linux-kernel - Re: [PATCH v3 4/8] memcg: reduce memory for the lruvec and memcg stats

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Date: Tue, 30 Apr 2024 16:07:05 -0700
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Shakeel Butt <shakeel.butt@...ux.dev>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Johannes Weiner <hannes@...xchg.org>, 
	Michal Hocko <mhocko@...nel.org>, Roman Gushchin <roman.gushchin@...ux.dev>, 
	Muchun Song <muchun.song@...ux.dev>, "T . J . Mercier" <tjmercier@...gle.com>, kernel-team@...a.com, 
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 4/8] memcg: reduce memory for the lruvec and memcg stats

On Tue, Apr 30, 2024 at 4:00 PM Shakeel Butt <shakeel.butt@...ux.dev> wrote:
>
> On Tue, Apr 30, 2024 at 10:41:01AM -0700, Yosry Ahmed wrote:
> > On Tue, Apr 30, 2024 at 10:38 AM Shakeel Butt <shakeel.butt@...ux.dev> wrote:
> > >
> > > On Tue, Apr 30, 2024 at 01:41:38AM -0700, Yosry Ahmed wrote:
> > > > On Mon, Apr 29, 2024 at 11:06 PM Shakeel Butt <shakeel.butt@...ux.dev> wrote:
> > > > >
> > > [...]
> > > > > +
> > > > > +#define NR_MEMCG_NODE_STAT_ITEMS ARRAY_SIZE(memcg_node_stat_items)
> > > > > +#define NR_MEMCG_STATS (NR_MEMCG_NODE_STAT_ITEMS + ARRAY_SIZE(memcg_stat_items))
> > > > > +static int8_t mem_cgroup_stats_index[MEMCG_NR_STAT] __read_mostly;
> > > >
> > > > NR_MEMCG_STATS and MEMCG_NR_STAT are awfully close and have different
> > > > meanings. I think we should come up with better names (sorry nothing
> > > > comes to mind) or add a comment to make the difference more obvious.
> > > >
> > >
> > > How about the following comment?
> >
> > The comment LGTM. I prefer renaming them though if someone can come up
> > with better names.
> >
>
> I will be posting v4 and will change the name (still thinking about the
> name) becasuse:
>
> > > > > +static void init_memcg_stats(void)
> > > > > +{
> > > > > +       int8_t i, j = 0;
> > > > > +
> > > > > +       /* Switch to short once this failure occurs. */
> > > > > +       BUILD_BUG_ON(NR_MEMCG_STATS >= 127 /* INT8_MAX */);
>
> The above should be MEMCG_NR_STAT instead of NR_MEMCG_STATS.

Yeah it's pretty confusing :)

How about something explicit like:

NR_MEMCG_POSSIBLE_STAT_ITEMS / MEMCG_MAX_STAT_ITEM
NR_MEMCG_ACTUAL_STAT_ITEMS / MEMCG_ACTUAL_NR_STAT

They look ugly, but I can't think of anything better. Maybe they will
inspire something better :)