Re: [PATCH 1/5] sched,time: count actually elapsed irq & softirq time

[Peter Zijlstra]

On Thu, Jun 16, 2016 at 12:06:03PM -0400, r...@redhat.com wrote:
> +static unsigned long irqtime_account_hi_update(unsigned long max_jiffies)
>  {
>       u64 *cpustat = kcpustat_this_cpu->cpustat;
> +     unsigned long irq_jiffies;
>       unsigned long flags;
> +     u64 irq;
>  
>       local_irq_save(flags);
> +     irq = this_cpu_read(cpu_hardirq_time) - cpustat[CPUTIME_IRQ];
> +     irq_jiffies = min(cputime_to_jiffies(irq), max_jiffies);

cputime_to_jiffies is a division, could we not avoid that by doing
something like:

        irq_jiffies = min(irq, jiffies_to_cputime(max_jiffies));
        while (irq_jiffies > cputime_one_jiffy) {
                irq_jiffies -= cputime_one_jiffy;
                cpustat[CPUTIME_IRQ] += cputime_one_jiffy;
        }

assuming that the loop is 'rare' etc.. If not, only do the division on
that same > cputime_one_jiffy condition.

> +     if (irq_jiffies)
> +             cpustat[CPUTIME_IRQ] += jiffies_to_cputime(irq_jiffies);
>       local_irq_restore(flags);
> +     return irq_jiffies;
>  }