Re: [PATCH V3 1/2] sched/fair: Fix load_balance() affinity redo path

[Dietmar Eggemann <dietmar.eggemann@arm.com>]

On 19/05/17 14:31, Dietmar Eggemann wrote:
> On 18/05/17 20:36, Jeffrey Hugo wrote:
> 
> [...]
> 
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index d711093..a5d41b1 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -8220,7 +8220,24 @@ static int load_balance(int this_cpu, struct rq *this_rq,
>>  		/* All tasks on this runqueue were pinned by CPU affinity */
>>  		if (unlikely(env.flags & LBF_ALL_PINNED)) {
>>  			cpumask_clear_cpu(cpu_of(busiest), cpus);
>> -			if (!cpumask_empty(cpus)) {
>> +			/*
>> +			 * dst_cpu is not a valid busiest cpu in the following
>> +			 * check since load cannot be pulled from dst_cpu to be
>> +			 * put on dst_cpu.
>> +			 */
>> +			cpumask_clear_cpu(env.dst_cpu, cpus);
>> +			/*
>> +			 * Go back to "redo" iff the load-balance cpumask
>> +			 * contains other potential busiest cpus for the
>> +			 * current sched domain.
>> +			 */
>> +			if (cpumask_intersects(cpus, sched_domain_span(env.sd))) {
>> +				/*
>> +				 * Now that the check has passed, reenable
>> +				 * dst_cpu so that load can be calculated on
>> +				 * it in the redo path.
>> +				 */
>> +				cpumask_set_cpu(env.dst_cpu, cpus);
> 
> IMHO, this will work nicely and its way easier.

This was too quick ... if we still have other potential dst cpus
available and cpu_of(busiest) is the latest src cpu then this will fail.

It does work on sd with 'group_weight == 1', e.g. your MC sd 'sd->child
== NULL'.

But IMHO 'group_imbalance' propagation has to work on higher sd levels
as well.

> Another idea might be to check if the LBF_ALL_PINNED is set when we
> check if we should clean the imbalance flag.
> 
> @@ -8307,14 +8307,13 @@ static int load_balance(int this_cpu, struct rq *this_rq,
>          * We reach balance although we may have faced some affinity
>          * constraints. Clear the imbalance flag if it was set.
>          */
> -       if (sd_parent) {
> +       if (sd_parent && !(env.flags & LBF_ALL_PINNED)) {
>                 int *group_imbalance = &sd_parent->groups->sgc->imbalance;
> 
>                 if (*group_imbalance)
>                         *group_imbalance = 0;
>         }

[...]