linux-kernel - Re: [PATCH] cgroup: rstat: optimize flush through speculative test

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Date:   Mon, 4 Oct 2021 07:44:24 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Michal Koutný <mkoutny@...e.com>,
        Cgroups <cgroups@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] cgroup: rstat: optimize flush through speculative test

On Mon, Oct 04, 2021 at 10:25:12AM -0700, Shakeel Butt wrote:
> > > 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.

Sorry for being so slow but can you point to the exact call path which gets
slowed down so significantly? I'm mostly wondering whether we need some sort
of time-batched flushes because even with lock avoidance the flush path
really isn't great when called frequently. We can mitigate it further if
necessary - e.g. by adding an "updated" bitmap so that the flusher doesn't
have to go around touching the cachelines for all the cpus.

Thanks.

-- 
tejun