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Date:   Tue, 8 Jan 2019 09:32:14 +1100
From:   Dave Chinner <david@fromorbit.com>
To:     Waiman Long <longman@redhat.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Luis Chamberlain <mcgrof@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Jonathan Corbet <corbet@lwn.net>, linux-kernel@vger.kernel.org,
        linux-doc@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        Davidlohr Bueso <dave@stgolabs.net>,
        Miklos Szeredi <miklos@szeredi.hu>,
        Daniel Colascione <dancol@google.com>,
        Randy Dunlap <rdunlap@infradead.org>
Subject: Re: [PATCH 0/2] /proc/stat: Reduce irqs counting performance overhead
Message-ID: <20190107223214.GZ6311@dastard>
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On Mon, Jan 07, 2019 at 10:12:56AM -0500, Waiman Long wrote:
> As newer systems have more and more IRQs and CPUs available in their
> system, the performance of reading /proc/stat frequently is getting
> worse and worse.

Because the "roll-your-own" per-cpu counter implementaiton has been
optimised for low possible addition overhead on the premise that
summing the counters is rare and isn't a performance issue. This
patchset is a direct indication that this "summing is rare and can
be slow" premise is now invalid.

We have percpu counter infrastructure that trades off a small amount
of addition overhead for zero-cost reading of the counter value.
i.e. why not just convert this whole mess to percpu_counters and
then just use percpu_counter_read_positive()? Then we just don't
care how often userspace reads the /proc file because there is no
summing involved at all...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com