netdev - Re: [PATCH] mm/memcontrol: Add the drop

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Message-ID: <20200921104257.GA632859@chrisdown.name>
Date:   Mon, 21 Sep 2020 11:42:57 +0100
From:   Chris Down <chris@...isdown.name>
To:     Michal Hocko <mhocko@...e.com>
Cc:     zangchunxin@...edance.com, hannes@...xchg.org,
        vdavydov.dev@...il.com, akpm@...ux-foundation.org, tj@...nel.org,
        lizefan@...wei.com, corbet@....net, ast@...nel.org,
        daniel@...earbox.net, kafai@...com, songliubraving@...com,
        yhs@...com, andriin@...com, john.fastabend@...il.com,
        kpsingh@...omium.org, cgroups@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        bpf@...r.kernel.org
Subject: Re: [PATCH] mm/memcontrol: Add the drop_cache interface for cgroup v2

Michal Hocko writes:
>On Mon 21-09-20 16:02:55, zangchunxin@...edance.com wrote:
>> From: Chunxin Zang <zangchunxin@...edance.com>
>>
>> In the cgroup v1, we have 'force_mepty' interface. This is very
>> useful for userspace to actively release memory. But the cgroup
>> v2 does not.
>>
>> This patch reuse cgroup v1's function, but have a new name for
>> the interface. Because I think 'drop_cache' may be is easier to
>> understand :)
>
>This should really explain a usecase. Global drop_caches is a terrible
>interface and it has caused many problems in the past. People have
>learned to use it as a remedy to any problem they might see and cause
>other problems without realizing that. This is the reason why we even
>log each attempt to drop caches.
>
>I would rather not repeat the same mistake on the memcg level unless
>there is a very strong reason for it.

I agree with Michal. We already have ways to do best-effort memory release on 
cgroup v2, primarily with memory.high. Singling out a category of memory for 
reclaim has historically proved to be a fool's errand.