If a cpu is idle and another cpu has more than one runnable task, pull one of them without considering cpu_power source or target. This allows low cpu_power cpus to offload potentially oversubscribed high cpu_power cpus.
In heterogeneous systems containing cpus with different cpu_power, the load-balancer will put more tasks on sched_domains with high (above default) cpu_power cpus and fewer on sched_domains with low cpu_power cpus. Hence, if the number of running tasks is equal to the number of cpus, the load-balancer may decide to leave low cpu_power idle and placing more than one task on each high cpu_power cpu. This is not optimal use of the available compute resources.
Placing one task on each cpu before adding more to any of the high cpu_power cpus should generally give a better overall throughput regardless of the cpu_power of the cpus.
Signed-off-by: Morten Rasmussen morten.rasmussen@arm.com Reviewed-by: Vincent Guittot vincent.guittot@linaro.org --- kernel/sched/fair.c | 21 ++++++++++++++++++--- 1 file changed, 18 insertions(+), 3 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 4781cdd..095885c 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -4039,7 +4039,8 @@ static int move_tasks(struct lb_env *env) if (sched_feat(LB_MIN) && load < 16 && !env->sd->nr_balance_failed) goto next;
- if ((load / 2) > env->imbalance) + if ((load / 2) > env->imbalance && + (env->idle != CPU_IDLE && env->idle != CPU_NEWLY_IDLE)) goto next;
if (!can_migrate_task(p, env)) @@ -4539,6 +4540,15 @@ static inline void update_sg_lb_stats(struct lb_env *env, if (overloaded_cpu) sgs->group_imb = 1;
+ /* + * When idle balancing pull tasks if more than one task per cpu + * in group + */ + if (env->idle == CPU_IDLE || env->idle == CPU_NEWLY_IDLE) { + if (group->group_weight < sgs->sum_nr_running) + sgs->group_imb = 1; + } + sgs->group_capacity = DIV_ROUND_CLOSEST(group->sgp->power, SCHED_POWER_SCALE); if (!sgs->group_capacity) @@ -4766,8 +4776,13 @@ void fix_small_imbalance(struct lb_env *env, struct sd_lb_stats *sds) min(sds->this_load_per_task, sds->this_load + tmp); pwr_move /= SCHED_POWER_SCALE;
- /* Move if we gain throughput */ - if (pwr_move > pwr_now) + /* + * Move if we gain throughput, or if we have cpus idling while others + * are running more than one task. + */ + if ((pwr_move > pwr_now) || + (sds->busiest_group_weight < sds->busiest_nr_running && + (env->idle == CPU_IDLE || env->idle == CPU_NEWLY_IDLE))) env->imbalance = sds->busiest_load_per_task; }