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

[Johannes Weiner <hannes@cmpxchg.org>]

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@linux.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@linux.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@linux.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