Re: [Xen-devel] [PATCH 1/2] xen: credit2: avoid using cpumask_weight() in hot-paths

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 20/04/2019 16:24, Dario Faggioli wrote:
> cpumask_weight() is known to be expensive. In Credit2, we use it in
> load-balancing, but only for knowing how many CPUs are active in a
> runqueue.

With a few small changes it can be improved substantially (by using
POPCNT when available), but it is still fundamentally an O(n) operation.

Definitely +1 to algorithm changes which avoid its use entirely.

A few cosmetic observations.

> diff --git a/xen/common/sched_credit2.c b/xen/common/sched_credit2.c
> index 6958b265fc..7034325243 100644
> --- a/xen/common/sched_credit2.c
> +++ b/xen/common/sched_credit2.c
> @@ -466,6 +466,7 @@ struct csched2_runqueue_data {
>      spinlock_t lock;           /* Lock for this runqueue                     
> */
>  
>      struct list_head runq;     /* Ordered list of runnable vms               
> */
> +    int nr_cpus;               /* How many CPUs are sharing this runqueue    
> */

Unsigned int.

> @@ -3829,8 +3833,11 @@ init_pdata(struct csched2_private *prv, struct 
> csched2_pcpu *spc,
>                  __cpumask_set_cpu(rcpu, &csched2_pcpu(cpu)->sibling_mask);
>              }
>  
> -    if ( cpumask_weight(&rqd->active) == 1 )
> +    if ( rqd->nr_cpus == 1 )
> +    {
> +     ASSERT(cpumask_weight(&rqd->active) == 1);

Tab.

>          rqd->pick_bias = cpu;
> +    }
>  
>      return spc->runq_id;
>  }
> @@ -3944,8 +3951,12 @@ csched2_deinit_pdata(const struct scheduler *ops, void 
> *pcpu, int cpu)
>      __cpumask_clear_cpu(cpu, &rqd->smt_idle);
>      __cpumask_clear_cpu(cpu, &rqd->active);
>  
> -    if ( cpumask_empty(&rqd->active) )
> +    rqd->nr_cpus--;
> +    ASSERT(cpumask_weight(&rqd->active) == rqd->nr_cpus);
> +
> +    if ( rqd->nr_cpus == 0 )
>      {
> +     ASSERT(cpumask_empty(&rqd->active));

And here.

Is it really worth having both asserts?  The second is redundant.

~Andrew

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