From 4e4846aafb09473f1ca69fa061e72437b55f78c7 Mon Sep 17 00:00:00 2001 From: Vincent Guittot Date: Thu, 8 Dec 2016 17:56:53 +0100 Subject: [PATCH 80/92] sched/core: Fix find_idlest_group() for fork During fork, the utilization of a task is init once the rq has been selected because the current utilization level of the rq is used to set the utilization of the fork task. As the task's utilization is still 0 at this step of the fork sequence, it doesn't make sense to look for some spare capacity that can fit the task's utilization. Furthermore, I can see perf regressions for the test: hackbench -P -g 1 because the least loaded policy is always bypassed and tasks are not spread during fork. With this patch and the fix below, we are back to same performances as for v4.8. The fix below is only a temporary one used for the test until a smarter solution is found because we can't simply remove the test which is useful for others benchmarks | @@ -5708,13 +5708,6 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t | | avg_cost = this_sd->avg_scan_cost; | | - /* | - * Due to large variance we need a large fuzz factor; hackbench in | - * particularly is sensitive here. | - */ | - if ((avg_idle / 512) < avg_cost) | - return -1; | - | time = local_clock(); | | for_each_cpu_wrap(cpu, sched_domain_span(sd), target, wrap) { Tested-by: Matt Fleming Signed-off-by: Vincent Guittot Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: Matt Fleming Acked-by: Morten Rasmussen Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Thomas Gleixner Cc: dietmar.eggemann@arm.com Cc: kernellwp@gmail.com Cc: umgwanakikbuti@gmail.com Cc: yuyang.du@intel.comc Link: http://lkml.kernel.org/r/1481216215-24651-2-git-send-email-vincent.guittot@linaro.org Signed-off-by: Ingo Molnar (cherry picked from commit f2e4ac502860c2adcc5ce56e0064fc4bcab90af4) Signed-off-by: Gaku Inami --- kernel/sched/fair.c | 8 ++++++++ 1 file changed, 8 insertions(+) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 489f6d3..fea3ca8 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -5809,13 +5809,21 @@ static unsigned long capacity_spare_wake(int cpu, struct task_struct *p) * utilized systems if we require spare_capacity > task_util(p), * so we allow for some task stuffing by using * spare_capacity > task_util(p)/2. + * + * Spare capacity can't be used for fork because the utilization has + * not been set yet, we must first select a rq to compute the initial + * utilization. */ + if (sd_flag & SD_BALANCE_FORK) + goto skip_spare; + if (this_spare > task_util(p) / 2 && imbalance*this_spare > 100*most_spare) return NULL; else if (most_spare > task_util(p) / 2) return most_spare_sg; +skip_spare: if (!idlest || 100*this_load < imbalance*min_load) return NULL; return idlest; -- 1.9.1