From 6b94780e45c17b83e3e75f8aaca5a328db583c74 Mon Sep 17 00:00:00 2001 From: Vincent Guittot Date: Thu, 8 Dec 2016 17:56:54 +0100 Subject: [PATCH] sched/core: Use load_avg for selecting idlest group find_idlest_group() only compares the runnable_load_avg when looking for the least loaded group. But on fork intensive use case like hackbench where tasks blocked quickly after the fork, this can lead to selecting the same CPU instead of other CPUs, which have similar runnable load but a lower load_avg. When the runnable_load_avg of 2 CPUs are close, we now take into account the amount of blocked load as a 2nd selection factor. There is now 3 zones for the runnable_load of the rq: - [0 .. (runnable_load - imbalance)]: Select the new rq which has significantly less runnable_load - [(runnable_load - imbalance) .. (runnable_load + imbalance)]: The runnable loads are close so we use load_avg to chose between the 2 rq - [(runnable_load + imbalance) .. ULONG_MAX]: Keep the current rq which has significantly less runnable_load The scale factor that is currently used for comparing runnable_load, doesn't work well with small value. As an example, the use of a scaling factor fails as soon as this_runnable_load == 0 because we always select local rq even if min_runnable_load is only 1, which doesn't really make sense because they are just the same. So instead of scaling factor, we use an absolute margin for runnable_load to detect CPUs with similar runnable_load and we keep using scaling factor for blocked load. For use case like hackbench, this enable the scheduler to select different CPUs during the fork sequence and to spread tasks across the system. Tests have been done on a Hikey board (ARM based octo cores) for several kernel. The result below gives min, max, avg and stdev values of 18 runs with each configuration. The patches depend on the "no missing update_rq_clock()" work. hackbench -P -g 1 ea86cb4b7621 7dc603c9028e v4.8 v4.8+patches min 0.049 0.050 0.051 0,048 avg 0.057 0.057(0%) 0.057(0%) 0,055(+5%) max 0.066 0.068 0.070 0,063 stdev +/-9% +/-9% +/-8% +/-9% More performance numbers here: https://lkml.kernel.org/r/20161203214707.GI20785@codeblueprint.co.uk Tested-by: Matt Fleming Signed-off-by: Vincent Guittot Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: Matt Fleming Cc: Linus Torvalds Cc: Morten.Rasmussen@arm.com 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/1481216215-24651-3-git-send-email-vincent.guittot@linaro.org Signed-off-by: Ingo Molnar --- kernel/sched/fair.c | 55 ++++++++++++++++++++++++++++++++++++++++++----------- 1 file changed, 44 insertions(+), 11 deletions(-) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index ebb815f..6559d19 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -5405,16 +5405,20 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p, { struct sched_group *idlest = NULL, *group = sd->groups; struct sched_group *most_spare_sg = NULL; - unsigned long min_load = ULONG_MAX, this_load = 0; + unsigned long min_runnable_load = ULONG_MAX, this_runnable_load = 0; + unsigned long min_avg_load = ULONG_MAX, this_avg_load = 0; unsigned long most_spare = 0, this_spare = 0; int load_idx = sd->forkexec_idx; - int imbalance = 100 + (sd->imbalance_pct-100)/2; + int imbalance_scale = 100 + (sd->imbalance_pct-100)/2; + unsigned long imbalance = scale_load_down(NICE_0_LOAD) * + (sd->imbalance_pct-100) / 100; if (sd_flag & SD_BALANCE_WAKE) load_idx = sd->wake_idx; do { - unsigned long load, avg_load, spare_cap, max_spare_cap; + unsigned long load, avg_load, runnable_load; + unsigned long spare_cap, max_spare_cap; int local_group; int i; @@ -5431,6 +5435,7 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p, * the group containing the CPU with most spare capacity. */ avg_load = 0; + runnable_load = 0; max_spare_cap = 0; for_each_cpu(i, sched_group_cpus(group)) { @@ -5440,7 +5445,9 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p, else load = target_load(i, load_idx); - avg_load += load; + runnable_load += load; + + avg_load += cfs_rq_load_avg(&cpu_rq(i)->cfs); spare_cap = capacity_spare_wake(i, p); @@ -5449,14 +5456,31 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p, } /* Adjust by relative CPU capacity of the group */ - avg_load = (avg_load * SCHED_CAPACITY_SCALE) / group->sgc->capacity; + avg_load = (avg_load * SCHED_CAPACITY_SCALE) / + group->sgc->capacity; + runnable_load = (runnable_load * SCHED_CAPACITY_SCALE) / + group->sgc->capacity; if (local_group) { - this_load = avg_load; + this_runnable_load = runnable_load; + this_avg_load = avg_load; this_spare = max_spare_cap; } else { - if (avg_load < min_load) { - min_load = avg_load; + if (min_runnable_load > (runnable_load + imbalance)) { + /* + * The runnable load is significantly smaller + * so we can pick this new cpu + */ + min_runnable_load = runnable_load; + min_avg_load = avg_load; + idlest = group; + } else if ((runnable_load < (min_runnable_load + imbalance)) && + (100*min_avg_load > imbalance_scale*avg_load)) { + /* + * The runnable loads are close so take the + * blocked load into account through avg_load. + */ + min_avg_load = avg_load; idlest = group; } @@ -5482,14 +5506,23 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p, goto skip_spare; if (this_spare > task_util(p) / 2 && - imbalance*this_spare > 100*most_spare) + imbalance_scale*this_spare > 100*most_spare) return NULL; - else if (most_spare > task_util(p) / 2) + + if (most_spare > task_util(p) / 2) return most_spare_sg; skip_spare: - if (!idlest || 100*this_load < imbalance*min_load) + if (!idlest) + return NULL; + + if (min_runnable_load > (this_runnable_load + imbalance)) return NULL; + + if ((this_runnable_load < (min_runnable_load + imbalance)) && + (100*this_avg_load < imbalance_scale*min_avg_load)) + return NULL; + return idlest; } -- 2.7.4