <mhocko@kernel.org>
To: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Andrew Morton <akpm@linux-foundation.org>,
David Rientjes <rientjes@google.com>,
Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
Oleg Nesterov <oleg@redhat.com>, Hugh Dickins <hughd@google.com>,
Andrea Arcangeli <aarcange@redhat.com>,
linux-mm@kvack.org, LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] mm, oom: allow oom reaper to race with exit_mmap
Date: Mon, 24 Jul 2017 16:15:26 +0200 [thread overview]
Message-ID: <20170724141526.GM25221@dhcp22.suse.cz> (raw)
In-Reply-To: <20170724140008.sd2n6af6izjyjtda@node.shutemov.name>
On Mon 24-07-17 17:00:08, Kirill A. Shutemov wrote:
> On Mon, Jul 24, 2017 at 09:23:32AM +0200, Michal Hocko wrote:
> > From: Michal Hocko <mhocko@suse.com>
> >
> > David has noticed that the oom killer might kill additional tasks while
> > the exiting oom victim hasn't terminated yet because the oom_reaper marks
> > the curent victim MMF_OOM_SKIP too early when mm->mm_users dropped down
> > to 0. The race is as follows
> >
> > oom_reap_task do_exit
> > exit_mm
> > __oom_reap_task_mm
> > mmput
> > __mmput
> > mmget_not_zero # fails
> > exit_mmap # frees memory
> > set_bit(MMF_OOM_SKIP)
> >
> > The victim is still visible to the OOM killer until it is unhashed.
> >
> > Currently we try to reduce a risk of this race by taking oom_lock
> > and wait for out_of_memory sleep while holding the lock to give the
> > victim some time to exit. This is quite suboptimal approach because
> > there is no guarantee the victim (especially a large one) will manage
> > to unmap its address space and free enough memory to the particular oom
> > domain which needs a memory (e.g. a specific NUMA node).
> >
> > Fix this problem by allowing __oom_reap_task_mm and __mmput path to
> > race. __oom_reap_task_mm is basically MADV_DONTNEED and that is allowed
> > to run in parallel with other unmappers (hence the mmap_sem for read).
> >
> > The only tricky part is to exclude page tables tear down and all
> > operations which modify the address space in the __mmput path. exit_mmap
> > doesn't expect any other users so it doesn't use any locking. Nothing
> > really forbids us to use mmap_sem for write, though. In fact we are
> > already relying on this lock earlier in the __mmput path to synchronize
> > with ksm and khugepaged.
>
> That's true, but we take mmap_sem there for small portion of cases.
>
> It's quite different from taking the lock unconditionally. I'm worry about
> scalability implication of such move. On bigger machines it can be big
> hit.
What kind of scalability implication you have in mind? There is
basically a zero contention on the mmap_sem that late in the exit path
so this should be pretty much a fast path of the down_write. I agree it
is not 0 cost but the cost of the address space freeing should basically
make it a noise.
> Should we do performance/scalability evaluation of the patch before
> getting it applied?
What kind of test(s) would you be interested in?
--
Michal Hocko
SUSE Labs
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next prev parent reply other threads:[~2017-07-24 14:15 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-07-24 7:23 [PATCH] mm, oom: allow oom reaper to race with exit_mmap Michal Hocko
2017-07-24 14:00 ` Kirill A. Shutemov
2017-07-24 14:15 ` Michal Hocko [this message]
2017-07-24 14:51 ` Kirill A. Shutemov
2017-07-24 16:11 ` Michal Hocko
2017-07-25 14:17 ` Kirill A. Shutemov
2017-07-25 14:26 ` Michal Hocko
2017-07-25 15:07 ` Kirill A. Shutemov
2017-07-25 15:15 ` Michal Hocko
2017-07-25 14:26 ` Michal Hocko
2017-07-25 15:17 ` Kirill A. Shutemov
2017-07-25 15:23 ` Michal Hocko
2017-07-25 15:31 ` Kirill A. Shutemov
2017-07-25 16:04 ` Michal Hocko
2017-07-25 19:19 ` Andrea Arcangeli
2017-07-26 5:45 ` Michal Hocko
2017-07-26 16:29 ` Andrea Arcangeli
2017-07-26 16:43 ` Andrea Arcangeli
2017-07-27 6:50 ` Michal Hocko
2017-07-27 14:55 ` Andrea Arcangeli
2017-07-28 6:23 ` Michal Hocko
2017-07-28 1:58 ` [PATCH 1/1] mm: oom: let oom_reap_task and exit_mmap to run kbuild test robot
2017-08-15 0:20 ` [PATCH] mm, oom: allow oom reaper to race with exit_mmap David Rientjes
2017-07-24 15:27 ` Michal Hocko
2017-07-24 16:42 ` kbuild test robot
2017-07-24 18:12 ` Michal Hocko
2017-07-25 15:26 ` Andrea Arcangeli
2017-07-25 15:45 ` Michal Hocko
2017-07-25 18:26 ` Andrea Arcangeli
2017-07-26 5:45 ` Michal Hocko
2017-07-26 16:39 ` Andrea Arcangeli
2017-07-27 6:32 ` Michal Hocko
2017-08-10 8:16 Michal Hocko
2017-08-10 18:05 ` Andrea Arcangeli
2017-08-10 18:51 ` Michal Hocko
2017-08-10 20:36 ` Michal Hocko
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