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

[Glauber Costa]

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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