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Date:   Fri, 29 May 2020 12:14:12 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     Chris Down <chris@chrisdown.name>
Cc:     Johannes Weiner <hannes@cmpxchg.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Tejun Heo <tj@kernel.org>, linux-mm@kvack.org,
        cgroups@vger.kernel.org, linux-kernel@vger.kernel.org,
        kernel-team@fb.com
Subject: Re: [PATCH] mm, memcg: reclaim more aggressively before high
 allocator throttling
Message-ID: <20200529101412.GJ4406@dhcp22.suse.cz>
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On Fri 29-05-20 11:08:58, Chris Down wrote:
> Michal Hocko writes:
> > > > > task->memcg_nr_pages_over_high is not vague, it's a best-effort
> > > > > mechanism to distribute fairness. It's the current task's share of the
> > > > > cgroup's overage, and it allows us in the majority of situations to
> > > > > distribute reclaim work and sleeps in proportion to how much the task
> > > > > is actually at fault.
> > > >
> > > > Agreed. But this stops being the case as soon as the reclaim target has
> > > > been reached and new reclaim attempts are enforced because the memcg is
> > > > still above the high limit. Because then you have a completely different
> > > > reclaim target - get down to the limit. This would be especially visible
> > > > with a large memcg_nr_pages_over_high which could even lead to an over
> > > > reclaim.
> > > 
> > > We actually over reclaim even before this patch -- this patch doesn't bring
> > > much new in that regard.
> > > 
> > > Tracing try_to_free_pages for a cgroup at the memory.high threshold shows
> > > that before this change, we sometimes even reclaim on the order of twice the
> > > number of pages requested. For example, I see cases where we requested 1000
> > > pages to be reclaimed, but end up reclaiming 2000 in a single reclaim
> > > attempt.
> > 
> > This is interesting and worth looking into. I am aware that we can
> > reclaim potentially much more pages during the icache reclaim and that
> > there was a heated discussion without any fix merged in the end IIRC.
> > Do you have any details?
> 
> Sure, we can look into this more, but let's do it separately from this patch
> -- I don't see that its merging should be contingent on that discussion :-)

Yes that is a separate issue.
-- 
Michal Hocko
SUSE Labs