linux-kernel - Re: [patch v3 0/8] sched: use runnable avg in load balance

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Date:	Wed, 03 Apr 2013 14:01:18 +0800
From:	Alex Shi <alex.shi@...el.com>
To:	Michael Wang <wangyun@...ux.vnet.ibm.com>
CC:	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,
	morten.rasmussen@....com, vincent.guittot@...aro.org,
	gregkh@...uxfoundation.org, preeti@...ux.vnet.ibm.com,
	viresh.kumar@...aro.org, linux-kernel@...r.kernel.org,
	len.brown@...el.com, rafael.j.wysocki@...el.com, jkosina@...e.cz,
	clark.williams@...il.com, tony.luck@...el.com,
	keescook@...omium.org, mgorman@...e.de, riel@...hat.com
Subject: Re: [patch v3 0/8] sched: use runnable avg in load balance

On 04/03/2013 01:38 PM, Michael Wang wrote:
> On 04/03/2013 12:28 PM, Alex Shi wrote:
> [snip]
>>
>> but the patch may cause some unfairness if this/prev cpu are not burst at 
>> same time. So could like try the following patch?
> 
> I will try it later, some doubt below :)
> 
> [snip]
>> +
>> +	if (cpu_rq(this_cpu)->avg_idle < sysctl_sched_migration_cost ||
>> +		cpu_rq(prev_cpu)->avg_idle < sysctl_sched_migration_cost)
>> +		burst= 1;
>> +
>> +	/* use instant load for bursty waking up */
>> +	if (!burst) {
>> +		load = source_load(prev_cpu, idx);
>> +		this_load = target_load(this_cpu, idx);
>> +	} else {
>> +		load = cpu_rq(prev_cpu)->load.weight;
>> +		this_load = cpu_rq(this_cpu)->load.weight;
> 
> Ok, my understanding is, we want pull if 'prev_cpu' is burst, and don't
> want pull if 'this_cpu' is burst, correct?

Nope, as my thought, a burst cpu doesn't mean it has heavy load than
others, so we still need to compare their load on the same condition.
> 
> And we do this by guess the load higher or lower, is that right?

ditto.
> 
> And I think target_load() is capable enough to choose the higher load,
> if 'cpu_rq(cpu)->load.weight' is really higher, the results will be the
> same.
> 
> So what about this:
> 
> 	/* prefer higher load if burst */
> 	load = burst_prev ?
> 		target_load(prev_cpu, idx) : source_load(prev_cpu, idx);
> 
> 	this_load = target_load(this_cpu, idx);

here, the idx is zero, so source_load and target load is same.
> 
> Regards,
> Michael Wang
> 
>> +	}
>>
>>  	/*
>>  	 * If sync wakeup then subtract the (maximum possible)
>>
> 


-- 
Thanks Alex
--
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