linux-kernel - Re: [PATCH v10 08/11] sched: replace capacity

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Date:	Wed, 1 Apr 2015 17:57:59 +0200
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Xunlei Pang <pang.xunlei@...aro.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	lkml <linux-kernel@...r.kernel.org>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Morten Rasmussen <Morten.Rasmussen@....com>,
	Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
	Rik van Riel <riel@...hat.com>,
	Linaro Kernel Mailman List <linaro-kernel@...ts.linaro.org>,
	Mike Galbraith <efault@....de>,
	Dietmar Eggemann <dietmar.eggemann@....com>
Subject: Re: [PATCH v10 08/11] sched: replace capacity_factor by usage

On 1 April 2015 at 16:54, Xunlei Pang <pang.xunlei@...aro.org> wrote:
> Hi Vincent,
>
> On 1 April 2015 at 17:06, Vincent Guittot <vincent.guittot@...aro.org> wrote:
>> On 1 April 2015 at 05:37, Xunlei Pang <pang.xunlei@...aro.org> wrote:
>>> Hi Vincent,
>>>
>>> On 27 March 2015 at 23:59, Vincent Guittot <vincent.guittot@...aro.org> wrote:
>>>> On 27 March 2015 at 15:52, Xunlei Pang <pang.xunlei@...aro.org> wrote:
>>>>> Hi Vincent,
>>>>>
>>>>> On 27 February 2015 at 23:54, Vincent Guittot
>>>>> <vincent.guittot@...aro.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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