Re: [Xen-devel] [PATCH 03 of 11 v3] xen: sched_credit: when picking, make sure we get an idle one, if any

[Juergen Gross <juergen.gross@xxxxxxxxxxxxxx>]

Am 01.02.2013 12:01, schrieb Dario Faggioli:
> The pcpu picking algorithm treats two threads of a SMT core the same.
> More specifically, if one is idle and the other one is busy, they both
> will be assigned a weight of 1. Therefore, when picking begins, if the
> first target pcpu is the busy thread (and if there are no other idle
> pcpu than its sibling), that will never change.
>
> This change fixes this by ensuring that, before entering the core of
> the picking algorithm, the target pcpu is an idle one (if there is an
> idle pcpu at all, of course).
>
> Signed-off-by: Dario Faggioli<dario.faggioli@xxxxxxxxxx>
Acked-by: Juergen Gross <juergen.gross@xxxxxxxxxxxxxx>

> diff --git a/xen/common/sched_credit.c b/xen/common/sched_credit.c
> --- a/xen/common/sched_credit.c
> +++ b/xen/common/sched_credit.c
> @@ -526,6 +526,18 @@ static int
>       if ( vc->processor == cpu&&  IS_RUNQ_IDLE(cpu) )
>           cpumask_set_cpu(cpu,&idlers);
>       cpumask_and(&cpus,&cpus,&idlers);
> +
> +    /*
> +     * It is important that cpu points to an idle processor, if a suitable
> +     * one exists (and we can use cpus to check and, possibly, choose a new
> +     * CPU, as we just&&-ed it with idlers). In fact, if we are on SMT, and
> +     * cpu points to a busy thread with an idle sibling, both the threads
> +     * will be considered the same, from the "idleness" calculation point
> +     * of view", preventing vcpu from being moved to the thread that is
> +     * actually idle.
> +     */
> +    if ( !cpumask_empty(&cpus)&&  !cpumask_test_cpu(cpu,&cpus) )
> +        cpu = cpumask_cycle(cpu,&cpus);
>       cpumask_clear_cpu(cpu,&cpus);
>
>       while ( !cpumask_empty(&cpus) )
>
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--
Juergen Gross                 Principal Developer Operating Systems
PBG PDG ES&S SWE OS6                   Telephone: +49 (0) 89 3222 2967
Fujitsu Technology Solutions              e-mail: juergen.gross@xxxxxxxxxxxxxx
Domagkstr. 28                           Internet: ts.fujitsu.com
D-80807 Muenchen                 Company details: ts.fujitsu.com/imprint.html

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