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Date:   Fri, 8 Feb 2019 14:32:55 -0800
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Waiman Long <longman@redhat.com>,
        Matthew Wilcox <willy@infradead.org>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Kees Cook <keescook@chromium.org>,
        linux-fsdevel@vger.kernel.org, Davidlohr Bueso <dave@stgolabs.net>,
        Miklos Szeredi <miklos@szeredi.hu>,
        Daniel Colascione <dancol@google.com>,
        Dave Chinner <david@fromorbit.com>,
        Randy Dunlap <rdunlap@infradead.org>,
        Marc Zyngier <marc.zyngier@arm.com>
Subject: Re: [patch V2 1/2] genriq: Avoid summation loops for /proc/stat
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On Fri, 08 Feb 2019 14:48:03 +0100 Thomas Gleixner <tglx@linutronix.de> wrote:

> Waiman reported that on large systems with a large amount of interrupts the
> readout of /proc/stat takes a long time to sum up the interrupt
> statistics. In principle this is not a problem. but for unknown reasons
> some enterprise quality software reads /proc/stat with a high frequency.
> 
> The reason for this is that interrupt statistics are accounted per cpu. So
> the /proc/stat logic has to sum up the interrupt stats for each interrupt.
> 
> This can be largely avoided for interrupts which are not marked as
> 'PER_CPU' interrupts by simply adding a per interrupt summation counter
> which is incremented along with the per interrupt per cpu counter.
> 
> The PER_CPU interrupts need to avoid that and use only per cpu accounting
> because they share the interrupt number and the interrupt descriptor and
> concurrent updates would conflict or require unwanted synchronization.
> 
> ...
>
> --- a/include/linux/irqdesc.h
> +++ b/include/linux/irqdesc.h
> @@ -65,6 +65,7 @@ struct irq_desc {
>  	unsigned int		core_internal_state__do_not_mess_with_it;
>  	unsigned int		depth;		/* nested irq disables */
>  	unsigned int		wake_depth;	/* nested wake enables */
> +	unsigned int		tot_count;

Confused.  Isn't this going to quickly overflow?