Subject: Re: [PATCH 1/3] sched: Fix nohz_kick_needed to consider the nr_busy of the parent domain's group

From: Kamalesh Babulal <[email-masked]>
Date: Tue, 22 Oct 2013 20:05:59 +0530

* Vaidyanathan Srinivasan <[email-masked]> [2013-10-21 17:14:42]:

>  	for_each_domain(cpu, sd) {
> -		struct sched_group *sg = sd->groups;
> -		struct sched_group_power *sgp = sg->sgp;
> -		int nr_busy = atomic_read(&sgp->nr_busy_cpus);
> -
> -		if (sd->flags & SD_SHARE_PKG_RESOURCES && nr_busy > 1)
> -			goto need_kick_unlock;
> +		struct sched_domain *sd_parent = sd->parent;
> +		struct sched_group *sg;
> +		struct sched_group_power *sgp;
> +		int nr_busy;
> +
> +		if (sd_parent) {
> +			sg = sd_parent->groups;
> +			sgp = sg->sgp;
> +			nr_busy = atomic_read(&sgp->nr_busy_cpus);
> +
> +			if (sd->flags & SD_SHARE_PKG_RESOURCES && nr_busy > 1)
> +				goto need_kick_unlock;
> +		}
> 
>  		if (sd->flags & SD_ASYM_PACKING && nr_busy != sg->group_weight
>  		    && (cpumask_first_and(nohz.idle_cpus_mask,

CC'ing Suresh Siddha and Vincent Guittot

Please correct me, If my understanding of idle balancing is wrong.
With proposed approach will not idle load balancer kick in, even if
there are busy cpus across groups or if there are 2 busy cpus which
are spread across sockets.

Consider 2 socket machine with 4 processors each (MC and NUMA domains).
If the machine is partial loaded such that cpus 0,4,5,6,7 are busy, then too
nohz balancing is triggered because with this approach
(NUMA)->groups->sgp->nr_busy_cpus is taken in account for nohz kick, while
iterating over MC domain.

Isn't idle load balancer not suppose kick in, even in the case of two busy
cpu's in a dual-core single socket system.

Thanks,
Kamalesh.