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Date:   Thu, 21 May 2020 14:05:30 +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
Message-ID: <20200521130530.GE990580@chrisdown.name>
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Chris Down writes:
>>I believe I have asked in other email in this thread. Could you explain
>>why enforcint the requested target (memcg_nr_pages_over_high) is
>>insufficient for the problem you are dealing with? Because that would
>>make sense for large targets to me while it would keep relatively
>>reasonable semantic of the throttling - aka proportional to the memory
>>demand rather than the excess.
>
>memcg_nr_pages_over_high is related to the charge size. As such, if 
>you're way over memory.high as a result of transient reclaim failures, 
>but the majority of your charges are small, it's going to hard to make 
>meaningful progress:
>
>1. Most nr_pages will be MEMCG_CHARGE_BATCH, which is not enough to help;
>2. Large allocations will only get a single reclaim attempt to succeed.
>
>As such, in many cases we're either doomed to successfully reclaim a 
>paltry amount of pages, or fail to reclaim a lot of pages. Asking 
>try_to_free_pages() to deal with those huge allocations is generally 
>not reasonable, regardless of the specifics of why it doesn't work in 
>this case.

Oh, I somehow elided the "enforcing" part of your proposal. Still, there's no 
guarantee even if large allocations are reclaimed fully that we will end up 
going back below memory.high, because even a single other large allocation 
which fails to reclaim can knock us out of whack again.