linux-kernel - Re: [RFC][PATCH v5 01/14] sched: add a new arch_sd_local_flags for sched

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Date:	Thu, 14 Nov 2013 13:07:42 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Morten Rasmussen <morten.rasmussen@....com>
Cc:	Dietmar Eggemann <Dietmar.Eggemann@....com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>, Paul Turner <pjt@...gle.com>,
	"cmetcalf@...era.com" <cmetcalf@...era.com>,
	"tony.luck@...el.com" <tony.luck@...el.com>,
	Alex Shi <alex.shi@...el.com>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Jonathan Corbet <corbet@....net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Len Brown <len.brown@...el.com>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Amit Kucheria <amit.kucheria@...aro.org>,
	Lukasz Majewski <l.majewski@...sung.com>,
	"james.hogan@...tec.com" <james.hogan@...tec.com>,
	"heiko.carstens@...ibm.com" <heiko.carstens@...ibm.com>
Subject: Re: [RFC][PATCH v5 01/14] sched: add a new arch_sd_local_flags for
 sched_domain init

On Thu, Nov 14, 2013 at 10:49:02AM +0000, Morten Rasmussen wrote:
> We need a way to know which group of cpus the flag applies to. If we
> don't want to pass a pointer to the sched_domain and we want to replace
> the current named sched_domain levels with something more flexible, the
> only solution I can think of right now is to pass a cpumask to the arch
> code. Better suggestions?

That might work.

> If we let arch generate the topology it could set the flags as well. But
> that means that an arch would have to deal with generating the topology
> even if it just needs to flip a single flag in the default topology.
> 
> Another thing is if we want to put energy related information into the
> sched_domain hierarchy. If we want various energy costs (P and C state)
> to be represented here we would need to modify more than just flags.
> 
> One way to do that is to put the energy information into a sub-struct and
> have another arch_sd_energy() call that allows the arch to populate that
> struct with relevant information.

Right.. I didn't think that far ahead yet :-)

So one thing that could be done is something like:

struct sd_energy arch_sd_energy(const struct sched_domain * sd);

Where you have an entire sd available, but since its const you cannot
actually modify it and are restricted to the return value -- which we
can validate before applying.

And we can have one such a function per specific thing we want to allow
modifying.
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