Re: [PATCH v10 08/11] sched: replace capacity_factor by usage

[Xunlei Pang]

Hi Vincent,

On 1 April 2015 at 17:06, Vincent Guittot <vincent.guittot@xxxxxxxxxx> wrote:
> On 1 April 2015 at 05:37, Xunlei Pang <pang.xunlei@xxxxxxxxxx> wrote:
>> Hi Vincent,
>>
>> On 27 March 2015 at 23:59, Vincent Guittot <vincent.guittot@xxxxxxxxxx> wrote:
>>> On 27 March 2015 at 15:52, Xunlei Pang <pang.xunlei@xxxxxxxxxx> wrote:
>>>> Hi Vincent,
>>>>
>>>> On 27 February 2015 at 23:54, Vincent Guittot
>>>> <vincent.guittot@xxxxxxxxxx> 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.

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;
 }

Thanks,
-Xunlei
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