Date:	Mon, 06 Jan 2014 18:40:46 +0000
From:	Dietmar Eggemann <dietmar.eggemann@....com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	"mingo@...hat.com" <mingo@...hat.com>,
	"vincent.guittot@...aro.org" <vincent.guittot@...aro.org>,
	Morten Rasmussen <Morten.Rasmussen@....com>,
	Chris Redpath <Chris.Redpath@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 0/8] change scheduler domain hierarchy set-up

On 20/12/13 14:00, Peter Zijlstra wrote:
> On Fri, Dec 13, 2013 at 12:11:20PM +0000, dietmar.eggemann@....com wrote:
>> From: Dietmar Eggemann <dietmar.eggemann@....com>
>>
>> This patch-set cleans up the scheduler domain level initialization code.
>> It is based on the idea of Peter Zijlstra to use a single scheduler domain
>> init function sketched here: https://lkml.org/lkml/2013/11/5/239 
>>
>> What does the patch-set try to achieve:
>>
>> 1) Let the arch define the conventional (here defined to all levels except
>>    the NUMA levels) scheduler domain hierarchy.  The arch specifies per
>>    scheduler domain the pointer to the getter function of the
>>    corresponding cpu mask as well as the topology related scheduler
>>    domain flags.
>>
>> 2) Unify the set-up code for conventional and NUMA scheduler domains.
>>    All scheduler domain topology levels are now allocated in the same
>>    function and the scheduler does not rely on a default scheduler
>>    domain topology array any more.  All scheduler domains now use a
>>    common initialization function which makes the existing SD_FOO_INIT
>>    macros redundant.
> 
> Yeah, still a tad confused on what you did there, need to look in more
> detail.

I will come up w/ a V2 of this patch set. So don't worry to review this
bit. I just thought that we can unify the existing code in
sched_init_numa() function w/ the conventional sched domain set-up.


> 
>> 3) The arch is no longer limited to the existing scheduler domain levels
>>    (SMT, MC, BOOK, CPU) but can easily define additional levels.
>>
>> 4) Prepare the mechanics to make it easier to integrate the provision of
>>    additional topology related data (e.g. energy information) to the
>>    scheduler.
> 
> Right, I was hoping you'd have a little more on that, but we'll get
> there I suppose ;-)

I still think that sd_energy will be an (optional) additional column in
xxx_topology[] (besides cpu mask func ptr and topology flags).
We're currently in the process of deriving something like this starting
from the use-cases described in Morten's email-set sent out on linux-pm
on 20/12/13 'Energy-aware scheduling use-cases and scheduler issues' via
an appropriate energy model.
The current patch set is more a preparation and clean-up exercise for
this so far.

> 
>> Current limitations:
>>
>> 1) The arch interface for scheduler domain set-up is only implemented for
>>    the ARM and the x86 arch and tested on an ARM TC2 (2 clusters, one with
>>    2 Cortex A15 and the other with 3 Cortex A7) and an Intel i5-520M (2
>>    cores with 2 threads each) platform.   
>>
>> 2) For other archs it has only been compile tested for certain
>>    configurations (powerpc: chroma_defconfig, mips: ip27_defconfig,
>>    s390: defconfig, tile: tilegx_defconfig).  Obviously, linking these
>>    kernels doesn't succeed due to the missing arch interface for
>>    scheduler domain set-up implementation (undefined reference to
>>    arch_sched_domain_info).
>>
>> 3) It does not delete the arch specific SD_FOO_INIT macros for ia64,
>>    metag, s390 and tile arch.
>>
>> 4) It does not delete the arch_sd_sibling_asym_packing function which
>>    will be redundant once the arch interface for scheduler domain set-up
>>    has been implemented for powerpc arch.
>>
>> 5) There is no default set-up any more.  Each arch has to define a
>>    arch_sched_domain_info array, a circumstance which might not be
>>    desirable.
> 
> Yeah, that's sad, I think we want to keep the default thing to limit the
> amount of pointless duplication for all archs that are not special.
> 
> Also, like you point out above, breaking all archs isn't nice :-)
>

Understood. V2 patch set will have a default set-up again.


>> 6) It has to be specified what happens when an arch specifies an
>>    arch_sched_domain_info array with only a { NULL, } entry.
> 
> Crash hard on boot :-) Although I suppose since its all compile time
> constants we could try and be smart and make the build fail somehow.
> 
> The one thing I do dislike is that you mixed SDTL_flags and SD_flags
> into a single variable. Don't do that its bound to collide and give
> weird results at some point, and its not like any of these structures
> are space critical in any way shape or form.

Understood. Will change that.

-- Dietmar
> 


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