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

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Message-ID: <20240423183041.GA318022@cmpxchg.org>
Date: Tue, 23 Apr 2024 14:30:41 -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 Tue, Apr 23, 2024 at 10:44:07AM -0700, Shakeel Butt wrote:
> On Tue, Apr 23, 2024 at 09:58:44AM -0400, Johannes Weiner wrote:
> > 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.
> 
> Thanks for taking a look. I will look into the translation table
> approach. The reason I went with this approach was that I am in parallel
> looking into rearranging fields of important MM structs and also enums
> to improve cache locality. For example, the field NR_SWAPCACHE is always
> accessed together with NR_FILE_PAGES, so it makes sense to have them on
> same cacheline. So, is the rearrangement of vmstats a big NO or a little
> bit here and there is fine unlike what I did with this patch?

I'm curious what other folks think.

The cache optimization is a stronger argument, IMO, because it
directly benefits the users of /proc/vmstat. And it would be fairly
self contained inside the node_stat_item enum - "ordered for cache".

I was more hesitant about imposing a memcg requirement on the generic
vmstat ordering.