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

Thomas Gleixner
2019-08-27T14:42:41Z

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.

Thanks,

tglx