linux-kernel - Re: [PATCH 4/4] sched: bias to target cpu load to reduce task moving

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Date:	Fri, 20 Dec 2013 22:45:03 +0800
From:	Alex Shi <alex.shi@...aro.org>
To:	Morten Rasmussen <morten.rasmussen@....com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"vincent.guittot@...aro.org" <vincent.guittot@...aro.org>,
	"daniel.lezcano@...aro.org" <daniel.lezcano@...aro.org>,
	"fweisbec@...il.com" <fweisbec@...il.com>,
	"linux@....linux.org.uk" <linux@....linux.org.uk>,
	"tony.luck@...el.com" <tony.luck@...el.com>,
	"fenghua.yu@...el.com" <fenghua.yu@...el.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"arjan@...ux.intel.com" <arjan@...ux.intel.com>,
	"pjt@...gle.com" <pjt@...gle.com>,
	"fengguang.wu@...el.com" <fengguang.wu@...el.com>,
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	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"hanjun.guo@...aro.org" <hanjun.guo@...aro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/4] sched: bias to target cpu load to reduce task moving

On 12/20/2013 07:19 PM, Morten Rasmussen wrote:
>> @@ -4132,10 +4137,10 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p, int this_cpu)
>> >  
>> >  		for_each_cpu(i, sched_group_cpus(group)) {
>> >  			/* Bias balancing toward cpus of our domain */
>> > -			if (local_group)
>> > +			if (i == this_cpu)
> What is the motivation for changing the local_group load calculation?
> Now the load contributions of all cpus in the local group, except
> this_cpu, will contribute more as their contribution (this_load) is
> determined using target_load() instead.
> 
> If I'm not mistaken, that will lead to more frequent load balancing as
> the local_group bias has been reduced. That is the opposite of your
> intentions based on your comment in target_load().

Good catch. will reconsider this again. :)
> 
>> >  				load = source_load(i);
>> >  			else
>> > -				load = target_load(i);
>> > +				load = target_load(i, sd->imbalance_pct);
> You scale by sd->imbalance_pct instead of 100+(sd->imbalance_pct-100)/2
> that you removed above. sd->imbalance_pct may have been arbitrarily
> chosen in the past, but changing it may affect behavior. 
> 


-- 
Thanks
    Alex
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