linux-kernel - Re: [PATCH 1/4] mm: rearrange node_stat

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Message-ID: <20240423135844.GA21141@cmpxchg.org>
Date: Tue, 23 Apr 2024 09:58:44 -0400
From: Johannes Weiner <hannes@...xchg.org>
To: Shakeel Butt <shakeel.butt@...ux.dev>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Michal Hocko <mhocko@...nel.org>,
	Roman Gushchin <roman.gushchin@...ux.dev>,
	Muchun Song <muchun.song@...ux.dev>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4] mm: rearrange node_stat_item to put memcg stats at
 start

On Mon, Apr 22, 2024 at 10:18:23PM -0700, Shakeel Butt wrote:
> At the moment the memcg stats are sized based on the size of enum
> node_stat_item but not all fields in node_stat_item corresponds to memcg
> stats. So, rearrage the contents of node_stat_item such that all the
> memcg specific stats are at the top and then the later patches will make
> sure that the memcg code will not waste space for non-memcg stats.
> 
> Signed-off-by: Shakeel Butt <shakeel.butt@...ux.dev>

This series is a great idea and the savings speak for themselves.

But rearranging and splitting vmstats along the memcg-nomemcg line
seems like an undue burden on the non-memcg codebase and interface.

- It messes with user-visible /proc/vmstat ordering, and sets things
  up to do so on an ongoing basis as stats are added to memcg.

- It also separates related stats (like the workingset ones) in
  /proc/vmstat when memcg only accounts a subset.

Would it make more sense to have a translation table inside memcg?
Like we have with memcg1_events.