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

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Date:   Wed, 28 Aug 2019 21:50:55 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>
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, 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.

>  
> > > No. Talk to Daniel. There is not going to be yet another ad hoc thingy just
> > > to make block happy.
> > 
> > I just take a first glance at the code of 'interrupt timing', and its
> > motivation is to predicate of the next occurrence of interested interrupt
> > for use cases of PM, such as predicate wakeup time.
> > 
> > And predication could be one much more difficult thing, and its implementation
> > requires to record more info: such as irq number, recent multiple irq timestamps,
> > that means its overhead is a bit high. Meantime IRQS_TIMINGS should only be set
> > on interested interrupt(s).
> 
> 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

2) irq/timing doesn't cover softirq

Daniel, could you take a look and see if irq flood detection can be
implemented easily by irq/timing.c?


thanks,
Ming