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

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Message-ID: <alpine.DEB.2.21.1908281605190.23149@nanos.tec.linutronix.de>
Date:   Wed, 28 Aug 2019 16:07:19 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Ming Lei <ming.lei@...hat.com>
cc:     LKML <linux-kernel@...r.kernel.org>,
        Long Li <longli@...rosoft.com>, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Keith Busch <keith.busch@...el.com>, Jens Axboe <axboe@...com>,
        Christoph Hellwig <hch@....de>,
        Sagi Grimberg <sagi@...mberg.me>,
        John Garry <john.garry@...wei.com>,
        Hannes Reinecke <hare@...e.com>,
        linux-nvme@...ts.infradead.org, linux-scsi@...r.kernel.org,
        Daniel Lezcano <daniel.lezcano@...aro.org>
Subject: Re: [PATCH 1/4] softirq: implement IRQ flood detection mechanism

On Wed, 28 Aug 2019, Ming Lei wrote:
> On Wed, Aug 28, 2019 at 01:23:06PM +0200, Thomas Gleixner wrote:
> > On Wed, 28 Aug 2019, Ming Lei wrote:
> > > On Wed, Aug 28, 2019 at 01:09:44AM +0200, Thomas Gleixner wrote:
> > > > > > Also how is that supposed to work when sched_clock is jiffies based?
> > > > > 
> > > > > Good catch, looks ktime_get_ns() is needed.
> > > > 
> > > > And what is ktime_get_ns() returning when the only available clocksource is
> > > > jiffies?
> > > 
> > > IMO, it isn't one issue. If the only clocksource is jiffies, we needn't to
> > > expect high IO performance. Then it is fine to always handle the irq in
> > > interrupt context or thread context.
> > > 
> > > However, if it can be recognized runtime, irq_flood_detected() can
> > > always return true or false.
> > 
> > Right. The clocksource is determined at runtime. And if there is no high
> > resolution clocksource then that function will return crap.
> 
> This patch still works even though the only clocksource is jiffies.

Works by some definition of works, right?

> > Well, yes. But it's trivial enough to utilize parts of it for your
> > purposes.
> 
> >From the code of kernel/irq/timing.c:
> 
> 1) record_irq_time() only records the start time of one irq, and not
> consider the time taken in interrupt handler, so we can't figure out
> the real interval between two do_IRQ() on one CPU

I said utilize and that means that the infrastructure can be used and
extended. I did not say that it solves your problem, right?

> 2) irq/timing doesn't cover softirq

That's solvable, right?
 
> Daniel, could you take a look and see if irq flood detection can be
> implemented easily by irq/timing.c?

I assume you can take a look as well, right?

Thanks,

	tglx