linux-kernel - Re: [PATCH v2 0/2] mm: memcg: fix tracking of pending stats updates values

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Message-ID: <CAJD7tkZYd1u1s_s8hK4LHktWfPOFjPYdqhyXmsLEdY6KAeYuXA@mail.gmail.com>
Date:   Tue, 3 Oct 2023 01:49:19 -0700
From:   Yosry Ahmed <yosryahmed@...gle.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Muchun Song <muchun.song@...ux.dev>,
        Michal Koutný <mkoutny@...e.com>,
        linux-mm@...ck.org, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 0/2] mm: memcg: fix tracking of pending stats updates values

On Tue, Oct 3, 2023 at 1:09 AM Michal Hocko <mhocko@...e.com> wrote:
>
> On Tue 03-10-23 01:03:53, Yosry Ahmed wrote:
> > On Tue, Oct 3, 2023 at 12:57 AM Michal Hocko <mhocko@...e.com> wrote:
> > >
> > > On Mon 25-09-23 10:11:05, Yosry Ahmed wrote:
> > > > On Mon, Sep 25, 2023 at 6:50 AM Michal Hocko <mhocko@...e.com> wrote:
> > > > >
> > > > > On Fri 22-09-23 17:57:38, Yosry Ahmed wrote:
> > > > > > While working on adjacent code [1], I realized that the values passed
> > > > > > into memcg_rstat_updated() to keep track of the magnitude of pending
> > > > > > updates is consistent. It is mostly in pages, but sometimes it can be in
> > > > > > bytes or KBs. Fix that.
> > > > >
> > > > > What kind of practical difference does this change make? Is it worth
> > > > > additional code?
> > > >
> > > > As explained in patch 2's commit message, the value passed into
> > > > memcg_rstat_updated() is used for the "flush only if not worth it"
> > > > heuristic. As we have discussed in different threads in the past few
> > > > weeks, unnecessary flushes can cause increased global lock contention
> > > > and/or latency.
> > > >
> > > > Byte-sized paths (percpu, slab, zswap, ..) feed bytes into the
> > > > heuristic, but those are interpreted as pages, which means we will
> > > > flush earlier than we should. This was noticed by code inspection. How
> > > > much does this matter in practice? I would say it depends on the
> > > > workload: how many percpu/slab allocations are being made vs. how many
> > > > flushes are requested.
> > > >
> > > > On a system with 100 cpus, 25M of stat updates are needed for a flush
> > > > usually, but ~6K of slab/percpu updates will also (mistakenly) cause a
> > > > flush.
> > >
> > > This surely depends on workload and that is understandable. But it would
> > > be really nice to provide some numbers for typical workloads which
> > > exercise slab heavily.
> >
> > If you have a workload in mind I can run it and see how many flushes
> > we get with/without this patch. The first thing that pops into my head
> > is creating a bunch of empty files but I don't know if that's the best
> > thing to get numbers from.
>
> Let me remind you that you are proposing a performance optimization and
> such a change requires some numbers to actually show it is benefitial.
> There are cases where the resulting code is clearly an improvement and
> the performance benefit is just a nice side effect. I do not consider
> this to be the case. The whole thing is quite convoluted even without
> a better precision you are proposing. And let me be clear, I am not
> opposing your patch but I would rather see it based on more than just
> hand waving.

It is purely based on code inspection, and honestly I don't have
numbers to support it. I saw something wrong with the code and I tried
to fix it, I was working on something else when I noticed it. That
being said, I acknowledge it's not making the code any prettier :)

Feel free to suggest improvements to the code to make it more
bearable, otherwise if you don't like it I will just leave it to be
honest.

Thanks for taking a look!

> --
> Michal Hocko
> SUSE Labs