Re: [PATCH v6 1/7] Documentation: DT: arm: add support for sockets defining package boundaries

[Morten Rasmussen]

On Thu, May 30, 2019 at 08:56:03AM -0400, Andrew F. Davis wrote:
> On 5/30/19 7:51 AM, Morten Rasmussen wrote:
> >On Wed, May 29, 2019 at 07:39:17PM -0400, Andrew F. Davis wrote:
> >>On 5/29/19 5:13 PM, Atish Patra wrote:
> >>>From: Sudeep Holla <sudeep.holla@xxxxxxx>
> >>>
> >>>The current ARM DT topology description provides the operating system
> >>>with a topological view of the system that is based on leaf nodes
> >>>representing either cores or threads (in an SMT system) and a
> >>>hierarchical set of cluster nodes that creates a hierarchical topology
> >>>view of how those cores and threads are grouped.
> >>>
> >>>However this hierarchical representation of clusters does not allow to
> >>>describe what topology level actually represents the physical package or
> >>>the socket boundary, which is a key piece of information to be used by
> >>>an operating system to optimize resource allocation and scheduling.
> >>>
> >>
> >>Are physical package descriptions really needed? What does "socket" imply
> >>that a higher layer "cluster" node grouping does not? It doesn't imply a
> >>different NUMA distance and the definition of "socket" is already not well
> >>defined, is a dual chiplet processor not just a fancy dual "socket" or are
> >>dual "sockets" on a server board "slotket" card, will we need new names for
> >>those too..
> >
> >Socket (or package) just implies what you suggest, a grouping of CPUs
> >based on the physical socket (or package). Some resources might be
> >associated with packages and more importantly socket information is
> >exposed to user-space. At the moment clusters are being exposed to
> >user-space as sockets which is less than ideal for some topologies.
> >
> 
> I see the benefit of reporting the physical layout and packaging information
> to user-space for tracking reasons, but from software perspective this
> doesn't matter, and the resource partitioning should be described elsewhere
> (NUMA nodes being the go to example).

That would make defining a NUMA node mandatory even for non-NUMA
systems?

> >At the moment user-space is only told about hw threads, cores, and
> >sockets. In the very near future it is going to be told about dies too
> >(look for Len Brown's multi-die patch set).
> >
> 
> Seems my hypothetical case is already in the works :(

Indeed. IIUC, the reasoning behind it is related to actual multi-die
x86 packages and some rapl stuff being per-die or per-core.

> 
> >I don't see how we can provide correct information to user-space based
> >on the current information in DT. I'm not convinced it was a good idea
> >to expose this information to user-space to begin with but that is
> >another discussion.
> >
> 
> Fair enough, it's a little late now to un-expose this info to userspace so
> we should at least present it correctly. My worry was this getting out of
> hand with layering, for instance what happens when we need to add die nodes
> in-between cluster and socket?

If we want the die mask to be correct for arm/arm64/riscv we need die
information from somewhere. I'm not in favour of adding more topology
layers to the user-space visible topology description, but others might
have a valid reason and if it is exposed I would prefer if we try to
expose the right information.

Btw, for packages, we already have that information in ACPI/PPTT so it
would be nice if we could have that for DT based systems too.

Morten