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Date:   Thu, 21 May 2020 13:23:27 +0100
From:   Chris Down <chris@chrisdown.name>
To:     Michal Hocko <mhocko@kernel.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Johannes Weiner <hannes@cmpxchg.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
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(I'll leave the dirty throttling discussion to Johannes, because I'm not so 
familiar with that code or its history.)

Michal Hocko writes:
>> > The main problem I see with that approach is that the loop could easily
>> > lead to reclaim unfairness when a heavy producer which doesn't leave the
>> > kernel (e.g. a large read/write call) can keep a different task doing
>> > all the reclaim work. The loop is effectivelly unbound when there is a
>> > reclaim progress and so the return to the userspace is by no means
>> > proportional to the requested memory/charge.
>>
>> It's not unbound when there is reclaim progress, it stops when we are within
>> the memory.high throttling grace period. Right after reclaim, we check if
>> penalty_jiffies is less than 10ms, and abort and further reclaim or
>> allocator throttling:
>
>Just imagine that you have parallel producers increasing the high limit
>excess while somebody reclaims those. Sure in practice the loop will be
>bounded but the reclaimer might perform much more work on behalf of
>other tasks.

A cgroup is a unit and breaking it down into "reclaim fairness" for individual 
tasks like this seems suspect to me. For example, if one task in a cgroup is 
leaking unreclaimable memory like crazy, everyone in that cgroup is going to be 
penalised by allocator throttling as a result, even if they aren't 
"responsible" for that reclaim.

So the options here are as follows when a cgroup is over memory.high and a 
single reclaim isn't enough:

1. Decline further reclaim. Instead, throttle for up to 2 seconds.
2. Keep on reclaiming. Only throttle if we can't get back under memory.high.

The outcome of your suggestion to decline further reclaim is case #1, which is 
significantly more practically "unfair" to that task. Throttling is extremely 
disruptive to tasks and should be a last resort when we've exhausted all other 
practical options. It shouldn't be something you get just because you didn't 
try to reclaim hard enough.