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

[Jeffrey Hugo]

On 10/20/2017 10:14 AM, Jeremy Linton wrote:
> 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.

What do you mean, it can't be used?  We expect a followup series which 
uses PPTT to define scheduling domains/groups.

The scheduler supports 4 types of levels, with an arbitrary number of 
instances of each - NUMA, DIE (package, usually not used with NUMA), MC 
(multicore, typically cores which share resources like cache), SMT 
(threads).

Our particular platform has a single socket/package, with multiple 
"clusters", each cluster consisting of multiple cores that share caches. 
 We represent all of this in PPTT, and expect it to be used.  Leaf 
nodes are cores.  The level above is the cluster.  The top level is the 
package.  We expect eventually (and understand that Jeremy is not 
tackling this with his current series) that clusters get represented MC 
so that migrated processes prefer their cache-shared siblings, and the 
entire package is represented by DIE.

This will have to come from PPTT since you can't use core_siblings to 
derive this.  Additionally, if we had multiple layers of clustering, we 
would expect each layer to be represented by MC.  Topology.c has none of 
this support today.

PPTT can refer to SLIT/SRAT to determine if a hirearchy level 
corresponds to the "Cluster-on-Die" concept of other architectures 
(which end up as NUMA nodes in NUMA scheduling domains).

What PPTT will have to do is parse the tree(s), determine what each 
level is - SMT, MC, NUMA, DIE - and then use set_sched_topology() so 
that the scheduler can build up groups/domains appropriately.


Jeremy, we've tested v3 on our platform.  The topology part works as 
expected, we no longer see lstopo reporting sockets where there are 
none, but the scheduling groups are broken (expected).  Caches still 
don't work right (no sizes reported, and the sched caches are not 
attributed to the cores).  We will likely have additional comments as we 
delve into it.
> > Does this answer your question ?
> Yes, other than what to do with the two drivers.
>
> > Thanks,
> > Lorenzo


--
Jeffrey Hugo
Qualcomm Datacenter Technologies as an affiliate of Qualcomm 
Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the
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