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

From: Peter Zijlstra <peterz@infradead.org>
To: riel@redhat.com
Cc: linux-kernel@vger.kernel.org, mingo@kernel.org,
	pbonzini@redhat.com, fweisbec@gmail.com, wanpeng.li@hotmail.com,
	efault@gmx.de, tglx@linutronix.de, rkrcmar@redhat.com
Subject: Re: [PATCH 1/5] sched,time: count actually elapsed irq & softirq time
Date: Tue, 21 Jun 2016 23:21:42 +0200	[thread overview]
Message-ID: <20160621212142.GS30909@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <1466093167-27653-2-git-send-email-riel@redhat.com>

On Thu, Jun 16, 2016 at 12:06:03PM -0400, riel@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;
>  }