linux-kernel - Re: [PATCH v2] memcg: use ratelimited stats flush in the reclaim

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Message-ID: <CAKEwX=Mc9U_eEqoEYtwdfOUZTa=gboLtbF5FGy4pL--A54JJDw@mail.gmail.com>
Date: Wed, 14 Aug 2024 16:03:13 -0700
From: Nhat Pham <nphamcs@...il.com>
To: Shakeel Butt <shakeel.butt@...ux.dev>
Cc: Jesper Dangaard Brouer <hawk@...nel.org>, Yosry Ahmed <yosryahmed@...gle.com>, 
	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>, Yu Zhao <yuzhao@...gle.com>, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, Meta kernel team <kernel-team@...a.com>, 
	cgroups@...r.kernel.org
Subject: Re: [PATCH v2] memcg: use ratelimited stats flush in the reclaim

On Wed, Aug 14, 2024 at 9:32 AM Shakeel Butt <shakeel.butt@...ux.dev> wrote:
>
>
> Ccing Nhat
>
> On Wed, Aug 14, 2024 at 02:57:38PM GMT, Jesper Dangaard Brouer wrote:
> > I suspect the next whac-a-mole will be the rstat flush for the slab code
> > that kswapd also activates via shrink_slab, that via
> > shrinker->count_objects() invoke count_shadow_nodes().
> >
>
> Actually count_shadow_nodes() is already using ratelimited version.
> However zswap_shrinker_count() is still using the sync version. Nhat is
> modifying this code at the moment and we can ask if we really need most
> accurate values for MEMCG_ZSWAP_B and MEMCG_ZSWAPPED for the zswap
> writeback heuristic.

You are referring to this, correct:

mem_cgroup_flush_stats(memcg);
nr_backing = memcg_page_state(memcg, MEMCG_ZSWAP_B) >> PAGE_SHIFT;
nr_stored = memcg_page_state(memcg, MEMCG_ZSWAPPED);

It's already a bit less-than-accurate - as you pointed out in another
discussion, it takes into account the objects and sizes of the entire
subtree, rather than just the ones charged to the current (memcg,
node) combo. Feel free to optimize this away!

In fact, I should probably replace this with another (atomic?) counter
in zswap_lruvec_state struct, which tracks the post-compression size.
That way, we'll have a better estimate of the compression factor -
total post-compression size /  (length of LRU * page size), and
perhaps avoid the whole stat flushing path altogether...

>
> > --Jesper