On Tue, 2016-06-21 at 23:21 +0200, Peter Zijlstra wrote:
> 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.I suspect the loop is not rare on systems with nohz_idle, where it may be quite a while before a timer tick happens on an idle cpu. I can certainly make sure the division is only done when irq > 2*cputime_one_jiffy. I will do that in the next version. -- All Rights Reversed.
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