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

[Pierre Gondois]

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;

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