linux-kernel - Re: [PATCH v2 1/2] mm: memcg: refactor page state unit helpers

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Date:   Thu, 5 Oct 2023 11:06:29 +0200
From:   Michal Koutný <mkoutny@...e.com>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     Yosry Ahmed <yosryahmed@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Michal Hocko <mhocko@...nel.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Muchun Song <muchun.song@...ux.dev>, linux-mm@...ck.org,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] mm: memcg: refactor page state unit helpers

On Wed, Oct 04, 2023 at 02:36:19PM -0400, Johannes Weiner <hannes@...xchg.org> wrote:
> Yes, it's because of node resolution which event counters generally
> don't have. Some of the refault events influence node-local reclaim
> decisions, see mm/vmscan.c::snapshot_refaults().
> 
> There are a few other event counters in the stat array that people
> thought would be useful to have split out in
> /sys/devices/system/node/nodeN/vmstat to understand numa behavior
> better.
> 
> It's a bit messy.
> 
> Some events would be useful to move to 'stats' for the numa awareness,
> such as the allocator stats and reclaim activity.
> 
> Some events would be useful to move to 'stats' for the numa awareness,
> but don't have the zone resolution required by them, such as
> kswapd/kcompactd wakeups.

Thanks for the enlightenment.

> Some events aren't numa specific, such as oom kills, drop_pagecache.

These are oddballs indeed. As with the normalization patchset these are
counted as PAGE_SIZE^W 1 error but they should rather be an infinite
error (to warrant a flush).

So my feedback to this series is:
- patch 1/2 -- creating two classes of units is consequence of unclarity
  between state and events (as in event=Δstate/Δt) and resolution
  (global vs per-node), so the better approach would be to tidy this up,
- patch 2/2 -- it could use the single unit class that exists, 
  it'll bound the error of printed numbers afterall (and can be changed
  later depending on how it affects internal consumers).

My 0.02€,
Michal

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