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Subject: Re: [PATCH 1/4] softirq: implement IRQ flood detection mechanism
To:     Bart Van Assche <bvanassche@acm.org>,
        Ming Lei <ming.lei@redhat.com>
Cc:     Jens Axboe <axboe@fb.com>, Hannes Reinecke <hare@suse.com>,
        Sagi Grimberg <sagi@grimberg.me>, linux-scsi@vger.kernel.org,
        Peter Zijlstra <peterz@infradead.org>,
        Long Li <longli@microsoft.com>,
        John Garry <john.garry@huawei.com>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-nvme@lists.infradead.org,
        Keith Busch <keith.busch@intel.com>,
        Ingo Molnar <mingo@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Christoph Hellwig <hch@lst.de>
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From:   Daniel Lezcano <daniel.lezcano@linaro.org>
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Message-ID: <2a8bd278-5384-d82f-c09b-4fce236d2d95@linaro.org>
Date:   Wed, 4 Sep 2019 19:31:48 +0200
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Sender: linux-kernel-owner@vger.kernel.org
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Hi,

On 04/09/2019 19:07, Bart Van Assche wrote:
> On 9/3/19 12:50 AM, Daniel Lezcano wrote:
>> On 03/09/2019 09:28, Ming Lei wrote:
>>> On Tue, Sep 03, 2019 at 08:40:35AM +0200, Daniel Lezcano wrote:
>>>> It is a scheduler problem then ?
>>>
>>> Scheduler can do nothing if the CPU is taken completely by handling
>>> interrupt & softirq, so seems not a scheduler problem, IMO.
>>
>> Why? If there is a irq pressure on one CPU reducing its capacity, the
>> scheduler will balance the tasks on another CPU, no?
> 
> 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 ?

> See also irqtime_account_irq() in kernel/sched/cputime.c.

From my POV, this framework could be interesting to detect this situation.


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