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SubjectRe: [PATCH v10 08/11] sched: replace capacity_factor by usage
On 1 April 2015 at 16:54, Xunlei Pang <pang.xunlei@linaro.org> wrote:
> Hi Vincent,
>
> On 1 April 2015 at 17:06, Vincent Guittot <vincent.guittot@linaro.org> wrote:
>> On 1 April 2015 at 05:37, Xunlei Pang <pang.xunlei@linaro.org> wrote:
>>> Hi Vincent,
>>>
>>> On 27 March 2015 at 23:59, Vincent Guittot <vincent.guittot@linaro.org> wrote:
>>>> On 27 March 2015 at 15:52, Xunlei Pang <pang.xunlei@linaro.org> wrote:
>>>>> Hi Vincent,
>>>>>
>>>>> On 27 February 2015 at 23:54, Vincent Guittot
>>>>> <vincent.guittot@linaro.org> wrote:
>>>>>> /**
>>>>>> @@ -6432,18 +6435,19 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
>>>>>>
>>>>>> /*
>>>>>> * In case the child domain prefers tasks go to siblings
>>>>>> - * first, lower the sg capacity factor to one so that we'll try
>>>>>> + * first, lower the sg capacity so that we'll try
>>>>>> * and move all the excess tasks away. We lower the capacity
>>>>>> * of a group only if the local group has the capacity to fit
>>>>>> - * these excess tasks, i.e. nr_running < group_capacity_factor. The
>>>>>> - * extra check prevents the case where you always pull from the
>>>>>> - * heaviest group when it is already under-utilized (possible
>>>>>> - * with a large weight task outweighs the tasks on the system).
>>>>>> + * these excess tasks. The extra check prevents the case where
>>>>>> + * you always pull from the heaviest group when it is already
>>>>>> + * under-utilized (possible with a large weight task outweighs
>>>>>> + * the tasks on the system).
>>>>>> */
>>>>>> if (prefer_sibling && sds->local &&
>>>>>> - sds->local_stat.group_has_free_capacity) {
>>>>>> - sgs->group_capacity_factor = min(sgs->group_capacity_factor, 1U);
>>>>>> - sgs->group_type = group_classify(sg, sgs);
>>>>>> + group_has_capacity(env, &sds->local_stat) &&
>>>>>> + (sgs->sum_nr_running > 1)) {
>>>>>> + sgs->group_no_capacity = 1;
>>>>>> + sgs->group_type = group_overloaded;
>>>>>> }
>>>>>>
>>>>>
>>>>> For SD_PREFER_SIBLING, if local has 1 task and group_has_capacity()
>>>>> returns true(but not overloaded) for it, and assume sgs group has 2
>>>>> tasks, should we still mark this group overloaded?
>>>>
>>>> yes, the load balance will then choose if it's worth pulling it or not
>>>> depending of the load of each groups
>>>
>>> Maybe I didn't make it clearly.
>>> For example, CPU0~1 are SMT siblings, CPU2~CPU3 are another pair.
>>> CPU0 is idle, others each has 1 task. Then according to this patch,
>>> CPU2~CPU3(as one group) will be viewed as overloaded(CPU0~CPU1 as
>>> local group, and group_has_capacity() returns true here), so the
>>> balancer may initiate an active task moving. This is different from
>>> the current code as SD_PREFER_SIBLING logic does. Is this problematic?
>>
>> IMHO, it's not problematic, It's worth triggering a load balance if
>> there is an imbalance between the 2 groups (as an example CPU0~1 has
>> one low nice prio task but CPU1~2 have 2 high nice prio tasks) so the
>> decision will be done when calculating the imbalance
>
> Yes, but assuming the balancer calculated some imbalance, after moving
> like CPU0~CPU1 have 1 low prio task and 1 high prio task, CPU2~CPU3
> have 1 high piro task, seems it does no good because there's only 1
> task per CPU after all.

In this condition i agree that it doesn't worth to move a task and the
scheduler should not because the imbalance will be too small to found
a busiest queue.
So the decision is more linked to the weighted load of the tasks than
to the number of tasks

>
> So, is code below better( we may have more than 2 SMT siblings, like
> Broadcom XLP processor having 4 SMT per core)?
> if (prefer_sibling && sds->local &&
> group_has_capacity(env, &sds->local_stat) &&
> (sgs->sum_nr_running > sds->local_stat.sum_nr_running + 1)) {
> sgs->group_no_capacity = 1;
> sgs->group_type = group_overloaded;
> }

I would say no because it mainly depends of the weighted load of the
tasks so calculate_imbalance is the right place

Vincent
>
> Thanks,
> -Xunlei


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