Re: [PATCH] mm: memcg: use ratelimited stats flush in obj_cgroup_may_zswap()

[Shakeel Butt <shakeel.butt@linux.dev>]

On Mon, Aug 17, 2026 at 09:18:43PM +0800, Song Hu wrote:
> obj_cgroup_may_zswap() runs on every folio swapped out through
> zswap.  For each ancestor with a non-max zswap.max, it flushes the
> cgroup rstat hierarchy synchronously with force=true, which skips
> the ratelimit inside __mem_cgroup_flush_stats().  In a swap storm
> with zswap.max configured, a container takes the global rstat lock
> on every swapped-out folio.

Any reason you are limiting zswap through zswap.max?

> 
> zswap_shrinker_count() had the same pattern and switched to
> mem_cgroup_flush_stats_ratelimited() in commit ea80da363a1f
> ("mm/zswap: use ratelimited stats flush in zswap_shrinker_count()"),
> where the same flush on the shrinker side showed up at 2.88% of
> kernel cycles under osq_lock on a 96-core machine.
> 
> Measured on a KVM guest with a swap storm under a cgroup with
> zswap.max set: obj_cgroup_may_zswap() was entered 198,977 times
> before the patch and 198,968 times after, while
> __mem_cgroup_flush_stats() was entered 281,017 times before and
> 80,445 times after.  The removed 200,572 flushes match the store
> attempt count almost exactly; the remainder comes from other stats
> readers in the swap path.

This is a known issue. Using ratelimited interface also comes with a drawback
that the kernel may react on stale information and the consequences might be
unneeded oom-kills.

There was orthogonal discussion on moving zswap limit enforcement away from
rstat. Yosry, any updates on that?

> 
> The stats can now be up to one flusher cycle stale, so zswap.max
> admission can overshoot for one cycle in a storm; the overshoot is
> corrected as soon as the next flush lands and later stores see it,
> the same tradeoff the shrinker side made.
> 
> Fixes: f4840ccfca25 ("zswap: memcg accounting")
> Signed-off-by: Song Hu <husong@kylinos.cn>
> ---
>  mm/memcontrol.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 17da1f43b7d3..7a8f689055c6 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -6000,8 +6000,7 @@ bool obj_cgroup_may_zswap(struct obj_cgroup *objcg)
>  			break;
>  		}
>  
> -		/* Force flush to get accurate stats for charging */
> -		__mem_cgroup_flush_stats(memcg, true);
> +		mem_cgroup_flush_stats_ratelimited(memcg);
>  		pages = memcg_page_state(memcg, MEMCG_ZSWAP_B) / PAGE_SIZE;
>  		if (pages < max)
>  			continue;
> -- 
> 2.43.0
>