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

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Date:	Wed, 1 Apr 2015 11:06:47 +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 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

Vincent

>
>>
>>>
>>> -Xunlei
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