Re: [PATCH] sched/fair: Use all little CPUs for CPU-bound workload

[Pierre Gondois <pierre.gondois@arm.com>]

Hello Vincent,

On 11/17/23 15:17, Vincent Guittot wrote:
> Hi Pierre,
> 
> On Fri, 10 Nov 2023 at 13:59, Pierre Gondois <pierre.gondois@arm.com> wrote:
>>
>> Running n CPU-bound tasks on an n CPUs platform with asymmetric CPU
>> capacity might result in a task placement where two tasks run on a
>> big CPU and none on a little CPU. This placement could be more optimal
>> by using all CPUs.
>>
>> Testing platform:
>> Juno-r2:
>> - 2 big CPUs (1-2), maximum capacity of 1024
>> - 4 little CPUs (0,3-5), maximum capacity of 383
>>
>> Testing workload ([1]):
>> Spawn 6 CPU-bound tasks. During the first 100ms (step 1), each tasks
>> is affine to a CPU, except for:
>> - one little CPU which is left idle.
>> - one big CPU which has 2 tasks affine.
>> After the 100ms (step 2), remove the cpumask affinity.
>>
>> Before patch:
>> During step 2, the load balancer running from the idle CPU tags sched
>> domains as:
>> - little CPUs: 'group_has_spare'. Indeed, 3 CPU-bound tasks run on a
>>    4 CPUs sched-domain, and the idle CPU provides enough spare
>>    capacity.
>> - big CPUs: 'group_overloaded'. Indeed, 3 tasks run on a 2 CPUs
>>    sched-domain, so the following path is used:
>>    group_is_overloaded()
>>    \-if (sgs->sum_nr_running <= sgs->group_weight) return true;
> 
> This remembers me a similar discussion with Qais:
> https://lore.kernel.org/lkml/20230716014125.139577-1-qyousef@layalina.io/

Yes indeed, this is exactly the same case and same backstory actually.

> 
>>
>>    The following path which would change the migration type to
>>    'migrate_task' is not taken:
>>    calculate_imbalance()
>>    \-if (env->idle != CPU_NOT_IDLE && env->imbalance == 0)
>>    as the local group has some spare capacity, so the imbalance
>>    is not 0.
>>
>> The migration type requested is 'migrate_util' and the busiest
>> runqueue is the big CPU's runqueue having 2 tasks (each having a
>> utilization of 512). The idle little CPU cannot pull one of these
>> task as its capacity is too small for the task. The following path
>> is used:
>> detach_tasks()
>> \-case migrate_util:
>>    \-if (util > env->imbalance) goto next;
>>
>> After patch:
>> When the local group has spare capacity and the busiest group is at
>> least tagged as 'group_fully_busy', if the local group has more CPUs
> 
> the busiest group is more than  'group_fully_busy'
> 
>> than CFS tasks and the busiest group more CFS tasks than CPUs,
>> request a 'migrate_task' type migration.
>>
>> Improvement:
>> Running the testing workload [1] with the step 2 representing
>> a ~10s load for a big CPU:
>> Before patch: ~19.3s
>> After patch: ~18s (-6.7%)
>>
>> The issue only happens at the DIE level on platforms able to have
>> 'migrate_util' migration types, i.e. no DynamIQ systems where
>> SD_SHARE_PKG_RESOURCES is set.
>>
>> Signed-off-by: Pierre Gondois <pierre.gondois@arm.com>
>> ---
>>   kernel/sched/fair.c | 17 +++++++++++++++++
>>   1 file changed, 17 insertions(+)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index df348aa55d3c..5a215c96d420 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -10495,6 +10495,23 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
>>                          env->imbalance = max(local->group_capacity, local->group_util) -
>>                                           local->group_util;
>>
>> +                       /*
>> +                        * On an asymmetric system with CPU-bound tasks, a
>> +                        * migrate_util balance might not be able to migrate a
>> +                        * task from a big to a little CPU, letting a little
>> +                        * CPU unused.
>> +                        * If local has an empty CPU and busiest is overloaded,
> 
> group_has_spare doesn't mean that the local has an empty cpu but that
> one or more cpu might be idle some time which could not be the case
> when the load balance happen
> 
>> +                        * balance one task with a migrate_task migration type
>> +                        * instead.
>> +                        */
>> +                       if (env->sd->flags & SD_ASYM_CPUCAPACITY &&
>> +                           local->sum_nr_running < local->group_weight &&
>> +                           busiest->sum_nr_running > busiest->group_weight) {
>> +                               env->migration_type = migrate_task;
>> +                               env->imbalance = 1;
> 
> I wonder if this is too aggressive. We can have cases where
> (local->sum_nr_running < local->group_weight) at the time of the load
> balance because one cpu can be shortly idle and you will migrate the
> task that will then compete with another one on a little core. So

Ok right.

> maybe you should do something similar to the migrate_load in
> detach_tasks like:
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index fc8e9ced6aa8..3a04fa0f1eae 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8977,7 +8977,7 @@ static int detach_tasks(struct lb_env *env)
>                  case migrate_util:
>                          util = task_util_est(p);
> 
> -                       if (util > env->imbalance)
> +                       if (shr_bound(util,
> env->sd->nr_balance_failed) > env->imbalance)
>                                  goto next;
> 
>                          env->imbalance -= util;
> --
> 
> This should cover more intermediate cases and would benefit to more
> topology and cases

Your change also solves the issue. I'll try to see if this might
raise other issues,

Thanks for the suggestion,
Regards,
Pierre

> 
>> +                               return;
>> +                       }
>> +
>>                          /*
>>                           * In some cases, the group's utilization is max or even
>>                           * higher than capacity because of migrations but the
>> --
>> 2.25.1
>>