Re: [PATCH] proc/stat: Separate out individual irq counts into /proc/stat_irqs

[Alexey Dobriyan]

> Therefore, application performance can be impacted if the application
> reads /proc/stat rather frequently.

[nods]
Text interfaces can be designed in a very stupid way.

> For example, reading /proc/stat in a certain 2-socket Skylake server
> took about 4.6ms because it had over 5k irqs.

Is this top(1)? What is this application doing?
If it needs percpu usage stats, then maybe /proc/stat should be
converted away from single_open() so that core seq_file code doesn't
generate everything at once.

> -
> -	/* sum again ? it could be updated? */
> -	for_each_irq_nr(j)
> -		seq_put_decimal_ull(p, " ", kstat_irqs_usr(j));
> -

This is direct userspace breakage.

Proper fix is to start strategic switch away from /proc.
It is a fun toy but its time has come.