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Date:   Thu, 19 Apr 2018 12:43:19 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Waiman Long <longman@redhat.com>
Cc:     Kate Stewart <kstewart@linuxfoundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        linux-kernel@vger.kernel.org, Alexey Dobriyan <adobriyan@gmail.com>
Subject: Re: [PATCH] proc/stat: Separate out individual irq counts into
 /proc/stat_irqs
Message-Id: <20180419124319.8e329eb25234c045bf161cd5@linux-foundation.org>
In-Reply-To: <1524157769-30775-1-git-send-email-longman@redhat.com>
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On Thu, 19 Apr 2018 13:09:29 -0400 Waiman Long <longman@redhat.com> wrote:

> It was found that reading /proc/stat could be time consuming on
> systems with a lot of irqs. For example, reading /proc/stat in a
> certain 2-socket Skylake server took about 4.6ms because it had over
> 5k irqs. In that particular case, the majority of the CPU cycles for
> reading /proc/stat was spent in the kstat_irqs() function.  Therefore,
> application performance can be impacted if the application reads
> /proc/stat rather frequently.
> 
> The "intr" line within /proc/stat contains a sum total of all the irqs
> that have happened followed by a list of irq counts for each individual
> irq number. In many cases, the first number is good enough. The
> individual irq counts may not provide that much more information.
> 
> In order to avoid this kind of performance issue, all these individual
> irq counts are now separated into a new /proc/stat_irqs file. The
> sum total irq count will stay in /proc/stat and be duplicated in
> /proc/stat_irqs. Applications that need to look up individual irq counts
> will now have to look into /proc/stat_irqs instead of /proc/stat.
> 

(cc /proc maintainer)

It's a non-backward-compatible change.  For something which has
existing for so long, it would be a mighty task to demonstrate that no
existing userspace will be disrupted by this change.

So we need to think again.  A new interface which omits the per-IRQ
stats might be acceptable.

Or, conceivably, a new /proc knob which disables the per-IRQ stats in
/proc/stat.  That would allow operators to opt in to this disabling and
would avoid the need to alter
whatever-application-it-is-that-is-having-trouble.  This seems a bit ugly
though.

Also, the changelog is rather vague.  "application performance can be
impacted".  Well, *are* applications impacted?  What is the real-world
performance gain which this change provides, in a real-world workload?