linux-kernel - Re: [PATCH 08/18] sched: consider runnable load average in move

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Date:	Wed, 12 Dec 2012 14:26:44 +0800
From:	Alex Shi <alex.shi@...el.com>
To:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
CC:	rob@...dley.net, mingo@...hat.com, peterz@...radead.org,
	gregkh@...uxfoundation.org, andre.przywara@....com, rjw@...k.pl,
	paul.gortmaker@...driver.com, akpm@...ux-foundation.org,
	paulmck@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org,
	pjt@...gle.com, vincent.guittot@...aro.org
Subject: Re: [PATCH 08/18] sched: consider runnable load average in move_tasks

On 12/12/2012 12:41 PM, Preeti U Murthy wrote:
> Hi Alex,
> On 12/10/2012 01:52 PM, Alex Shi wrote:
>> Except using runnable load average in background, move_tasks is also
>> the key functions in load balance. We need consider the runnable load
>> average in it in order to the apple to apple load comparison.
>>
>> Signed-off-by: Alex Shi <alex.shi@...el.com>
>> ---
>>  kernel/sched/fair.c |   11 ++++++++++-
>>  1 files changed, 10 insertions(+), 1 deletions(-)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 6d893a6..bbb069c 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -3741,6 +3741,15 @@ static unsigned long task_h_load(struct task_struct *p);
>>  
>>  static const unsigned int sched_nr_migrate_break = 32;
>>  
>> +static unsigned long task_h_load_avg(struct task_struct *p)
>> +{
>> +	u32 period = p->se.avg.runnable_avg_period;
>> +	if (!period)
>> +		return 0;
>> +
>> +	return task_h_load(p) * p->se.avg.runnable_avg_sum / period;
>                         ^^^^^^^^^^^^
> This might result in an overflow,considering you are multiplying two 32
> bit integers.Below is how this is handled in
> __update_task_entity_contrib in kernel/sched/fair.c
> 
> u32 contrib;
> /* avoid overflowing a 32-bit type w/ SCHED_LOAD_SCALE */
> contrib = se->avg.runnable_avg_sum * scale_load_down(se->load.weight);
> contrib /= (se->avg.runnable_avg_period + 1);
> se->avg.load_avg_contrib = scale_load(contrib);

scale_load_down is do nothing.
> 
> Also why can't p->se.load_avg_contrib be used directly? as a return
> value for task_h_load_avg? since this is already updated in
> update_task_entity_contrib and update_group_entity_contrib.

No, only non task entity goes to  update_group_entity_contrib. not task
entity.


>> +}
>> +
>>  /*
>>   * move_tasks tries to move up to imbalance weighted load from busiest to
>>   * this_rq, as part of a balancing operation within domain "sd".
>> @@ -3776,7 +3785,7 @@ static int move_tasks(struct lb_env *env)
>>  		if (throttled_lb_pair(task_group(p), env->src_cpu, env->dst_cpu))
>>  			goto next;
>>  
>> -		load = task_h_load(p);
>> +		load = task_h_load_avg(p);
>>  
>>  		if (sched_feat(LB_MIN) && load < 16 && !env->sd->nr_balance_failed)
>>  			goto next;
>>
> 
> Regards
> Preeti U Murthy
> 

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