linux-kernel - Re: [PATCH] mm: memcontrol: don't count limit-setting reclaim as memory pressure

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Message-ID: <20200728194447.GB196042@chrisdown.name>
Date:   Tue, 28 Jul 2020 20:44:47 +0100
From:   Chris Down <chris@...isdown.name>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>, Roman Gushchin <guro@...com>,
        linux-mm@...ck.org, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH] mm: memcontrol: don't count limit-setting reclaim as
 memory pressure

Johannes Weiner writes:
>When an outside process lowers one of the memory limits of a cgroup
>(or uses the force_empty knob in cgroup1), direct reclaim is performed
>in the context of the write(), in order to directly enforce the new
>limit and have it being met by the time the write() returns.
>
>Currently, this reclaim activity is accounted as memory pressure in
>the cgroup that the writer(!) belongs to. This is unexpected. It
>specifically causes problems for senpai
>(https://github.com/facebookincubator/senpai), which is an agent that
>routinely adjusts the memory limits and performs associated reclaim
>work in tens or even hundreds of cgroups running on the host. The
>cgroup that senpai is running in itself will report elevated levels of
>memory pressure, even though it itself is under no memory shortage or
>any sort of distress.
>
>Move the psi annotation from the central cgroup reclaim function to
>callsites in the allocation context, and thereby no longer count any
>limit-setting reclaim as memory pressure. If the newly set limit
>causes the workload inside the cgroup into direct reclaim, that of
>course will continue to count as memory pressure.

Seems totally reasonable, and the patch looks fine too.

>Signed-off-by: Johannes Weiner <hannes@...xchg.org>

Acked-by: Chris Down <chris@...isdown.name>