linux-kernel - Re: [PATCH v2 3/3] sched: clean-up struct sd_lb

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Date:	Tue, 06 Aug 2013 14:52:19 +0530
From:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>
CC:	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, Mike Galbraith <efault@....de>,
	Paul Turner <pjt@...gle.com>, Alex Shi <alex.shi@...el.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Namhyung Kim <namhyung@...nel.org>
Subject: Re: [PATCH v2 3/3] sched: clean-up struct sd_lb_stat

Hi Joonsoo,

On 08/05/2013 01:02 PM, Joonsoo Kim wrote:
>>> +	if (busiest->group_imb) {
>>> +		busiest->sum_weighted_load =
>>> +			min(busiest->sum_weighted_load, sds->sd_avg_load);
>>
>> Right here we get confused as to why the total load is being compared
>> against load per task (although you are changing it to load per task above).
> 
> Yes, you are right. I will add load_per_task to struct sg_lb_stats.
> 
> 
>>> @@ -4771,12 +4763,13 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
>>>  	 * Be careful of negative numbers as they'll appear as very large values
>>>  	 * with unsigned longs.
>>>  	 */
>>> -	max_pull = min(sds->max_load - sds->avg_load, load_above_capacity);
>>> +	max_pull = min(busiest->avg_load - sds->sd_avg_load,
>>> +						load_above_capacity);
>>
>> This is ok, but readability wise max_load is much better. max_load
>> signifies the maximum load per task on a group in this sd, and avg_load
>> signifies the total load per task across the sd. You are checking if
>> there is imbalance in the total load in the sd and try to even it out
>> across the sd.  Here "busiest" does not convey this immediately.
> 
> You may be confused. max_load doesn't means the maximum load per task.
> It means that the busiest group's load per cpu power. And here max doesn't
> mean maximum load in this sd, instead, load of busiest sg. IMO,
> busiest->avg_load convey proper meaning.

Yes sorry about that. It is load per cpu power. My point was that 
max_load helped in readability. Look at the these two lines in your patch.

-	max_pull = min(sds->max_load - sds->avg_load, load_above_capacity);
+	max_pull = min(busiest->avg_load - sds->sd_avg_load,
>>> +						load_above_capacity);

The deleted line says that there is an imbalance in the load/cpu in some 
part of the sd(max_load), which is above the average load of the sd, which
we are trying to even out. Something like the below diagram for the sd load.

  max_load
_/\________ avg_load.

I believe the line you have added makes it hard to understand this.

Anyway this is a minor issue, you can ignore it.

Regards
Preeti U Murthy

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