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

Bart Van Assche
2019-09-04T19:47:19Z

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?

Does anyone know which CPUs the following text from
Documentation/admin-guide/kernel-parameters.txt refers to?

tsc= [ ... ]
[x86] noirqtime: Do not use TSC to do irq accounting.
Used to run time disable IRQ_TIME_ACCOUNTING on any
platforms where RDTSC is slow and this accounting
can add overhead.

Bart.