Re: [PATCH 6/6] xen: sched: optimize exclusive pinning case (Credit1 & 2)

[Dario Faggioli <dario.faggioli@citrix.com>]

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On Fri, 2017-07-21 at 18:19 +0100, George Dunlap wrote:
> On 06/23/2017 11:55 AM, Dario Faggioli wrote:
> > diff --git a/xen/common/sched_credit.c b/xen/common/sched_credit.c
> > index 4f6330e..85e014d 100644
> > --- a/xen/common/sched_credit.c
> > +++ b/xen/common/sched_credit.c
> > @@ -429,6 +429,24 @@ static inline void __runq_tickle(struct
> > csched_vcpu *new)
> >      idlers_empty = cpumask_empty(&idle_mask);
> >  
> >      /*
> > +     * Exclusive pinning is when a vcpu has hard-affinity with
> > only one
> > +     * cpu, and there is no other vcpu that has hard-affinity with
> > that
> > +     * same cpu. This is infrequent, but if it happens, is for
> > achieving
> > +     * the most possible determinism, and least possible overhead
> > for
> > +     * the vcpus in question.
> > +     *
> > +     * Try to identify the vast majority of these situations, and
> > deal
> > +     * with them quickly.
> > +     */
> > +    if ( unlikely(cpumask_cycle(cpu, new->vcpu->cpu_hard_affinity) 
> > == cpu &&
> 
> Won't this check entail a full "loop" of the cpumask?  It's cheap
> enough
> if nr_cpu_ids is small; but don't we support (theoretically) 4096
> logical cpus?
> 
> It seems like having a vcpu flag that identifies a vcpu as being
> pinned
> would be a more efficient way to do this.  That way we could run this
> check once whenever the hard affinity changed, rather than every time
> we
> want to think about where to run this vcpu.
> 
> What do you think?
> 
Right. We actually should get some help from the hardware (ffs &
firends)... but I think you're right. Implementing this with a flag, as
 you're suggesting, is most likely better, and easy enough.

I'll go for that!

Regards,
Dario
-- 
<<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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