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

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 06/23/2017 11:55 AM, Dario Faggioli wrote:
> Exclusive pinning of vCPUs is used, sometimes, for
> achieving the highest level of determinism, and the
> least possible overhead, for the vCPUs in question.
> 
> Although static 1:1 pinning is not recommended, for
> general use cases, optimizing the tickling code (of
> Credit1 and Credit2) is easy and cheap enough, so go
> for it.
> 
> Signed-off-by: Dario Faggioli <dario.faggioli@xxxxxxxxxx>
> ---
> Cc: George Dunlap <george.dunlap@xxxxxxxxxx>
> Cc: Anshul Makkar <anshulmakkar@xxxxxxxxx>
> ---
>  xen/common/sched_credit.c    |   19 +++++++++++++++++++
>  xen/common/sched_credit2.c   |   21 ++++++++++++++++++++-
>  xen/include/xen/perfc_defn.h |    1 +
>  3 files changed, 40 insertions(+), 1 deletion(-)
> 
> 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?

 -George

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