Re: [PATCH 2/7] node: Add heterogenous memory performance

[Keith Busch <keith.busch@intel.com>]

On Mon, Nov 26, 2018 at 11:00:09PM -0800, Dan Williams wrote:
> On Wed, Nov 14, 2018 at 2:53 PM Keith Busch <keith.busch@intel.com> wrote:
> >
> > Heterogeneous memory systems provide memory nodes with latency
> > and bandwidth performance attributes that are different from other
> > nodes. Create an interface for the kernel to register these attributes
> > under the node that provides the memory. If the system provides this
> > information, applications can query the node attributes when deciding
> > which node to request memory.
> >
> > When multiple memory initiators exist, accessing the same memory target
> > from each may not perform the same as the other. The highest performing
> > initiator to a given target is considered to be a local initiator for
> > that target. The kernel provides performance attributes only for the
> > local initiators.
> >
> > The memory's compute node should be symlinked in sysfs as one of the
> > node's initiators.
> >
> > The following example shows the new sysfs hierarchy for a node exporting
> > performance attributes:
> >
> >   # tree /sys/devices/system/node/nodeY/initiator_access
> >   /sys/devices/system/node/nodeY/initiator_access
> >   |-- read_bandwidth
> >   |-- read_latency
> >   |-- write_bandwidth
> >   `-- write_latency
> 
> With the expectation that there will be nodes that are initiator-only,
> target-only, or both I think this interface should indicate that. The
> 1:1 "local" designation of HMAT should not be directly encoded in the
> interface, it's just a shortcut for finding at least one initiator in
> the set that can realize the advertised performance. At least if the
> interface can enumerate the set of initiators then it becomes clear
> whether sysfs can answer a performance enumeration question or if the
> application needs to consult an interface with specific knowledge of a
> given initiator-target pairing.
> 
> It seems a precursor to these patches is arranges for offline node
> devices to be created for the ACPI proximity domains that are
> offline-by default for reserved memory ranges.

The intention is that all initiators symlinked to the memory node share
the initiator_access attributes, as well as itself the node is its own
initiator. There's no limit to how many the new kernel interface in
patch 1/7 allows you to register, so it's not really a 1:1 relationship.

Either instead or in addition to the symlinks, we can export a node_mask
in the initiator_access directory for which these access attributes
apply if that makes the intention more clear.