Messages in this thread Patch in this message | | | From | Pierre Gondois <> | Subject | [PATCH] sched/fair: Use all little CPUs for CPU-bound workload | Date | Fri, 10 Nov 2023 13:59:02 +0100 |
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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@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, + * 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
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