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Date:   Fri, 29 Jun 2018 15:26:38 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc:     Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
        David Rientjes <rientjes@google.com>, linux-mm@kvack.org,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm,oom: Bring OOM notifier callbacks to outside of OOM
 killer.
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On Fri 29-06-18 05:52:18, Paul E. McKenney wrote:
> On Fri, Jun 29, 2018 at 11:04:19AM +0200, Michal Hocko wrote:
> > On Thu 28-06-18 14:31:05, Paul E. McKenney wrote:
> > > On Thu, Jun 28, 2018 at 01:39:42PM +0200, Michal Hocko wrote:
[...]
> > > > Well, I am not really sure what is the objective of the oom notifier to
> > > > point you to the right direction. IIUC you just want to kick callbacks
> > > > to be handled sooner under a heavy memory pressure, right? How is that
> > > > achieved? Kick a worker?
> > > 
> > > That is achieved by enqueuing a non-lazy callback on each CPU's callback
> > > list, but only for those CPUs having non-empty lists.  This causes
> > > CPUs with lists containing only lazy callbacks to be more aggressive,
> > > in particular, it prevents such CPUs from hanging out idle for seconds
> > > at a time while they have callbacks on their lists.
> > > 
> > > The enqueuing happens via an IPI to the CPU in question.
> > 
> > I am afraid this is too low level for my to understand what is going on
> > here. What are lazy callbacks and why do they need any specific action
> > when we are getting close to OOM? I mean, I do understand that we might
> > have many callers of call_rcu and free memory lazily. But there is quite
> > a long way before we start the reclaim until we reach the OOM killer path.
> > So why don't those callbacks get called during that time period? How are
> > their triggered when we are not hitting the OOM path? They surely cannot
> > sit there for ever, right? Can we trigger them sooner? Maybe the
> > shrinker is not the best fit but we have a retry feedback loop in the page
> > allocator, maybe we can kick this processing from there.
> 
> The effect of RCU's current OOM code is to speed up callback invocation
> by at most a few seconds (assuming no stalled CPUs, in which case
> it is not possible to speed up callback invocation).
> 
> Given that, I should just remove RCU's OOM code entirely?

Yeah, it seems so. I do not see how this would really help much. If we
really need some way to kick callbacks then we should do so much earlier
in the reclaim process - e.g. when we start struggling to reclaim any
memory.

I am curious. Has the notifier been motivated by a real world use case
or it was "nice thing to do"?
-- 
Michal Hocko
SUSE Labs