linux-kernel - Re: [PATCH] mm: memcg: add swapcache stat for memcg v2

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Date:   Wed, 6 Jan 2021 15:53:49 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Johannes Weiner <hannes@...xchg.org>, Roman Gushchin <guro@...com>,
        Yang Shi <shy828301@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: memcg: add swapcache stat for memcg v2

On Thu 31-12-20 18:39:55, Shakeel Butt wrote:
> This patch adds swapcache stat for the cgroup v2. The swapcache
> represents the memory that is accounted against both the memory and the
> swap limit of the cgroup. The main motivation behind exposing the
> swapcache stat is for enabling users to gracefully migrate from cgroup
> v1's memsw counter to cgroup v2's memory and swap counters.
> 
> Cgroup v1's memsw limit allows users to limit the memory+swap usage of a
> workload but without control on the exact proportion of memory and swap.
> Cgroup v2 provides separate limits for memory and swap which enables
> more control on the exact usage of memory and swap individually for the
> workload.
> 
> With some little subtleties, the v1's memsw limit can be switched with
> the sum of the v2's memory and swap limits. However the alternative for
> memsw usage is not yet available in cgroup v2. Exposing per-cgroup
> swapcache stat enables that alternative. Adding the memory usage and
> swap usage and subtracting the swapcache will approximate the memsw
> usage. This will help in the transparent migration of the workloads
> depending on memsw usage and limit to v2' memory and swap counters.

Could you expand a bit more on why memsw usage is important even in
cgroup v2 land? How are you going to use the approximated value?

I am not really objecting to adding this counter. We do export it for
the global case and having a memcg view sounds useful for analysis.
-- 
Michal Hocko
SUSE Labs