Re: [PATCH] mm,page_alloc: wait for oom_lock than back off

[Michal Hocko <mhocko@kernel.org>]

On Wed 21-02-18 23:27:05, Tetsuo Handa wrote:
> Michal Hocko wrote:
> > On Tue 20-02-18 22:32:56, Tetsuo Handa wrote:
> > > >From c3b6616238fcd65d5a0fdabcb4577c7e6f40d35e Mon Sep 17 00:00:00 2001
> > > From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> > > Date: Tue, 20 Feb 2018 11:07:23 +0900
> > > Subject: [PATCH] mm,page_alloc: wait for oom_lock than back off
> > > 
> > > This patch fixes a bug which is essentially same with a bug fixed by
> > > commit 400e22499dd92613 ("mm: don't warn about allocations which stall for
> > > too long").
> > > 
> > > Currently __alloc_pages_may_oom() is using mutex_trylock(&oom_lock) based
> > > on an assumption that the owner of oom_lock is making progress for us. But
> > > it is possible to trigger OOM lockup when many threads concurrently called
> > > __alloc_pages_slowpath() because all CPU resources are wasted for pointless
> > > direct reclaim efforts. That is, schedule_timeout_uninterruptible(1) in
> > > __alloc_pages_may_oom() does not always give enough CPU resource to the
> > > owner of the oom_lock.
> > > 
> > > It is possible that the owner of oom_lock is preempted by other threads.
> > > Preemption makes the OOM situation much worse. But the page allocator is
> > > not responsible about wasting CPU resource for something other than memory
> > > allocation request. Wasting CPU resource for memory allocation request
> > > without allowing the owner of oom_lock to make forward progress is a page
> > > allocator's bug.
> > > 
> > > Therefore, this patch changes to wait for oom_lock in order to guarantee
> > > that no thread waiting for the owner of oom_lock to make forward progress
> > > will not consume CPU resources for pointless direct reclaim efforts.
> > 
> > So instead we will have many tasks sleeping on the lock and prevent the
> > oom reaper to make any forward progress. This is not a solution without
> > further steps. Also I would like to see a real life workload that would
> > benefit from this.
> 
> Of course I will propose follow-up patches.

The patch in its current form will cause a worse behavior than we have
currently, because pending oom waiters simply block the oom reaper. So I
do not really see any reason to push this forward without other changes.

So NAK to this patch in its current form.
-- 
Michal Hocko
SUSE Labs

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