Re: [PATCH] SEDF: avoid gathering vCPU-s on pCPU0

[Keir Fraser <keir@xen.org>]

On 01/03/2013 15:35, "Jan Beulich" <JBeulich@suse.com> wrote:

> The introduction of vcpu_force_reschedule() in 14320:215b799fa181 was
> incompatible with the SEDF scheduler: Any vCPU using
> VCPUOP_stop_periodic_timer (e.g. any vCPU of half way modern PV Linux
> guests) ends up on pCPU0 after that call. Obviously, running all PV
> guests' (and namely Dom0's) vCPU-s on pCPU0 causes problems for those
> guests rather sooner than later.
> 
> So the main thing that was clearly wrong (and bogus from the beginning)
> was the use of cpumask_first() in sedf_pick_cpu(). It is being replaced
> by a construct that prefers to put back the vCPU on the pCPU that it
> got launched on.
> 
> However, there's one more glitch: When reducing the affinity of a vCPU
> temporarily, and then widening it again to a set that includes the pCPU
> that the vCPU was last running on, the generic scheduler code would not
> force a migration of that vCPU, and hence it would forever stay on the
> pCPU it last ran on. Since that can again create a load imbalance, the
> SEDF scheduler wants a migration to happen regardless of it being
> apparently unnecessary.
> 
> Of course, an alternative to checking for SEDF explicitly in
> vcpu_set_affinity() would be to introduce a flags field in struct
> scheduler, and have SEDF set a "always-migrate-on-affinity-change"
> flag.
> 
> Signed-off-by: Jan Beulich <jbeulich@suse.com>

Acked-by: Keir Fraser <keir@xen.org>

> --- a/xen/common/sched_sedf.c
> +++ b/xen/common/sched_sedf.c
> @@ -397,7 +397,8 @@ static int sedf_pick_cpu(const struct sc
>  
>      online = cpupool_scheduler_cpumask(v->domain->cpupool);
>      cpumask_and(&online_affinity, v->cpu_affinity, online);
> -    return cpumask_first(&online_affinity);
> +    return cpumask_cycle(v->vcpu_id % cpumask_weight(&online_affinity) - 1,
> +                         &online_affinity);
>  }
>  
>  /*
> --- a/xen/common/schedule.c
> +++ b/xen/common/schedule.c
> @@ -605,7 +605,8 @@ int vcpu_set_affinity(struct vcpu *v, co
>      vcpu_schedule_lock_irq(v);
>  
>      cpumask_copy(v->cpu_affinity, affinity);
> -    if ( !cpumask_test_cpu(v->processor, v->cpu_affinity) )
> +    if ( VCPU2OP(v)->sched_id == XEN_SCHEDULER_SEDF ||
> +         !cpumask_test_cpu(v->processor, v->cpu_affinity) )
>          set_bit(_VPF_migrating, &v->pause_flags);
>  
>      vcpu_schedule_unlock_irq(v);
> 
> 
> 
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