Re: [Xen-devel] [PATCH] xen, credit2: Avoid extra c2t calcuation in csched_runtime

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 24.01.13 at 16:24, George Dunlap <george.dunlap@xxxxxxxxxxxxx> wrote:
> --- a/xen/common/sched_credit2.c
> +++ b/xen/common/sched_credit2.c
> @@ -1505,31 +1505,42 @@ csched_dom_destroy(const struct schedule
>  static s_time_t
>  csched_runtime(const struct scheduler *ops, int cpu, struct csched_vcpu 
> *snext)
>  {
> -    s_time_t time = CSCHED_MAX_TIMER;
> +    s_time_t rt_credit, time; /* Proposed runtime measured in credits */

Does rt_credit really need to be s_time_t rather than int?

>      struct csched_runqueue_data *rqd = RQD(ops, cpu);
>      struct list_head *runq = &rqd->runq;
>  
>      if ( is_idle_vcpu(snext->vcpu) )
>          return CSCHED_MAX_TIMER;
>  
> -    /* Basic time */
> -    time = c2t(rqd, snext->credit, snext);
> +    /* General algorithm:
> +     * 1) Run until snext's credit will be 0
> +     * 2) But if someone is waiting, run until snext's credit is equal
> +     * to his
> +     * 3) But never run longer than MAX_TIMER or shorter than MIN_TIMER.
> +     */
>  
> -    /* Next guy on runqueue */
> +    /* 1) Basic time: Run until credit is 0. */
> +    rt_credit = snext->credit;
> +
> +    /* 2) If there's someone waiting whose credit is positive,

... who's ...?

> +     * run until your credit ~= his */
>      if ( ! list_empty(runq) )
>      {
> -        struct csched_vcpu *svc = __runq_elem(runq->next);
> -        s_time_t ntime;
> +        struct csched_vcpu *swait = __runq_elem(runq->next);
>  
> -        if ( ! is_idle_vcpu(svc->vcpu) )
> +        if ( ! is_idle_vcpu(swait->vcpu)
> +             && swait->credit > 0 )
>          {
> -            ntime = c2t(rqd, snext->credit - svc->credit, snext);
> -
> -            if ( time > ntime )
> -                time = ntime;
> +            rt_credit = snext->credit - swait->credit;
>          }
>      }
>  
> +    /* FIXME: See if we can eliminate this conversion if we know time
> +     * will be outside (MIN,MAX).  Probably requires pre-calculating
> +     * credit values of MIN,MAX per vcpu, since each vcpu burns credit
> +     * at a different rate. */

The obvious situation is when rt_credit <= 0, which you could
deal with right away.

Jan

> +    time = c2t(rqd, rt_credit, snext);
> +
>      /* Check limits */
>      if ( time < CSCHED_MIN_TIMER )
>          time = CSCHED_MIN_TIMER;




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