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

[Jeremy Linton]

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