linux-kernel - Re: [PATCH 4/4] sched/topology: the group balance cpu must be a cpu where the group is installed

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Date:   Mon, 24 Apr 2017 16:27:39 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Lauro Ramos Venancio <lvenanci@...hat.com>
Cc:     lwang@...hat.com, riel@...hat.com, Mike Galbraith <efault@....de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/4] sched/topology: the group balance cpu must be a cpu
 where the group is installed

On Mon, Apr 24, 2017 at 04:19:44PM +0200, Peter Zijlstra wrote:
> On Mon, Apr 24, 2017 at 03:03:26PM +0200, Peter Zijlstra wrote:
> 
> > Also, would it not make sense to re-order patch 2 to come after this,
> > such that we _do_ have the group_mask available and don't have to jump
> > through hoops in order to link up the sgc? Afaict we don't actually use
> > the sgc until the above (reverse) loop computing the CPU capacities.
> 
> That is, if I force 4 on without 2, then doesn't something like the
> below also do the right thing? (without duplicating part of the magic
> already contained in build_group_mask)
> 
> ---
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -498,13 +498,16 @@ enum s_alloc {
>   *
>   * This function can only be used when all the groups are already built.
>   */
> -static void build_group_mask(struct sched_domain *sd, struct sched_group *sg)
> +static void
> +build_group_mask(struct sched_domain *sd, struct sched_group *sg, struct cpumask *mask)
>  {
>  	const struct cpumask *sg_span = sched_group_cpus(sg);
>  	struct sd_data *sdd = sd->private;
>  	struct sched_domain *sibling;
>  	int i;
>  
> +	cpumask_clear(mask);
> +
>  	for_each_cpu(i, sg_span) {
>  		sibling = *per_cpu_ptr(sdd->sd, i);
>  
> @@ -514,7 +517,7 @@ static void build_group_mask(struct sche
>  		if (!cpumask_equal(sg_span, sched_group_cpus(sibling->groups)))
>  			continue;
>  
> -		cpumask_set_cpu(i, sched_group_mask(sg));
> +		cpumask_set_cpu(i, mask);
>  	}
>  }
>  
> @@ -549,14 +552,19 @@ build_group_from_child_sched_domain(stru
>  }
>  
>  static void init_overlap_sched_group(struct sched_domain *sd,
> -				     struct sched_group *sg, int cpu)
> +				     struct sched_group *sg)
>  {
> +	struct cpumask *mask = sched_domains_tmpmask;
>  	struct sd_data *sdd = sd->private;
>  	struct cpumask *sg_span;
> +	int cpu;
> +
> +	build_group_mask(sd, sg, mask);
> +	cpu = cpumask_first_and(sched_group_mask(sg), mask); /* balance cpu */

s/group_mask/group_span/

>  
>  	sg->sgc = *per_cpu_ptr(sdd->sgc, cpu);
>  	if (atomic_inc_return(&sg->sgc->ref) == 1)
> -		build_group_mask(sd, sg);
> +		cpumask_copy(sched_group_mask(sg), mask);
>  
>  	/*
>  	 * Initialize sgc->capacity such that even if we mess up the