Message-ID: <531B0FDA.2040302@arm.com>
Date:	Sat, 08 Mar 2014 12:40:58 +0000
From:	Dietmar Eggemann <dietmar.eggemann@....com>
To:	Vincent Guittot <vincent.guittot@...aro.org>,
	"peterz@...radead.org" <peterz@...radead.org>
CC:	"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 07/03/14 02:47, Vincent Guittot wrote:
> On 6 March 2014 20:31, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
>> On 06/03/14 09:04, Vincent Guittot wrote:
>>>
>>> On 6 March 2014 07:17, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
>>>>
>>>> On 05/03/14 07:18, Vincent Guittot wrote:
>>>>>
>>>>>
>>>>> This patchset was previously part of the larger tasks packing patchset
>>>>> [1].
>>>>> I have splitted the latter in 3 different patchsets (at least) to make
>>>>> the
>>>>> thing easier.
>>>>> -configuration of sched_domain topology (this patchset)
>>>>> -update and consolidation of cpu_power
>>>>> -tasks packing algorithm
>>>>>
>>>>> Based on Peter Z's proposal [2][3], this patchset modifies the way to
>>>>> configure
>>>>> the sched_domain level in order to let architectures to add specific
>>>>> level
>>>>> like
>>>>> the current BOOK level or the proposed power gating level for ARM
>>>>> architecture.
>>>>>
>>>>> [1] https://lkml.org/lkml/2013/10/18/121
>>>>> [2] https://lkml.org/lkml/2013/11/5/239
>>>>> [3] https://lkml.org/lkml/2013/11/5/449
>>>>>
>>>>> Vincent Guittot (6):
>>>>>      sched: remove unused SCHED_INIT_NODE
>>>>>      sched: rework of sched_domain topology definition
>>>>>      sched: s390: create a dedicated topology table
>>>>>      sched: powerpc: create a dedicated topology table
>>>>>      sched: add a new SD_SHARE_POWERDOMAIN for sched_domain
>>>>>      sched: ARM: create a dedicated scheduler topology table
>>>>>
>>>>>     arch/arm/kernel/topology.c        |   26 ++++
>>>>>     arch/ia64/include/asm/topology.h  |   24 ----
>>>>>     arch/metag/include/asm/topology.h |   27 -----
>>>>>     arch/powerpc/kernel/smp.c         |   35 ++++--
>>>>>     arch/s390/include/asm/topology.h  |   13 +-
>>>>>     arch/s390/kernel/topology.c       |   25 ++++
>>>>>     arch/tile/include/asm/topology.h  |   33 ------
>>>>>     include/linux/sched.h             |   30 +++++
>>>>>     include/linux/topology.h          |  128 ++------------------
>>>>>     kernel/sched/core.c               |  235
>>>>> ++++++++++++++++++-------------------
>>>>>     10 files changed, 237 insertions(+), 339 deletions(-)
>>>>>
>>>>
>>>> Hi Vincent,
>>>>
>>>> I reviewed your patch-set carefully (including test runs on TC2),
>>>> especially
>>>> due to the fact that we want to build our sd_energy stuff on top of it.
>>>
>>>
>>> Thanks
>>>
>>>>
>>>>
>>>> One thing I'm still not convinced of is the fact that specifying
>>>> additional
>>>> sd levels in the struct sched_domain_topology_level table has an
>>>> advantage
>>>> over a function pointer for sd topology flags similar to the one we're
>>>> already using for the cpu mask in struct sched_domain_topology_level.
>>>>
>>>>     int (*sched_domain_flags_f)(int cpu);
>>>>
>>>
>>> We have to create additional level for some kind of topology as
>>> described in my trial https://lkml.org/lkml/2013/12/18/279 which is
>>> not possible with function pointer.
>>
>>
>> This is what I don't understand at the moment. In your example in the link
>> above, (2 cluster of 4 cores with SMT), cpu 0-7 can powergate while cpu 8-15
>> can't). Why can't we have
>
> The 2nd example is a bit more complicated and needs an additional
> level to describe the topology

I see. In the 2. example you want to have an additional MC level for cpu 
2-7 because you want to do load balance between those cpus more often 
than for cpu 0-7 for dst cpus from the set (2-7). Not sure if such 
topologies (only a subset of cpus inside a cluster can't be powergated) 
exists today in the real world though?

from https://lkml.org/lkml/2013/12/18/279:

CPU2:
domain 0: span 2-3 level: SMT
     flags: SD_SHARE_CPUPOWER | SD_SHARE_PKG_RESOURCES | 
SD_SHARE_POWERDOMAIN
     groups: 0 1              <-- Doesn't this have to be 'groups: 2 3'
   domain 1: span 2-7 level: MC
       flags: SD_SHARE_PKG_RESOURCES | SD_SHARE_POWERDOMAIN
       groups: 2-7 4-5 6-7
     domain 2: span 0-7 level: MC
         flags: SD_SHARE_PKG_RESOURCES
         groups: 2-7 0-1
       domain 3: span 0-15 level: CPU
           flags:
           groups: 0-7 8-15

>
>>
>> static inline int cpu_coregroup_flags(int cpu)
>> {
>>      int flags = SD_SHARE_PKG_RESOURCES;
>>
>>      if (cpu >= 8)
>>          flags |= SD_SHARE_POWERDOMAIN;
>>
>>      return flags;
>> }
>>
>> inside the arch specific topology file and use it in the MC level as the int
>> (*sched_domain_flags_f)(int cpu) member of struct
>> sched_domain_topology_level?
>>
>>
>>>
>>> Have you got a situation in mind where it will be necessary to use the
>>> function pointer with cpu number as an argument ?
>>
>>
>> The one above. Fundamentally all situations where you want to set sd
>> topology flags differently for different cpus in the same sd level.
>> big.LITTLE is another example but it's the same as powergated/!powergated in
>> this perspective.
>
> You see the flag of a level as something that can be changed per cpu
> whereas the proposal is to define a number of level with interesting
> properties for the scheduler and to associate a mask of cpus to this
> properties.

That's right.

>
> 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?

>
> Peter,
>
> Was the use of the cpu as a parameter in the initialization of
> sched_domain's flag a reason for asking for reworking the
> initialization of sched_domain ?
>
>>
>>
>>>
>>> In the current example of this patchset, the flags are statically set
>>> in the table but nothing prevents an architecture to update the flags
>>> value before being given to the scheduler
>>
>>
>> What will be the infrastructure if the arch wants to do so?
>
> I'm not sure to catch your point. The arch is now able to defines its
> own table and fill it before giving it to the scheduler so nothing
> prevents to set/update the flags field according the system
> configuration during boot sequence.

Sorry, misunderstanding from my side in the first place. You just said 
that the arch is able to change those flags (due to the arch specific 
topology table) and I related it to the possibility for the arch to set 
the topology flags per cpu.

>
> Thanks,
> Vincent
>>
>> Thanks,
>>
>> -- Dietmar
>>
>>
>>>
>>>> This function pointer would be simply another member of struct
>>>> sched_domain_topology_level and would replace int sd_flags.  AFAICS, you
>>>> have to create additional cpu mask functions anyway for the additional sd
>>>> levels, like cpu_corepower_mask() for the  GMC level in the ARM case.
>>>> There
>>>> could be a set of standard sd topology flags function for the default sd
>>>> layer and archs which want to pass in something special define those
>>>> function locally since they will use them only in their arch specific
>>>> struct
>>>> sched_domain_topology_level table anyway.  I know that you use the
>>>> existing
>>>> sd degenerate functionality for this and that the freeing of the
>>>> redundant
>>>> data structures (sched_domain, sched_group and sched_group_power) is
>>>> there
>>>> too but it still doesn't seem to me to be the right thing to do.
>>>>
>>>> The problem that we now expose internal data structures (struct sd_data
>>>> and
>>>> struct sched_domain_topology_level) could be dealt with later.
>>>>
>>>> -- Dietmar
>>>>
>>>
>>
>>
>


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