linux-kernel - Re: [PATCH] proc/stat: Separate out individual irq counts into /proc/stat

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Message-Id: <20180501170203.d2b0a99acf0b5288476a11a7@linux-foundation.org>
Date:   Tue, 1 May 2018 17:02:03 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Alexey Dobriyan <adobriyan@...il.com>
Cc:     David Rientjes <rientjes@...gle.com>,
        "Joel Fernandes (Google)" <joel.opensrc@...il.com>,
        Waiman Long <longman@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        rdunlap@...radead.org
Subject: Re: [PATCH] proc/stat: Separate out individual irq counts into
 /proc/stat_irqs

On Tue, 24 Apr 2018 09:18:59 +0300 Alexey Dobriyan <adobriyan@...il.com> wrote:

> On Mon, Apr 23, 2018 at 10:54:18PM -0700, David Rientjes wrote:
> > On Sat, 21 Apr 2018, Alexey Dobriyan wrote:
> > 
> > > > On Thu, Apr 19, 2018 at 04:23:02PM -0700, Joel Fernandes (Google) wrote:
> > > > > Can we not just remove per-IRQ stats from /proc/stat (since I gather
> > > > > from this discussion it isn't scalable), and just have applications
> > > > > that need per-IRQ stats use /proc/interrupts ?
> > > > 
> > > > If you can prove noone is using them in /proc/stat...
> > > 
> > > And you can't even stick WARN into /proc/stat to find out.
> > > 
> > 
> > FWIW, removing per irq counts from /proc/stat would break some of our
> > scripts.  We could adapt to that, but everybody else would have to as
> > well, so I'm afraid it's not going to be possible.
> 
> Excellent!
> 
> > It would probably be better to extract out the stats that you're actually
> > interested in to a new file.
> 
> This is the worst scenario. Individual IRQ stats are going to live in 2 places.
> And /proc/stat still would be slow.

No, a new /proc/stat2 which omits the `intr' line would be fast(er). 
Although if we're going to do such a thing we might choose to
reorganize and prune /proc/stat2 even further, and actually put some
thought into it - the current one is a bit of a dog's breakfast.

Dumb question(s):

a) if we moved the `intr' line to the very end of /proc/stat, would
   anything break?  Maybe.

b) if an application were then to read stuff from /proc/stat and
   were to stop reading before it read the `intr' line, would
   its read from /proc/stat still be slow?

c) if the answer to b) is "yes" then can we change that?  Only go
   off and prepare the `intr' line if the application is really truly
   reading it in?