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From:   Shakeel Butt <shakeelb@google.com>
Date:   Mon, 4 Oct 2021 10:25:12 -0700
Message-ID: <CALvZod5OKz=7pFpxCt1CONPyJO4wR5t+PH0nzdbFBT1SYpjrsg@mail.gmail.com>
Subject: Re: [PATCH] cgroup: rstat: optimize flush through speculative test
To:     Tejun Heo <tj@kernel.org>
Cc:     Johannes Weiner <hannes@cmpxchg.org>,
        =?UTF-8?Q?Michal_Koutn=C3=BD?= <mkoutny@suse.com>,
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On Mon, Oct 4, 2021 at 10:00 AM Tejun Heo <tj@kernel.org> wrote:
>
> Hello, Shakeel.
>
> On Wed, Sep 29, 2021 at 04:59:36PM -0700, Shakeel Butt wrote:
> > Currently cgroup_rstat_updated() has a speculative already-on-list test
> > to check if the given cgroup is already part of the rstat update tree.
> > This helps in reducing the contention on the rstat cpu lock. This patch
> > adds the similar speculative not-on-list test on the rstat flush
> > codepath.
> >
> > Recently the commit aa48e47e3906 ("memcg: infrastructure to flush memcg
> > stats") added periodic rstat flush. On a large system which is not much
> > busy, most of the per-cpu rstat tree would be empty. So, the speculative
> > not-on-list test helps in eliminating unnecessary work and potentially
> > reducing contention on the rstat cpu lock. Please note this might
> > introduce temporary inaccuracy but with the frequent and periodic flush
> > this would not be an issue.
> >
> > To evaluate the impact of this patch, an 8 GiB tmpfs file is created on
> > a system with swap-on-zram and the file was pushed to swap through
> > memory.force_empty interface. On reading the whole file, the memcg stat
> > flush in the refault code path is triggered. With this patch, we
> > observed 38% reduction in the read time of 8 GiB file.
>
> The patch looks fine to me but that's a lot of reduction in read time. Can
> you elaborate a bit on why this makes such a huge difference? Who's hitting
> on that lock so hard?
>

It was actually due to machine size. I ran a single threaded workload
without any interference on a 112 cpus machine. So, most of the time
the flush was acquiring and releasing the per-cpu rstat lock for empty
trees.

Shakeel