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Date:	Mon, 10 Mar 2014 14:21:59 +0100
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Dietmar Eggemann <dietmar.eggemann@....com>
Cc:	"peterz@...radead.org" <peterz@...radead.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 8 March 2014 13:40, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
> 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?

Be sure that such topology is studied and considered by HW guys

>
> 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?

With such function, an arch could set differently some topology flags
like SD_SHARE_PKG_RESOURCES and SD_SHARE_CPUPOWER in the same sd level
which make the notion of sd level meaningless.

>
>
>>
>> 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.

OK, i was not sure that your point was only about the cpu argument for
the SD's flags configuration

Vincent

>
>
>>
>> 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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