linux-kernel - Re: [RFC 0/6] rework sched_domain topology description

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Message-ID: <20140311131719.GY9987@twins.programming.kicks-ass.net>
Date:	Tue, 11 Mar 2014 14:17:19 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Dietmar Eggemann <dietmar.eggemann@....com>
Cc:	Vincent Guittot <vincent.guittot@...aro.org>,
	"mingo@...nel.org" <mingo@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"tony.luck@...el.com" <tony.luck@...el.com>,
	"fenghua.yu@...el.com" <fenghua.yu@...el.com>,
	"schwidefsky@...ibm.com" <schwidefsky@...ibm.com>,
	"james.hogan@...tec.com" <james.hogan@...tec.com>,
	"cmetcalf@...era.com" <cmetcalf@...era.com>,
	"benh@...nel.crashing.org" <benh@...nel.crashing.org>,
	"linux@....linux.org.uk" <linux@....linux.org.uk>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"preeti@...ux.vnet.ibm.com" <preeti@...ux.vnet.ibm.com>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>
Subject: Re: [RFC 0/6] rework sched_domain topology description

On Sat, Mar 08, 2014 at 12:40:58PM +0000, Dietmar Eggemann wrote:
> >
> >I don't have a strong opinion about using or not a cpu argument for
> >setting the flags of a level (it was part of the initial proposal
> >before we start to completely rework the build of sched_domain)
> >Nevertheless, I see one potential concern that you can have completely
> >different flags configuration of the same sd level of 2 cpus.
> 
> Could you elaborate a little bit further regarding the last sentence? Do you
> think that those completely different flags configuration would make it
> impossible, that the load-balance code could work at all at this sd?

So a problem with such an interfaces is that is makes it far too easy to
generate completely broken domains.

You can, for two cpus in the same domain provide, different flags; such
a configuration doesn't make any sense at all.

Now I see why people would like to have this; but unless we can make it
robust I'd be very hesitant to go this route.


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