Hello stable folk,

This patch was merged as: commit 3af7524b1419 ("sched/fair: Use all little CPUs for CPU-bound workloads") into 6.7, improving the following: commit 0b0695f2b34a ("sched/fair: Rework load_balance()")

Would it be possible to port it to the 6.1 stable branch ? The patch should apply cleanly by cherry-picking onto v6.1.94,

Regards, Pierre

On 12/6/23 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

---

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(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index d7a3c63a2171..9481b8cff31b 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -9060,7 +9060,7 @@ 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;