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

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Date:   Wed, 28 Aug 2019 07:04:52 +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 Tue, Aug 27, 2019 at 06:19:00PM +0200, Thomas Gleixner wrote:
> On Tue, 27 Aug 2019, Thomas Gleixner wrote:
> > On Tue, 27 Aug 2019, Ming Lei wrote:
> > > +/*
> > > + * Update average irq interval with the Exponential Weighted Moving
> > > + * Average(EWMA)
> > > + */
> > > +static void irq_update_interval(void)
> > > +{
> > > +#define IRQ_INTERVAL_EWMA_WEIGHT	128
> > > +#define IRQ_INTERVAL_EWMA_PREV_FACTOR	127
> > > +#define IRQ_INTERVAL_EWMA_CURR_FACTOR	(IRQ_INTERVAL_EWMA_WEIGHT - \
> > > +		IRQ_INTERVAL_EWMA_PREV_FACTOR)
> > 
> > Please do not stick defines into a function body. That's horrible.
> > 
> > > +
> > > +	int cpu = raw_smp_processor_id();
> > > +	struct irq_interval *inter = per_cpu_ptr(&avg_irq_interval, cpu);
> > > +	u64 delta = sched_clock_cpu(cpu) - inter->last_irq_end;
> > 
> > Why are you doing that raw_smp_processor_id() dance? The call site has
> > interrupts and preemption disabled.
> > 
> > Also how is that supposed to work when sched_clock is jiffies based?
> > 
> > > +	inter->avg = (inter->avg * IRQ_INTERVAL_EWMA_PREV_FACTOR +
> > > +		delta * IRQ_INTERVAL_EWMA_CURR_FACTOR) /
> > > +		IRQ_INTERVAL_EWMA_WEIGHT;
> > 
> > We definitely are not going to have a 64bit multiplication and division on
> > every interrupt. Asided of that this breaks 32bit builds all over the place.
> 
> That said, we already have infrastructure for something like this in the
> core interrupt code which is optimized to be lightweight in the fast path.
> 
> kernel/irq/timings.c

irq/timings.c is much more complicated, especially it applies EWMA to
compute each irq's average interval. However, we only need it for
do_IRQ() to cover all interrupt & softirq handler.

Also CONFIG_IRQ_TIMINGS is usually disabled on distributions.

thanks,
Ming