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

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Date:   Wed, 4 Sep 2019 20:02:11 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Bart Van Assche <bvanassche@....org>
Cc:     Daniel Lezcano <daniel.lezcano@...aro.org>,
        Ming Lei <ming.lei@...hat.com>,
        Keith Busch <keith.busch@...el.com>,
        Hannes Reinecke <hare@...e.com>,
        Sagi Grimberg <sagi@...mberg.me>, linux-scsi@...r.kernel.org,
        Long Li <longli@...rosoft.com>,
        John Garry <john.garry@...wei.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-nvme@...ts.infradead.org, Jens Axboe <axboe@...com>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH 1/4] softirq: implement IRQ flood detection mechanism

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 :/