Re: [PATCH v2] sched/cfs: make util/load_avg more stable

[Vincent Guittot]

On 25 April 2017 at 16:53, Dietmar Eggemann <dietmar.eggemann@xxxxxxx> wrote:
> On 25/04/17 13:40, Vincent Guittot wrote:
>> On 25 April 2017 at 13:05, Dietmar Eggemann <dietmar.eggemann@xxxxxxx> wrote:
>>> On 19/04/17 17:54, Vincent Guittot wrote:
>>>> In the current implementation of load/util_avg, we assume that the ongoing
>>>> time segment has fully elapsed, and util/load_sum is divided by LOAD_AVG_MAX,
>>>> even if part of the time segment still remains to run. As a consequence, this
>>>> remaining part is considered as idle time and generates unexpected variations
>>>> of util_avg of a busy CPU in the range ]1002..1024[ whereas util_avg should
>>>
>>> Why do you use the square brackets the other way around? Just curious.
>>
>> This refers to the very beginning and very end of time segment formulas below.
>> That being said, 1024 is not reachable because at very end we have :
>>  1024*MAX_LOAD_AVG*y+1024*1023 = 1023,9997
>>
>> 1002 is not reachable because at very beg we have
>> 1024*MAX_LOAD_AVG*y+ 1024*0 = 1002,0577
>>
>> But we are working with integer so [1002..1024[ is probably more correct
>
> OK, this is with y = 32nd-rt(0.5) exactly, understood.
>
> I assume you mean LOAD_AVG_MAX instead of MAX_LOAD_AVG.

correct

>
>>> 1002 stands for 1024*y^1 w/ y = 4008/4096 or y^32 = 0.5, right ? Might
>>> be worth mentioning.
>>>
>>>> stay at 1023.
>>>>
>>>> In order to keep the metric stable, we should not consider the ongoing time
>>>> segment when computing load/util_avg but only the segments that have already
>>>> fully elapsed. Bu to not consider the current time segment adds unwanted
>>>> latency in the load/util_avg responsivness especially when the time is scaled
>>>> instead of the contribution. Instead of waiting for the current time segment
>>>> to have fully elapsed before accounting it in load/util_avg, we can already
>>>> account the elapsed part but change the range used to compute load/util_avg
>>>> accordingly.
>>>>
>>>> At the very beginning of a new time segment, the past segments have been
>>>> decayed and the max value is MAX_LOAD_AVG*y. At the very end of the current
>>>> time segment, the max value becomes 1024(us) + MAX_LOAD_AVG*y which is equal
>>>> to MAX_LOAD_AVG. In fact, the max value is
>>>> sa->period_contrib + MAX_LOAD_AVG*y at any time in the time segment.
>
> s/MAX_LOAD_AVG/LOAD_AVG_MAX

ditto

>
>>>>
>>>> Taking advantage of the fact that MAX_LOAD_AVG*y == MAX_LOAD_AVG-1024, the
>>>> range becomes [0..MAX_LOAD_AVG-1024+sa->period_contrib].
>>>>
>>>> As the elapsed part is already accounted in load/util_sum, we update the max
>>>> value according to the current position in the time segment instead of
>>>> removing its contribution.
>>>
>>> Removing its contribution stands for '- 1024' of 'LOAD_AVG_MAX - 1024'
>>> which was added in patch 1/2?
>>
>> removing its contribution refers to  "- sa->period_contrib * weight"
>> and "- (running * sa->period_contrib << SCHED_CAPACITY_SHIFT))"  in
>> patch 1/2 of the previous version
>
> Yup, makes sense, so the '-1024' is the influence of the current 'time
> segment' (n = 0) then.

Yes

>
> IMHO, the removing of contribution in patch 1/2 wouldn't take freq and
> cpu scaling of contribution (which is still in accumulate_sum()) into
> consideration.

Yes you're right but as everything has been put in 1 single patch in
v2, it doesn't make any difference now

>
>>>> Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>>>> Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
>>>> ---
>>>>
>>>> Fold both patches in one
>>>>
>>>>  kernel/sched/fair.c | 6 +++---
>>>>  1 file changed, 3 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>>>> index 3f83a35..c3b8f0f 100644
>>>> --- a/kernel/sched/fair.c
>>>> +++ b/kernel/sched/fair.c
>>>> @@ -3017,12 +3017,12 @@ ___update_load_avg(u64 now, int cpu, struct sched_avg *sa,
>>>>       /*
>>>>        * Step 2: update *_avg.
>>>>        */
>>>> -     sa->load_avg = div_u64(sa->load_sum, LOAD_AVG_MAX);
>>>> +     sa->load_avg = div_u64(sa->load_sum, LOAD_AVG_MAX - 1024 + sa->period_contrib);
>>>>       if (cfs_rq) {
>>>>               cfs_rq->runnable_load_avg =
>>>> -                     div_u64(cfs_rq->runnable_load_sum, LOAD_AVG_MAX);
>>>> +                     div_u64(cfs_rq->runnable_load_sum, LOAD_AVG_MAX - 1024 + sa->period_contrib);
>>>>       }
>>>> -     sa->util_avg = sa->util_sum / LOAD_AVG_MAX;
>>>> +     sa->util_avg = sa->util_sum / (LOAD_AVG_MAX - 1024 + sa->period_contrib);
>>>>
>>>>       return 1;
>>>>  }
>>>>