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

On Mon, Aug 17, 2026 at 6:53 PM Song Hu <[email protected]> wrote:
>
>
>
> 在 2026/8/18 00:04, Shakeel Butt 写道:
> > 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?
> >
>
> Mostly fairness on a shared pool: zswap.max_pool_percent is global
> only, so on a multi-tenant host one cgroup's cold anonymous memory
> can soak the pool and crowd out the others.  zswap.max is the only
> per-cgroup control over that share; memory.max bounds the total
> footprint, not the share of the pool.
>
> >>
> >> 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?

I am not actively looking into that, but Joanne was looking into
AFAICT. I will respond to the thread there and CC Song as well.

> >
>
> Fair point.  The direction I had considered is the benign one -
> stale low after a burst of stores overshoots admission for a cycle.
> But after a burst of writebacks the folded value can be stale high
> too, and premature rejection with zswap.writeback off or swap.max
> tight ends in a memcg OOM kill that accurate stats would have
> avoided.  may_zswap() is admission control rather than a hint, so
> the guarantee matters more here than for the shrinker side
> of ea80da363a1f.
>
> If moving the limit enforcement off rstat is happening, that
> supersedes this patch and I am happy to drop it.  Otherwise it can
> serve as a stopgap for the per-folio lock cost - your call, and
> curious where Yosry's discussion stands.

Not sure how much progress is being made there, but I would really
like to stop the proliferation of ratelimited flushing, especially in
this case where it can cause OOM kills.