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
