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SubjectRe: [PATCH v6 1/7] Documentation: DT: arm: add support for sockets defining package boundaries
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@arm.com>
> >
> >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.

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).

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.

Morten

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