linux-kernel - Re: [patch v5 02/15] sched: set initial load avg of new forked task

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Date:	Mon, 25 Feb 2013 14:00:56 +0800
From:	Alex Shi <alex.shi@...el.com>
To:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
CC:	torvalds@...ux-foundation.org, mingo@...hat.com,
	peterz@...radead.org, tglx@...utronix.de,
	akpm@...ux-foundation.org, arjan@...ux.intel.com, bp@...en8.de,
	pjt@...gle.com, namhyung@...nel.org, efault@....de,
	vincent.guittot@...aro.org, gregkh@...uxfoundation.org,
	viresh.kumar@...aro.org, linux-kernel@...r.kernel.org,
	morten.rasmussen@....com
Subject: Re: [patch v5 02/15] sched: set initial load avg of new forked task

On 02/24/2013 06:57 PM, Preeti U Murthy wrote:
> Hi Alex,
> 
> On 02/20/2013 11:50 AM, Alex Shi wrote:
>> On 02/18/2013 01:07 PM, Alex Shi wrote:
>>> New task has no runnable sum at its first runnable time, so its
>>> runnable load is zero. That makes burst forking balancing just select
>>> few idle cpus to assign tasks if we engage runnable load in balancing.
>>>
>>> Set initial load avg of new forked task as its load weight to resolve
>>> this issue.
>>>
>>
>> patch answering PJT's update here. that merged the 1st and 2nd patches 
>> into one. other patches in serial don't need to change.
>>
>> =========
>> From 89b56f2e5a323a0cb91c98be15c94d34e8904098 Mon Sep 17 00:00:00 2001
>> From: Alex Shi <alex.shi@...el.com>
>> Date: Mon, 3 Dec 2012 17:30:39 +0800
>> Subject: [PATCH 01/14] sched: set initial value of runnable avg for new
>>  forked task
>>
>> We need initialize the se.avg.{decay_count, load_avg_contrib} for a
>> new forked task.
>> Otherwise random values of above variables cause mess when do new task
>> enqueue:
>>     enqueue_task_fair
>>         enqueue_entity
>>             enqueue_entity_load_avg
>>
>> and make forking balancing imbalance since incorrect load_avg_contrib.
>>
>> set avg.decay_count = 0, and avg.load_avg_contrib = se->load.weight to
>> resolve such issues.
>>
>> Signed-off-by: Alex Shi <alex.shi@...el.com>
>> ---
>>  kernel/sched/core.c | 3 +++
>>  kernel/sched/fair.c | 4 ++++
>>  2 files changed, 7 insertions(+)
>>
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index 26058d0..1452e14 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -1559,6 +1559,7 @@ static void __sched_fork(struct task_struct *p)
>>  #if defined(CONFIG_SMP) && defined(CONFIG_FAIR_GROUP_SCHED)
>>  	p->se.avg.runnable_avg_period = 0;
>>  	p->se.avg.runnable_avg_sum = 0;
>> +	p->se.avg.decay_count = 0;
>>  #endif
>>  #ifdef CONFIG_SCHEDSTATS
>>  	memset(&p->se.statistics, 0, sizeof(p->se.statistics));
>> @@ -1646,6 +1647,8 @@ void sched_fork(struct task_struct *p)
>>  		p->sched_reset_on_fork = 0;
>>  	}
>>
> I think the following comment will help here.
> /* All forked tasks are assumed to have full utilization to begin with */
>> +	p->se.avg.load_avg_contrib = p->se.load.weight;
>> +
>>  	if (!rt_prio(p->prio))
>>  		p->sched_class = &fair_sched_class;
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 81fa536..cae5134 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -1509,6 +1509,10 @@ static inline void enqueue_entity_load_avg(struct cfs_rq *cfs_rq,
>>  	 * We track migrations using entity decay_count <= 0, on a wake-up
>>  	 * migration we use a negative decay count to track the remote decays
>>  	 * accumulated while sleeping.
>> +	 *
>> +	 * When enqueue a new forked task, the se->avg.decay_count == 0, so
>> +	 * we bypass update_entity_load_avg(), use avg.load_avg_contrib initial
>> +	 * value: se->load.weight.
> 
> I disagree with the comment.update_entity_load_avg() gets called for all
> forked tasks.
> enqueue_task_fair->update_entity_load_avg() during the second
> iteration.But __update_entity_load_avg() in update_entity_load_avg()
> 

When goes 'enqueue_task_fair->update_entity_load_avg()' during the
second iteration. the se is changed.
That is different se.


-- 
Thanks Alex
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