linux-kernel - Re: [PATCH 5/7] KVM-GST: KVM Steal time accounting

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Date:	Tue, 14 Jun 2011 22:08:03 -0300
From:	Glauber Costa <glommer@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	Rik van Riel <riel@...hat.com>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	Avi Kivity <avi@...hat.com>,
	Anthony Liguori <aliguori@...ibm.com>,
	Eric B Munson <emunson@...bm.net>
Subject: Re: [PATCH 5/7] KVM-GST: KVM Steal time accounting

On 06/14/2011 07:10 AM, Peter Zijlstra wrote:
> On Mon, 2011-06-13 at 19:31 -0400, Glauber Costa wrote:
>> +static inline int touch_steal_time(int is_idle)
>> +{
>> +       u64 steal, st = 0;
>> +
>> +       if (static_branch(&paravirt_steal_enabled)) {
>> +
>> +               steal = paravirt_steal_clock(smp_processor_id());
>> +
>> +               steal -= this_rq()->prev_steal_time;
>> +               this_rq()->prev_steal_time += steal;
>
> If you move this addition below this test:
>
>> +               if (is_idle || (steal<  TICK_NSEC))
>> +                       return 0;
>
> that is, right here, then you don't loose tiny steal deltas and
> subsequent ticks accumulate their steal time until you really
> have a full steal tick to account.

true
> I guess you want something different for the idle case though.

definitely.

>> +               while (steal>  TICK_NSEC) {
>
> 			/* really, if we wanted a division we'd have written one */
> 			asm("" : "+rm" (steal));

Out of curiosity, have we seen any compiler de-optimize it to a 
division, or are you just being careful ?

>> +                       steal -= TICK_NSEC;
>> +                       st++;
>> +               }
>> +
>> +               account_steal_time(st);
>> +               return 1;
>> +       }
>> +       return 0;
>> +}
>
>

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