Re: [PATCH v3 6/7] arm64: topology: Enable ACPI/PPTT based CPU topology.

[Jeremy Linton <jeremy.linton@arm.com>]

Hi,

On 10/20/2017 04:14 AM, Lorenzo Pieralisi wrote:
> On Thu, Oct 19, 2017 at 11:13:27AM -0500, Jeremy Linton wrote:
>> On 10/19/2017 10:56 AM, Lorenzo Pieralisi wrote:
>>> On Thu, Oct 12, 2017 at 02:48:55PM -0500, Jeremy Linton wrote:
>>>> Propagate the topology information from the PPTT tree to the
>>>> cpu_topology array. We can get the thread id, core_id and
>>>> cluster_id by assuming certain levels of the PPTT tree correspond
>>>> to those concepts. The package_id is flagged in the tree and can be
>>>> found by passing an arbitrary large level to setup_acpi_cpu_topology()
>>>> which terminates its search when it finds an ACPI node flagged
>>>> as the physical package. If the tree doesn't contain enough
>>>> levels to represent all of thread/core/cod/package then the package
>>>> id will be used for the missing levels.
>>>>
>>>> Since server/ACPI machines are more likely to be multisocket and NUMA,
>>>
>>> I think this stuff is vague enough already so to start with I would drop
>>> patch 4 and 5 and stop assuming what machines are more likely to ship
>>> with ACPI than DT.
>>>
>>> I am just saying, for the umpteenth time, that these levels have no
>>> architectural meaning _whatsoever_, level is a hierarchy concept
>>> with no architectural meaning attached.
>>
>> ?
>>
>> Did anyone say anything about that? No, I think the only thing being
>> guaranteed here is that the kernel's physical_id maps to an ACPI
>> defined socket. Which seems to be the mindset of pretty much the
>> entire !arm64 community meaning they are optimizing their software
>> and the kernel with that concept in mind.
>>
>> Are you denying the existence of non-uniformity between threads
>> running on different physical sockets?
> 
> No, I have not explained my POV clearly, apologies.
> 
> AFAIK, the kernel currently deals with 2 (3 - if SMT) topology layers.
> 
> 1) thread
> 2) core
> 3) package
> 
> What I wanted to say is, that, to simplify this series, you do not need
> to introduce the COD topology level, since it is just another arbitrary
> topology level (ie there is no way you can pinpoint which level
> corresponds to COD with PPTT - or DT for the sake of this discussion)
> that would not be used in the kernel (apart from big.LITTLE cpufreq
> driver and PSCI checker whose usage of topology_physical_package_id() is
> questionable anyway).

Oh! But, i'm at a loss as to what to do with those two users if I set 
the node which has the physical socket flag set, as the "cluster_id" in 
the topology.

Granted, this being ACPI I don't expect the cpufreq driver to be active 
(given CPPC) and the psci checker might be ignored? Even so, its a bit 
of a misnomer what is actually happening. Are we good with this?


> 
> PPTT allows you to define what level corresponds to a package, use
> it to initialize the package topology level (that on ARM internal
> variables we call cluster) and be done with it.
> 
> I do not think that adding another topology level improves anything as
> far as ACPI topology detection is concerned, you are not able to use it
> in the scheduler or from userspace to group CPUs anyway.

Correct, and AFAIK after having poked a bit at the scheduler its sort of 
redundant as the generic cache sharing levels are more useful anyway.

> 
> Does this answer your question ?
Yes, other than what to do with the two drivers.

> 
> Thanks,
> Lorenzo
>