Re: [RFC] sched: CPU topology try

[Preeti U Murthy]

Hi Vincent,

On 12/18/2013 06:43 PM, 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] https://lkml.org/lkml/2013/11/5/239
> [2] https://lkml.org/lkml/2013/11/5/449
> 
> Based on the results of this tests, my feeling about this new way to init the
> sched_domain is a bit mitigated.
> 
> The good point is that I have been able to create the same sched_domain
> topologies than before and even more complex ones (where a subset of the cores
> in a cluster share their powergating capabilities). I have described various
> topology results below.
> 
> I use a system that is made of a dual cluster of quad cores with hyperthreading
> for my examples.
> 
> If one cluster (0-7) can powergate its cores independantly but not the other
> cluster (8-15) we have the following topology, which is equal to what I had
> previously:
> 
> CPU0:
> domain 0: span 0-1 level: SMT
>     flags: SD_SHARE_CPUPOWER | SD_SHARE_PKG_RESOURCES | SD_SHARE_POWERDOMAIN
>     groups: 0 1
>   domain 1: span 0-7 level: MC
>       flags: SD_SHARE_PKG_RESOURCES
>       groups: 0-1 2-3 4-5 6-7
>     domain 2: span 0-15 level: CPU
>         flags:
>         groups: 0-7 8-15
> 
> CPU8
> domain 0: span 8-9 level: SMT
>     flags: SD_SHARE_CPUPOWER | SD_SHARE_PKG_RESOURCES | SD_SHARE_POWERDOMAIN
>     groups: 8 9
>   domain 1: span 8-15 level: MC
>       flags: SD_SHARE_PKG_RESOURCES | SD_SHARE_POWERDOMAIN
>       groups: 8-9 10-11 12-13 14-15
>     domain 2: span 0-15 level CPU
>         flags:
>         groups: 8-15 0-7
> 
> We can even describe some more complex topologies if a susbset (2-7) of the
> cluster can't powergate independatly:
> 
> CPU0:
> domain 0: span 0-1 level: SMT
>     flags: SD_SHARE_CPUPOWER | SD_SHARE_PKG_RESOURCES | SD_SHARE_POWERDOMAIN
>     groups: 0 1
>   domain 1: span 0-7 level: MC
>       flags: SD_SHARE_PKG_RESOURCES
>       groups: 0-1 2-7
>     domain 2: span 0-15 level: CPU
>         flags:
>         groups: 0-7 8-15
> 
> CPU2:
> domain 0: span 2-3 level: SMT
>     flags: SD_SHARE_CPUPOWER | SD_SHARE_PKG_RESOURCES | SD_SHARE_POWERDOMAIN
>     groups: 0 1
>   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
> 
> In this case, we have an aditionnal sched_domain MC level for this subset (2-7)
> of cores so we can trigger some load balance in this subset before doing that
> on the complete cluster (which is the last level of cache in my example)
> 
> 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)

The design looks good to me. In my opinion information like P-states and
C-states dependency can be kept separate from the topology levels, it
might get too complicated unless the information is tightly coupled to
the topology.

> 
> 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

I do not feel this is a problem since the levels are not duplicated,
rather they have different properties within them which is best
represented by flags like you have introduced in this patch.

> 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

The sched domain levels which have SD_SHARE_POWERDOMAIN set is expected
to have cpus which are a subset of the cpus that this domain would have
included had this flag not been set. In addition to this every higher
domain, irrespective of SD_SHARE_POWERDOMAIN being set, will include all
cpus of the lower domains. As far as I see, this patch does not change
these assumptions. Hence I am unable to imagine a scenario when the
parent might not include all cpus of its children domain. Do you have
such a scenario in mind which can arise due to this patch ?

Thanks

Regards
Preeti U Murthy

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