Re: [Xen-devel] [PATCH 02/24] xen: credit1: fix mask to be used for tickling in Credit1

[Dario Faggioli <dario.faggioli@xxxxxxxxxx>]

On Wed, 2016-08-17 at 19:17 +0200, Dario Faggioli wrote:
> If there are idle pcpus inside the waking vcpu's
> soft-affinity mask, we should really tickle one
> of them (this is one of the purposes of the
> __runq_tickle() function itself!), not just
> any idle pcpu.
> 
> The issue has been introduced in 02ea5031825d
> ("credit1: properly deal with pCPUs not in any cpupool"),
> where the usage of idle_mask is changed, without
> updating the bottom of the function, where it
> is also referenced.
> 
> Signed-off-by: Dario Faggioli <dario.faggioli@xxxxxxxxxx>
> ---
> Cc: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
>
Oops!

Sorry George, got your email address wrong for this one.
Dario

> Cc: Anshul Makkar <anshul.makkar@xxxxxxxxxx>
> Cc: David Vrabel <david.vrabel@xxxxxxxxxx>
> ---
> David, a while ago you asked what could have been that was causing
> awful
> results for Credit1, for CPU bound workloads, in the overloaded
> scenario of one
> of my benchmarks. I think the bug fixed either here or in next patch
> (but I'd
> be rather sure it's this one) is where the problem was. :-)
> ---
>  xen/common/sched_credit.c |    5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/xen/common/sched_credit.c b/xen/common/sched_credit.c
> index 6eccf09..3d4f223 100644
> --- a/xen/common/sched_credit.c
> +++ b/xen/common/sched_credit.c
> @@ -454,11 +454,12 @@ static inline void __runq_tickle(struct
> csched_vcpu *new)
>                  if ( opt_tickle_one_idle )
>                  {
>                      this_cpu(last_tickle_cpu) =
> -                        cpumask_cycle(this_cpu(last_tickle_cpu),
> &idle_mask);
> +                        cpumask_cycle(this_cpu(last_tickle_cpu),
> +                                      cpumask_scratch_cpu(cpu));
>                      __cpumask_set_cpu(this_cpu(last_tickle_cpu),
> &mask);
>                  }
>                  else
> -                    cpumask_or(&mask, &mask, &idle_mask);
> +                    cpumask_or(&mask, &mask,
> cpumask_scratch_cpu(cpu));
>              }
>  
>              /* Did we find anyone? */
> 
-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

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