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Date:   Wed, 4 Sep 2019 20:02:11 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Bart Van Assche <bvanassche@acm.org>
Cc:     Daniel Lezcano <daniel.lezcano@linaro.org>,
        Ming Lei <ming.lei@redhat.com>,
        Keith Busch <keith.busch@intel.com>,
        Hannes Reinecke <hare@suse.com>,
        Sagi Grimberg <sagi@grimberg.me>, linux-scsi@vger.kernel.org,
        Long Li <longli@microsoft.com>,
        John Garry <john.garry@huawei.com>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-nvme@lists.infradead.org, Jens Axboe <axboe@fb.com>,
        Ingo Molnar <mingo@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH 1/4] softirq: implement IRQ flood detection mechanism
Message-ID: <20190904180211.GX2332@hirez.programming.kicks-ass.net>
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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 :/