Re: [RFC] sched: CPU topology try

From: Vincent Guittot
Date: Tue Jan 07 2014 - 03:23:10 EST

On 6 January 2014 17:21, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Wed, Dec 18, 2013 at 02:13:51PM +0100, Vincent Guittot wrote:
>> This patch applies on top of the two patches [1][2] that have been proposed by
>> Peter for creating a new way to initialize sched_domain. It includes some minor
>> compilation fixes and a trial of using this new method on ARM platform.
>> [1]
>> [2]
>> Based on the results of this tests, my feeling about this new way to init the
>> sched_domain is a bit mitigated.
> Yay :-)
>> We can add more levels that will describe other dependency/independency like
>> the frequency scaling dependency and as a result the final sched_domain
>> topology will have additional levels (if they have not been removed during
>> the degenerate sequence)
> Yeah, this 'creative' use of degenerate domains is pretty neat ;-)

thanks :-)

>> My concern is about the configuration of the table that is used to create the
>> sched_domain. Some levels are "duplicated" with different flags configuration
>> which make the table not easily readable and we must also take care of the
>> order because parents have to gather all cpus of its childs. So we must
>> choose which capabilities will be a subset of the other one. The order is
>> almost straight forward when we describe 1 or 2 kind of capabilities
>> (package ressource sharing and power sharing) but it can become complex if we
>> want to add more.
> I think I see what you're saying, although I hope that won't actually
> happen in real hardware -- that said, people do tend to do crazy with
> these ARM chips :/

it should be ok for ARM chip because the cores in a cluster share the
same clock but it doesn't mean that it will not be possible in a near
future or on other arch.

> We should also try and be conservative in the topology flags we want to
> add, which should further reduce the amount of pain here.

yes, i see a interest for powerdomain sharing and clock sharing flags
so it should minimize the complexity

> For now I do think this is a viable approach.. Yes its a bit cumbersome
> for these asymmetric systems but it does give us enough to start
> playing.


> I yet have to read Morton's emails on the P and C states, will try to
> have a look at those tomorrow with a hopefully fresher brain -- somehow
> its the end of the day already..
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