Re: [Patch v2 3/6] sched/fair: Implement prefer sibling imbalance calculation between asymmetric groups

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Jun 08, 2023 at 03:32:29PM -0700, Tim Chen wrote:

> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 03573362274f..0b0904263d51 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9372,6 +9372,65 @@ static inline bool smt_balance(struct lb_env *env, struct sg_lb_stats *sgs,
>  	return false;
>  }
>  
> +static inline long sibling_imbalance(struct lb_env *env,
> +				    struct sd_lb_stats *sds,
> +				    struct sg_lb_stats *busiest,
> +				    struct sg_lb_stats *local)
> +{
> +	int ncores_busiest, ncores_local;
> +	long imbalance;
> +
> +	if (env->idle == CPU_NOT_IDLE)
> +		return 0;
> +
> +	ncores_busiest = sds->busiest->cores;
> +	ncores_local = sds->local->cores;
> +
> +	if (ncores_busiest == ncores_local &&
> +	    (!(env->sd->flags & SD_ASYM_PACKING) ||
> +	      sched_asym_equal(env->dst_cpu,
> +			      sds->busiest->asym_prefer_cpu))) {
> +		imbalance = busiest->sum_nr_running;
> +		lsub_positive(&imbalance, local->sum_nr_running);
> +		return imbalance;
> +	}
> +
> +	/* Balance such that nr_running/ncores ratio are same on both groups */
> +	imbalance = ncores_local * busiest->sum_nr_running;
> +	lsub_positive(&imbalance, ncores_busiest * local->sum_nr_running);
> +	/* Normalize imbalance to become tasks to be moved to restore balance */
> +	imbalance /= ncores_local + ncores_busiest;
> +
> +	if (env->sd->flags & SD_ASYM_PACKING) {
> +		int limit;
> +
> +		if (!busiest->sum_nr_running)
> +			goto out;

This seems out-of-place, shouldn't we have terminate sooner if busiest
is empty?

> +
> +		if (sched_asym_prefer(env->dst_cpu, sds->busiest->asym_prefer_cpu)) {
> +			/* Don't leave preferred core idle */
> +			if (imbalance == 0 && local->sum_nr_running < ncores_local)
> +				imbalance = 1;
> +			goto out;
> +		}
> +
> +		/* Limit tasks moved from preferred group, don't leave cores idle */
> +		limit = busiest->sum_nr_running;
> +		lsub_positive(&limit, ncores_busiest);
> +		if (imbalance > limit)
> +			imbalance = limit;

How does this affect the server parts that have larger than single core
turbo domains?

> +
> +		goto out;
> +	}
> +
> +	/* Take advantage of resource in an empty sched group */
> +	if (imbalance == 0 && local->sum_nr_running == 0 &&
> +	    busiest->sum_nr_running > 1)
> +		imbalance = 1;
> +out:
> +	return imbalance << 1;
> +}


But basically you have:

        LcBn - BcLn
  imb = -----------
           LcBc

Which makes sense, except you then return:

  imb * 2

which then made me wonder about rounding.

Do we want to to add (LcBc -1) or (LcBc/2) to resp. ceil() or round()
the thing before division? Because currently it uses floor().

If you evaludate it like:


        2 * (LcBn - BcLn)
  imb = -----------------
              LcBc

The result is different from what you have now.

What actual behaviour is desired in these low imbalance cases? and can
you write a comment as to why we do as we do etc..?