Re: [Xen-devel] [PATCH] small csched_cpu_pick() adjustments

[George Dunlap <dunlapg@xxxxxxxxx>]

Acked-by: George Dunlap <george.dunlap@xxxxxxxxxxxxx>

On Tue, Oct 27, 2009 at 1:37 PM, Jan Beulich <JBeulich@xxxxxxxxxx> wrote:
> When csched_cpu_pick() decides to move a vCPU to a different pCPU, so
> far in the vast majority of cases it selected the first core/thread of
> the most idle socket/core. When there are many short executing
> entities, this will generally lead to them not getting evenly
> distributed (since primary cores/threads will be preferred), making the
> need for subsequent migration more likely. Instead, candidate
> cores/threads should get treated as symmetrically as possible, and
> hence this changes the selection logic to cycle through all candidates.
>
> Further, since csched_cpu_pick() will never move a vCPU between threads
> of the same core (and since the weights calculated for individual
> threads of the same core are always identical), rather than removing
> just the selected pCPU from the mask that still needs looking at, all
> siblings of the chosen pCPU can be removed at once without affecting
> the outcome.
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxxxx>
>
> --- 2009-10-27.orig/xen/common/sched_credit.c   2009-10-27 12:07:32.000000000 
> +0100
> +++ 2009-10-27/xen/common/sched_credit.c        2009-10-27 14:27:28.000000000 
> +0100
> @@ -110,6 +110,7 @@ struct csched_pcpu {
>     uint32_t runq_sort_last;
>     struct timer ticker;
>     unsigned int tick;
> +    unsigned int idle_bias;
>  };
>
>  /*
> @@ -298,6 +299,7 @@ csched_pcpu_init(int cpu)
>     init_timer(&spc->ticker, csched_tick, (void *)(unsigned long)cpu, cpu);
>     INIT_LIST_HEAD(&spc->runq);
>     spc->runq_sort_last = csched_priv.runq_sort;
> +    spc->idle_bias = NR_CPUS - 1;
>     per_cpu(schedule_data, cpu).sched_priv = spc;
>
>     /* Start off idling... */
> @@ -379,7 +381,7 @@ __csched_vcpu_is_migrateable(struct vcpu
>  }
>
>  static int
> -csched_cpu_pick(struct vcpu *vc)
> +_csched_cpu_pick(struct vcpu *vc, bool_t commit)
>  {
>     cpumask_t cpus;
>     cpumask_t idlers;
> @@ -438,8 +440,10 @@ csched_cpu_pick(struct vcpu *vc)
>         if ( ( (weight_cpu < weight_nxt) ^ sched_smt_power_savings )
>                 && (weight_cpu != weight_nxt) )
>         {
> -            cpu = nxt;
> -            cpu_clear(cpu, cpus);
> +            cpu = cycle_cpu(CSCHED_PCPU(nxt)->idle_bias, nxt_idlers);
> +            if ( commit )
> +               CSCHED_PCPU(nxt)->idle_bias = cpu;
> +            cpus_andnot(cpus, cpus, per_cpu(cpu_sibling_map, cpu));
>         }
>         else
>         {
> @@ -450,6 +454,12 @@ csched_cpu_pick(struct vcpu *vc)
>     return cpu;
>  }
>
> +static int
> +csched_cpu_pick(struct vcpu *vc)
> +{
> +    return _csched_cpu_pick(vc, 1);
> +}
> +
>  static inline void
>  __csched_vcpu_acct_start(struct csched_vcpu *svc)
>  {
> @@ -529,7 +539,7 @@ csched_vcpu_acct(unsigned int cpu)
>     {
>         __csched_vcpu_acct_start(svc);
>     }
> -    else if ( csched_cpu_pick(current) != cpu )
> +    else if ( _csched_cpu_pick(current, 0) != cpu )
>     {
>         CSCHED_VCPU_STAT_CRANK(svc, migrate_r);
>         CSCHED_STAT_CRANK(migrate_running);
>
>
>
>
> _______________________________________________
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> http://lists.xensource.com/xen-devel
>
>

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