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

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Date:	Wed, 22 Jun 2016 12:40:31 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>, riel@...hat.com
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org, 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 21/06/2016 23:21, Peter Zijlstra wrote:
> 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.

It's a division by a constant, it ought to become a multiplication.  For
64-bit it will, and context tracking is only enabled for 64-bit.

BTW, for 32-bit there's a monster of a macro to turn do_div with
constant divisor into multiplications in include/asm-generic/div64.h.
However, x86-32 doesn't use it.

Paolo