6.1-stable review patch. If anyone has any objections, please let me know.
------------------
From: Pierre Gondois pierre.gondois@arm.com
commit 3af7524b14198f5159a86692d57a9f28ec9375ce upstream.
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.
Behavior before the 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 the 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/
Suggested-by: Vincent Guittot vincent.guittot@linaro.org Signed-off-by: Pierre Gondois pierre.gondois@arm.com Signed-off-by: Ingo Molnar mingo@kernel.org Reviewed-by: Vincent Guittot vincent.guittot@linaro.org Reviewed-by: Dietmar Eggemann dietmar.eggemann@arm.com Acked-by: Qais Yousef qyousef@layalina.io Link: https://lore.kernel.org/r/20231206090043.634697-1-pierre.gondois@arm.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- kernel/sched/fair.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
--- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -8524,7 +8524,7 @@ static int detach_tasks(struct lb_env *e 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;