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

[Ming Lei]

On Wed, Sep 04, 2019 at 12:47:13PM -0700, Bart Van Assche wrote:
> On 9/4/19 11:02 AM, Peter Zijlstra wrote:
> > On Wed, Sep 04, 2019 at 10:38:59AM -0700, Bart Van Assche wrote:
> > > 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 :/
> 
> Thanks Peter for having looked up these numbers. These numbers are much
> better than last time I checked. Ming, would CONFIG_IRQ_TIME_ACCOUNTING help
> your workload?

In my fio test on azure L80sv2, IRQ_TIME_ACCOUNTING isn't enabled.
However the irq flood detection introduces two RDTSC for each do_IRQ(),
not see obvious IOPS difference.

Thanks,
Ming