Re: [patch V2 1/2] genriq: Avoid summation loops for /proc/stat

Andrew Morton
2019-02-08T22:32:59Z

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?