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 12:11:59 -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 10:03 AM, Peter Zijlstra wrote:
> On Thu, Apr 20, 2017 at 04:51:43PM -0300, Lauro Ramos Venancio wrote:
>
>> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
>> index e77c93a..694e799 100644
>> --- a/kernel/sched/topology.c
>> +++ b/kernel/sched/topology.c
>> @@ -505,7 +507,11 @@ static void build_group_mask(struct sched_domain *sd, struct sched_group *sg)
>>  
>>  	for_each_cpu(i, sg_span) {
>>  		sibling = *per_cpu_ptr(sdd->sd, i);
>> -		if (!cpumask_test_cpu(i, sched_domain_span(sibling)))
>> +
>> +		if (!sibling->groups)
>> +			continue;
> How can this happen?
This happens on machines with asymmetric topologies. For more details see:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=c1174876874dcf8986806e4dad3d7d07af20b439
>
>> +
>> +		if (!cpumask_equal(sg_span, sched_group_cpus(sibling->groups)))
>>  			continue;
>>  
>>  		cpumask_set_cpu(i, sched_group_mask(sg));
>
>> @@ -1482,6 +1502,14 @@ struct sched_domain *build_sched_domain(struct sched_domain_topology_level *tl,
>>  		}
>>  	}
>>  
>> +	/* Init overlap groups */
>> +	for_each_cpu(i, cpu_map) {
>> +		for (sd = *per_cpu_ptr(d.sd, i); sd; sd = sd->parent) {
>> +			if (sd->flags & SD_OVERLAP)
>> +				init_overlap_sched_groups(sd);
>> +		}
>> +	}
> Why does this have to be a whole new loop? This is because in
> build_group_mask() we could encounter @sibling that were not constructed
> yet?
That is right. We can only build the group mask when all siblings are
constructed.

>
> So this is the primary fix?
Yes.
>
>> +
>>  	/* Calculate CPU capacity for physical packages and nodes */
>>  	for (i = nr_cpumask_bits-1; i >= 0; i--) {
>>  		if (!cpumask_test_cpu(i, cpu_map))
>
> 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.