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Subject: Re: [PATCH 0/2] /proc/stat: Reduce irqs counting performance overhead
To:     Dave Chinner <david@fromorbit.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>
References: <1546873978-27797-1-git-send-email-longman@redhat.com>
 <20190107223214.GZ6311@dastard>
From:   Waiman Long <longman@redhat.com>
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Message-ID: <9b4208b7-f97b-047c-4dab-15bd3791e7de@redhat.com>
Date:   Mon, 7 Jan 2019 17:41:39 -0500
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On 01/07/2019 05:32 PM, Dave Chinner wrote:
> 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.

Yes, percpu_counter_read_positive() is cheap. However, you still need to
pay the price somewhere. In the case of percpu_counter, the update is
more expensive.

I would say the percentage of applications that will hit this problem is
small. But for them, this problem has some significant performance overhead.

Cheers,
Longman