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

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Message-ID: <CAJD7tkZW8OYrEHkuONFt4wGiYSAsyVsA30zroOxc=opi5r5iBQ@mail.gmail.com>
Date: Wed, 14 Aug 2024 17:22:33 -0700
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Nhat Pham <nphamcs@...il.com>
Cc: Shakeel Butt <shakeel.butt@...ux.dev>, Jesper Dangaard Brouer <hawk@...nel.org>, 
	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 5:19 PM Nhat Pham <nphamcs@...il.com> wrote:
>
> On Wed, Aug 14, 2024 at 4:49 PM Yosry Ahmed <yosryahmed@...gle.com> wrote:
> >
> >
> > We can also use such atomic counters in obj_cgroup_may_zswap() and
> > eliminate the rstat flush there as well. Same for zswap_current_read()
> > probably.
>
> zswap/zswapped are subtree-cumulative counters. Would that be a problem?

For obj_cgroup_may_zswap() we iterate the parents anyway, so I think
it should be fine. We will also iterate the nodes on each level, but
this is usually a small number and is probably cheaper than an rstat
flush (but I can easily be wrong).

For zswap_current_read() we need to iterate the children, not the
parents. At this point the rstat flush is probably much better, so we
can leave this one alone. It's a userspace read anyway, so it
shouldn't be causing problems.

>
> >
> > Most in-kernel flushers really only need a few stats, so I am
> > wondering if it's better to incrementally move these ones outside of
> > the rstat framework and completely eliminate in-kernel flushers. For
> > instance, MGLRU does not require the flush that reclaim does as
> > Shakeel pointed out.
> >
> > This will solve so many scalability problems that all of us have
> > observed at some point or another and tried to optimize. I believe
> > using rstat for userspace reads was the original intention anyway.
>
> But yeah, the fewer in-kernel flushers we have, the fewer
> scalability/lock contention issues there will be. Not an expert in
> this area, but sounds like a worthwhile direction to pursue.