linux-kernel - Re: [PATCH 02/18] sched: fix find_idlest

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Date:	Tue, 11 Dec 2012 11:20:08 +0530
From:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
To:	Alex Shi <alex.shi@...el.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 02/18] sched: fix find_idlest_group mess logical

Hi Alex,
On 12/11/2012 10:59 AM, Alex Shi wrote:
> On 12/11/2012 01:08 PM, Preeti U Murthy wrote:
>> Hi Alex,
>>
>> On 12/10/2012 01:52 PM, Alex Shi wrote:
>>> There is 4 situations in the function:
>>> 1, no task allowed group;
>>> 	so min_load = ULONG_MAX, this_load = 0, idlest = NULL
>>> 2, only local group task allowed;
>>> 	so min_load = ULONG_MAX, this_load assigned, idlest = NULL
>>> 3, only non-local task group allowed;
>>> 	so min_load assigned, this_load = 0, idlest != NULL
>>> 4, local group + another group are task allowed.
>>> 	so min_load assigned, this_load assigned, idlest != NULL
>>>
>>> Current logical will return NULL in first 3 kinds of scenarios.
>>> And still return NULL, if idlest group is heavier then the
>>> local group in the 4th situation.
>>>
>>> Actually, I thought groups in situation 2,3 are also eligible to host
>>> the task. And in 4th situation, agree to bias toward local group.
>>> So, has this patch.
>>>
>>> Signed-off-by: Alex Shi <alex.shi@...el.com>
>>> ---
>>>  kernel/sched/fair.c |   12 +++++++++---
>>>  1 files changed, 9 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>>> index df99456..b40bc2b 100644
>>> --- a/kernel/sched/fair.c
>>> +++ b/kernel/sched/fair.c
>>> @@ -2953,6 +2953,7 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p,
>>>  		  int this_cpu, int load_idx)
>>>  {
>>>  	struct sched_group *idlest = NULL, *group = sd->groups;
>>> +	struct sched_group *this_group = NULL;
>>>  	unsigned long min_load = ULONG_MAX, this_load = 0;
>>>  	int imbalance = 100 + (sd->imbalance_pct-100)/2;
>>>  
>>> @@ -2987,14 +2988,19 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p,
>>>  
>>>  		if (local_group) {
>>>  			this_load = avg_load;
>>> -		} else if (avg_load < min_load) {
>>> +			this_group = group;
>>> +		}
>>> +		if (avg_load < min_load) {
>>>  			min_load = avg_load;
>>>  			idlest = group;
>>>  		}
>>>  	} while (group = group->next, group != sd->groups);
>>>  
>>> -	if (!idlest || 100*this_load < imbalance*min_load)
>>> -		return NULL;
>>> +	if (this_group && idlest != this_group)
>>> +		/* Bias toward our group again */
>>> +		if (100*this_load < imbalance*min_load)
>>> +			idlest = this_group;
>>
>> If the idlest group is heavier than this_group(or to put it better if
>> the difference in the loads of the local group and idlest group is less
>> than a threshold,it means there is no point moving the load from the
>> local group) you return NULL,that immediately means this_group is chosen
>> as the candidate group for the task to run,one does not have to
>> explicitly return that.
> 
> In situation 4, this_group is not NULL.

True.The return value of find_idlest_group() indicates that there is no
other idle group other than the local group(the group to which cpu
belongs to). it does not indicate that there is no host group for the
task.If this is the case,select_task_rq_fair() falls back to the
group(sd->child) to which the cpu chosen in the previous iteration
belongs to,This is nothing but this_group in the current iteration.

Regards
Preeti U Murthy

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