From eccc8acbe705a20e0911ea776371d84eba53cc8e Mon Sep 17 00:00:00 2001 From: Maria Yu Date: Fri, 26 Apr 2019 15:20:18 +0800 Subject: sched/fair: Avoid unnecessary active load balance When find busiest group, it will avoid load balance if it is only 1 task running on src cpu. Consider race when different cpus do newly idle load balance at the same time, check src cpu nr_running to avoid unnecessary active load balance again. See the race condition example here: 1) cpu2 have 2 tasks, so cpu2 rq->nr_running == 2 and cfs.h_nr_running ==2. 2) cpu4 and cpu5 doing newly idle load balance at the same time. 3) cpu4 and cpu5 both see cpu2 sched_load_balance_sg_stats sum_nr_run=2 so they are both see cpu2 as the busiest rq. 4) cpu5 did a success migration task from cpu2, so cpu2 only have 1 task left, cpu2 rq->nr_running == 1 and cfs.h_nr_running ==1. 5) cpu4 surely goes to no_move because currently cpu4 only have 1 task which is currently running. 6) and then cpu4 goes here to check if cpu2 need active load balance. Change-Id: Ia9539a43e9769c4936f06ecfcc11864984c50c29 Signed-off-by: Maria Yu --- kernel/sched/fair.c | 2 ++ 1 file changed, 2 insertions(+) (limited to 'kernel') diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 941424604fdd..42f05c742846 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -10456,8 +10456,10 @@ static int need_active_balance(struct lb_env *env) * It's worth migrating the task if the src_cpu's capacity is reduced * because of other sched_class or IRQs if more capacity stays * available on dst_cpu. + * Avoid pulling the CFS task if it is the only task running. */ if ((env->idle != CPU_NOT_IDLE) && + (env->src_rq->nr_running > 1) && (env->src_rq->cfs.h_nr_running == 1)) { if ((check_cpu_capacity(env->src_rq, sd)) && (capacity_of(env->src_cpu)*sd->imbalance_pct < capacity_of(env->dst_cpu)*100)) -- cgit v1.2.3