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

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Message-ID: <20140107134055.GA31570@twins.programming.kicks-ass.net>
Date:	Tue, 7 Jan 2014 14:40:55 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Vincent Guittot <vincent.guittot@...aro.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 Tue, Jan 07, 2014 at 02:32:07PM +0100, Vincent Guittot wrote:
> 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 ?

Yeah its for the fully loaded case. And like I said, there's not been
many complaints on this.

The 'problem' is that its always same cpu that does the most expensive
full machine balance; and always that cpu that is the one that gains
extra load to redistribute in the group. So its penalized twice.

Like said, really minor issue. Just something I thought I'd mention.
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