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

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Date:	Tue, 21 Jun 2016 18:20:23 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	pbonzini@...hat.com, fweisbec@...il.com, wanpeng.li@...mail.com,
	efault@....de, tglx@...utronix.de, rkrcmar@...hat.com
Subject: Re: [PATCH 1/5] sched,time: count actually elapsed irq & softirq
 time

On Tue, 2016-06-21 at 23:21 +0200, Peter Zijlstra wrote:
> On Thu, Jun 16, 2016 at 12:06:03PM -0400, riel@...hat.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.

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