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

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Date:   Thu, 5 Sep 2019 17:11:04 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Bart Van Assche <bvanassche@....org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        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 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