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

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Message-ID: <20200528164848.GB839178@chrisdown.name>
Date:   Thu, 28 May 2020 17:48:48 +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:
>> We send a simple bug fix: bring this instance of reclaim in line with
>> how everybody else is using the reclaim API, to meet the semantics as
>> they are intendend and documented.
>
>Here is where we are not on the same page though. Once you have identified
>that the main problem is that the reclaim fails too early to meet the
>target then the fix would be to enforce that target. I have asked why
>this hasn't been done and haven't got any real answer for that. Instead
>what you call "a simple bug fix" has larger consequences which are not
>really explained in the changelog and they are also not really trivial
>to see. If the changelog explicitly stated that the proportional memory
>reclaim is not sufficient because XYZ and the implementation has been
>changed to instead meet the high limit target then this would be a
>completely different story and I believe we could have saved some
>discussion.

I agree that the changelog can be made more clear. Any objection if I send v2 
with changelog changes to that effect, then? :-)

>> And somehow this is controversial, and we're just changing around user
>> promises as we see fit for our particular usecase?
>>
>> I don't even understand how the supposed alternate semantics you read
>> between the lines in the documentation would make for a useful
>> feature: It may fail to contain a group of offending tasks to the
>> configured limit, but it will be fair to those tasks while doing so?
>>
>> > But if your really want to push this through then let's do it
>> > properly at least. memcg->memcg_nr_pages_over_high has only very
>> > vague meaning if the reclaim target is the high limit.
>>
>> 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.