On Fri, 25 Nov, at 04:34:32PM, Vincent Guittot wrote:
> 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
> null 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) {
>
OK, I need to point out that I didn't apply the above hunk when
testing this patch series. But I wouldn't have expected that to impact
our fork-intensive workloads so much. Let me know if you'd like me to
re-run with it applied.
I don't see much of a difference, positive or negative, for the
majority of the test machines, it's mainly a wash.
However, the following 4-cpu Xeon E5504 machine does show a nice win,
with thread counts in the mid-range (note, the second column is number
of hackbench groups, where each group has 40 tasks),
hackbench-process-pipes
4.9.0-rc6 4.9.0-rc6 4.9.0-rc6
tip-sched fix-fig-for-fork fix-sig
Amean 1 0.2193 ( 0.00%) 0.2014 ( 8.14%) 0.1746 ( 20.39%)
Amean 3 0.4489 ( 0.00%) 0.3544 ( 21.04%) 0.3284 ( 26.83%)
Amean 5 0.6173 ( 0.00%) 0.4690 ( 24.02%) 0.4977 ( 19.37%)
Amean 7 0.7323 ( 0.00%) 0.6367 ( 13.05%) 0.6267 ( 14.42%)
Amean 12 0.9716 ( 0.00%) 1.0187 ( -4.85%) 0.9351 ( 3.75%)
Amean 16 1.2866 ( 0.00%) 1.2664 ( 1.57%) 1.2131 ( 5.71%)
