[v3] sched/fair: Use all little CPUs for CPU-bound workload

  Running n CPU-bound tasks on an n CPUs platform:
- with asymmetric CPU capacity
- not being a DynamIq system (i.e. having a PKG level sched domain
  without the SD_SHARE_PKG_RESOURCES flag set)
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'. Cf. group_has_capacity() and
  group_is_overloaded(), 3 CPU-bound tasks run on a 4 CPUs
  sched-domain, and the idle CPU provides enough spare capacity
  regarding the imbalance_pct
- 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;

  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:
As the number of failed balancing attempts grows (with
'nr_balance_failed'), progressively make it easier to migrate
a big task to the idling little CPU. A similar mechanism is
used for the 'migrate_load' migration type.

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%)

Similar issue reported at:
https://lore.kernel.org/lkml/20230716014125.139577-1-qyousef@layalina.io/

v1:
https://lore.kernel.org/all/20231110125902.2152380-1-pierre.gondois@arm.com/
v2:
https://lore.kernel.org/all/20231124153323.3202444-1-pierre.gondois@arm.com/

Suggested-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Pierre Gondois <pierre.gondois@arm.com>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
---

Notes:
    v2:
    - Used Vincent's approach.
    v3:
    - Updated commit message.
    - Added Reviewed-by tags

 kernel/sched/fair.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
  

On 12/06/23 10:00, Pierre Gondois wrote:
> Running n CPU-bound tasks on an n CPUs platform:
> - with asymmetric CPU capacity
> - not being a DynamIq system (i.e. having a PKG level sched domain
>   without the SD_SHARE_PKG_RESOURCES flag set)
> 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'. Cf. group_has_capacity() and
>   group_is_overloaded(), 3 CPU-bound tasks run on a 4 CPUs
>   sched-domain, and the idle CPU provides enough spare capacity
>   regarding the imbalance_pct
> - 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;
> 
>   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:
> As the number of failed balancing attempts grows (with
> 'nr_balance_failed'), progressively make it easier to migrate
> a big task to the idling little CPU. A similar mechanism is
> used for the 'migrate_load' migration type.
> 
> 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%)
> 
> Similar issue reported at:
> https://lore.kernel.org/lkml/20230716014125.139577-1-qyousef@layalina.io/
> 
> v1:
> https://lore.kernel.org/all/20231110125902.2152380-1-pierre.gondois@arm.com/
> v2:
> https://lore.kernel.org/all/20231124153323.3202444-1-pierre.gondois@arm.com/
> 
> Suggested-by: Vincent Guittot <vincent.guittot@linaro.org>
> Signed-off-by: Pierre Gondois <pierre.gondois@arm.com>
> Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
> Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
> ---

Thanks Pierre! I think this is a good candidate to stable. It is likely to help
some folks shipping with phantom domains on impacted stable kernels.


Cheers

--
Qais Yousef