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 10:37:56 -0700
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Yosry Ahmed <yosryahmed@...gle.com>
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 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?

/*
 * Please note that NR_MEMCG_STATS represents the number of memcg stats
 * we store in memory while MEMCG_NR_STAT represents the max enum value
 * of the memcg stats.
 */

> > +
> > +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 */);
> 
> Should we use S8_MAX here too?
> 

Yes. Andrew, can you please add the above comment and replacement of
127 with S8_MAX in the patch?

[...]
> >
> > -       pn = container_of(lruvec, struct mem_cgroup_per_node, lruvec);
> > -       x = READ_ONCE(pn->lruvec_stats->state[idx]);
> > +       i = memcg_stats_index(idx);
> > +       if (i >= 0) {
> 
> nit: we could return here if (i < 0) like you did in
> memcg_page_state() and others below, less indentation. Same for
> lruvec_page_state_local().
> 

I have fixed this in the following patch which adds warnings.


Thanks for the reviews.