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authorVincent Guittot <vincent.guittot@linaro.org>2016-12-08 17:56:53 +0100
committerIngo Molnar <mingo@kernel.org>2016-12-11 13:10:56 +0100
commitf519a3f1c6b7a990e5aed37a8f853c6ecfdee945 (patch)
treef8118444473a4320e42c0f4b0dd5bce32882f8f2
parent6643aab30f88e2929bcc401a9ae39bf6f1c0b61d (diff)
downloadlinux-f519a3f1c6b7a990e5aed37a8f853c6ecfdee945.tar.bz2
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 <matt@codeblueprint.co.uk> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Reviewed-by: Matt Fleming <matt@codeblueprint.co.uk> Acked-by: Morten Rasmussen <morten.rasmussen@arm.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> 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 <mingo@kernel.org>
-rw-r--r--kernel/sched/fair.c8
1 files changed, 8 insertions, 0 deletions
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 18d9e75f1f6e..ebb815f6bda7 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5473,13 +5473,21 @@ find_idlest_group(struct sched_domain *sd, 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;