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

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Date:	Wed, 22 Jun 2016 12:52:48 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	riel@...hat.com, 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 Wed, Jun 22, 2016 at 12:40:31PM +0200, Paolo Bonzini wrote:
> 
> 
> 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.

Right, not enabled on i386, however plenty 32bit archs (including ARM)
do have it enabled.

> 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.

Right, but some ARM chips don't exactly have a fast multiplier either,
so avoiding even the 64bit mult (which ends up being 3 or so actual
mult instructions) is worthwhile I'd say.