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SubjectRe: [PATCH] fix scheduler regression from "sched/fair: Rework load_balance()"
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On 26 Oct 2020, at 12:20, Vincent Guittot wrote:

> Le lundi 26 oct. 2020 à 12:04:45 (-0400), Rik van Riel a écrit :
>> On Mon, 26 Oct 2020 16:42:14 +0100
>> Vincent Guittot <vincent.guittot@linaro.org> wrote:
>>> On Mon, 26 Oct 2020 at 16:04, Rik van Riel <riel@surriel.com> wrote:
>>
>>>> Could utilization estimates be off, either lagging or
>>>> simply having a wrong estimate for a task, resulting
>>>> in no task getting pulled sometimes, while doing a
>>>> migrate_task imbalance always moves over something?
>>>
>>> task and cpu utilization are not always up to fully synced and may
>>> lag
>>> a bit which explains that sometimes LB can fail to migrate for a
>>> small
>>> diff
>>
>> OK, running with this little snippet below, I see latencies
>> improve back to near where they used to be:
>>
>> Latency percentiles (usec) runtime 150 (s)
>> 50.0th: 13
>> 75.0th: 31
>> 90.0th: 69
>> 95.0th: 90
>> *99.0th: 761
>> 99.5th: 2268
>> 99.9th: 9104
>> min=1, max=16158
>>
>> I suspect the right/cleaner approach might be to use
>> migrate_task more in !CPU_NOT_IDLE cases?
>>
>> Running a task to an idle CPU immediately, instead of refusing
>> to have the load balancer move it, improves latencies for fairly
>> obvious reasons.
>>
>> I am not entirely clear on why the load balancer should need to
>> be any more conservative about moving tasks than the wakeup
>> path is in eg. select_idle_sibling.
>
>
> what you are suggesting is something like:
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 4978964e75e5..3b6fbf33abc2 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9156,7 +9156,8 @@ static inline void calculate_imbalance(struct
> lb_env *env, struct sd_lb_stats *s
> * emptying busiest.
> */
> if (local->group_type == group_has_spare) {
> - if (busiest->group_type > group_fully_busy) {
> + if ((busiest->group_type > group_fully_busy) &&
> + !(env->sd->flags & SD_SHARE_PKG_RESOURCES)) {
> /*
> * If busiest is overloaded, try to fill spare
> * capacity. This might end up creating spare
> capacity
>
> which also fixes the problem for me and alignes LB with wakeup path
> regarding the migration
> in the LLC

Vincent’s patch on top of 5.10-rc1 looks pretty great:

Latency percentiles (usec) runtime 90 (s) (3320 total samples)
50.0th: 161 (1687 samples)
75.0th: 200 (817 samples)
90.0th: 228 (488 samples)
95.0th: 254 (164 samples)
*99.0th: 314 (131 samples)
99.5th: 330 (17 samples)
99.9th: 356 (13 samples)
min=29, max=358

Next we test in prod, which probably won’t have answers until
tomorrow. Thanks again Vincent!

-chris

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