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

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Message-ID: <20160621212142.GS30909@twins.programming.kicks-ass.net>
Date:	Tue, 21 Jun 2016 23:21:42 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	riel@...hat.com
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 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.

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