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

[Joshua Hahn <joshua.hahnjy@gmail.com>]

On Thu, 20 Aug 2026 15:18:39 -0700 Joanne Koong <joannelkoong@gmail.com> wrote:

> On Tue, Aug 18, 2026 at 11:35 AM Yosry Ahmed <yosry@kernel.org> wrote:
> >
> > On Mon, Aug 17, 2026 at 6:53 PM Song Hu <husong@kylinos.cn> 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.
> >
> 
> For this zswap stat, I ran some benchmarks comparing 4 approaches
> (switchable behind a runtime knob [1]):
> a) rstat + forced flush       (baseline aka what the tree does today)
> b) rstat + ratelimited         (Song's proposal)
> c) hierarchical per-CPU   (Yosry's idea from [2])
> d) page counters              (following what all the other memcg limits do)

Hi Joanne,

I hope you are doing well! Thank you for running all these experiments.
I'm not biased in any direction but I was curious what you did for the
page counter implementation. The reason I am asking is because I am
working on making page counters more scalable / performant by pushing
the memcg stock down to the page counter level [3]. I wonder if the
caching that stock provides can give (d) an upper hand over (c).

This is just an idea though, and I think this would depend on how your
(d) implementation looks like.

Sorry that I joined in this conversation late : -(

Thanks again, Joanne!
Joshua

[3] https://lore.kernel.org/all/20260623180124.868655-1-joshua.hahnjy@gmail.com/
(It's been a while since v4, I got a little sidetracked working on the
tiered memcg series. I have a v5 ready to go, just waiting on some
running some tests before sending it out.)