linux-kernel - Re: [PATCH] mm, memcg: reclaim more aggressively before high allocator throttling

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Date:   Fri, 29 May 2020 11:08:58 +0100
From:   Chris Down <chris@...isdown.name>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Tejun Heo <tj@...nel.org>, linux-mm@...ck.org,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com
Subject: Re: [PATCH] mm, memcg: reclaim more aggressively before high
 allocator throttling

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 :-)