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

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Message-ID: <9b924e48-e217-9c11-c1fb-46c92a82ea2d@acm.org>
Date:   Wed, 4 Sep 2019 12:47:13 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     Peter Zijlstra <peterz@...radead.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 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.