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

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Date:	Wed, 1 Apr 2015 11:37:45 +0800
From:	Xunlei Pang <pang.xunlei@...aro.org>
To:	Vincent Guittot <vincent.guittot@...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

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?

>
>>
>> -Xunlei
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