Re: [PATCH 1/4] softirq: implement IRQ flood detection mechanism

Peter Zijlstra
2019-09-04T18:23:42Z

On Wed, Sep 04, 2019 at 10:38:59AM -0700, Bart Van Assche wrote:
> On 9/4/19 10:31 AM, Daniel Lezcano wrote:
> > On 04/09/2019 19:07, Bart Van Assche wrote:
> > > Only if CONFIG_IRQ_TIME_ACCOUNTING has been enabled. However, I don't
> > > know any Linux distro that enables that option. That's probably because
> > > that option introduces two rdtsc() calls in each interrupt. Given the
> > > overhead introduced by this option, I don't think this is the solution
> > > Ming is looking for.
> >
> > Was this overhead reported somewhere ?
> I think it is widely known that rdtsc is a relatively slow x86 instruction.
> So I expect that using that instruction will cause a measurable overhead if
> it is called frequently enough. I'm not aware of any publicly available
> measurement data however.

https://www.agner.org/optimize/instruction_tables.pdf

RDTSC, Ryzen: ~36
RDTSC, Skylake: ~20

Sadly those same tables don't list the cost of actual exceptions or even
IRET :/