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 20:50:58 -0400
From: Johannes Weiner <hannes@...xchg.org>
To: Yosry Ahmed <yosryahmed@...gle.com>
Cc: Shakeel Butt <shakeel.butt@...ux.dev>,
	Andrew Morton <akpm@...ux-foundation.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 04:07:05PM -0700, Yosry Ahmed wrote:
> 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

NR is pretty common to mark the end of an enum range. It would be good
to keep that for enum memcg_stat_item.

The other one is about an array, where we usually use "size" or
"len". How about using one of those instead? I think it should be
sufficiently distinguished then:

- MEMORY_STAT_LEN
- MEMCG_VMSTAT_LEN
- MEMCG_VMSTAT_SIZE