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

[Michal Koutný]

On Wed, Oct 04, 2023 at 02:36:19PM -0400, Johannes Weiner <hannes@xxxxxxxxxxx> 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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