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

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Date:	Tue, 7 Jan 2014 14:32:07 +0100
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Morten Rasmussen <morten.rasmussen@....com>,
	Alex Shi <alex.shi@...aro.org>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"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>,
	"james.hogan@...tec.com" <james.hogan@...tec.com>,
	"jason.low2@...com" <jason.low2@...com>,
	"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 7 January 2014 14:15, Peter Zijlstra <peterz@...radead.org> wrote:
> On Tue, Jan 07, 2014 at 01:59:30PM +0100, Peter Zijlstra wrote:
>> On Tue, Jan 07, 2014 at 12:55:18PM +0000, Morten Rasmussen wrote:
>> > My understanding is that should_we_balance() decides which cpu is
>> > eligible for doing the load balancing for a given domain (and the
>> > domains above). That is, only one cpu in a group is allowed to load
>> > balance between the local group and other groups. That cpu would
>> > therefore be reponsible for pulling enough load that the groups are
>> > balanced even if it means temporarily overloading itself. The other cpus
>> > in the group will take care of load balancing the extra load within the
>> > local group later.
>>
>> Correct.
>
> On that; one of the things I wanted to (and previously did attempt but
> failed) is trying to rotate this cpu. Currently its always the first cpu
> (of the group) and that gives a noticeable bias.

Isn't the current policy (it's the 1st idle cpu in priority). a good
enough way to rotate the cpus ? Are you need the rotation for loaded
use case too ?

>
> If we could slowly rotate the cpu that does this that would alleviate
> both the load and cost bias.
>
> One thing I was thinking of is keeping a global counter maybe:
>  'x := jiffies >> n'
> might be good enough and using the 'x % nr_cpus_in_group'-th cpu
> instead.
>
> Then again, these are micro issue and not a lot of people complain
> about this.
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