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Date:   Thu, 5 Sep 2019 17:06:19 +0800
From:   Ming Lei <ming.lei@redhat.com>
To:     Daniel Lezcano <daniel.lezcano@linaro.org>
Cc:     Bart Van Assche <bvanassche@acm.org>, 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>
Subject: Re: [PATCH 1/4] softirq: implement IRQ flood detection mechanism
Message-ID: <20190905090617.GB4432@ming.t460p>
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On Wed, Sep 04, 2019 at 07:31:48PM +0200, Daniel Lezcano wrote:
> 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 ?

The syscall of gettimeofday() calls ktime_get_real_ts64() which finally
calls tk_clock_read() which calls rdtsc too.

But gettimeofday() is often used in fast path, and block IO_STAT needs to
read it too.

> 
> > See also irqtime_account_irq() in kernel/sched/cputime.c.
> 
> From my POV, this framework could be interesting to detect this situation.

Now we are talking about IRQ_TIME_ACCOUNTING instead of IRQ_TIMINGS, and the
former one could be used to implement the detection. And the only sharing
should be the read of timestamp.

Thanks,
Ming