Re: [PATCH 1/2 v2] sched: fix find_idlest_group for fork

[Vincent Guittot]

On 4 December 2016 at 00:25, Matt Fleming <matt@xxxxxxxxxxxxxxxxxxx> wrote:
> 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.

At least on my target ( hikey board :  dual quad cortex-A53 platform),
i can see additional perf improvements for the fork intensive test
"hackbench -P -g 1"
The patch above was there to explain any difference in perf results
with v4.8 but you don't need to re-run with it

>
> 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%)