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

Glauber Costa
2011-06-15T01:08:03Z

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;
>> +}
>
>