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 12:19:47 -0300
From:   Lauro Venancio <lvenanci@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
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 04/24/2017 11:27 AM, Peter Zijlstra wrote:
> 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)
Yes, it has the same result. I duplicated the build_group_mask magic to
avoid building the complete mask for all instances of a group.
Currently, the mask is built just once per group.
>>
>> ---
>> --- 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