[RFC PATCH] topology: Represent clusters of CPUs within a die.

[Greg Kroah-Hartman]

On Fri, Oct 16, 2020 at 11:27:02PM +0800, Jonathan Cameron wrote:
> Both ACPI and DT provide the ability to describe additional layers of
> topology between that of individual cores and higher level constructs
> such as the level at which the last level cache is shared.
> In ACPI this can be represented in PPTT as a Processor Hierarchy
> Node Structure [1] that is the parent of the CPU cores and in turn
> has a parent Processor Hierarchy Nodes Structure representing
> a higher level of topology.
> 
> For example Kunpeng 920 has clusters of 4 CPUs.  These do not share
> any cache resources, but the interconnect topology is such that
> the cost to transfer ownership of a cacheline between CPUs within
> a cluster is lower than between CPUs in different clusters on the same
> die.   Hence, it can make sense to deliberately schedule threads
> sharing data to a single cluster.
> 
> This patch simply exposes this information to userspace libraries
> like hwloc by providing cluster_cpus and related sysfs attributes.
> PoC of HWLOC support at [2].
> 
> Note this patch only handle the ACPI case.
> 
> Special consideration is needed for SMT processors, where it is
> necessary to move 2 levels up the hierarchy from the leaf nodes
> (thus skipping the processor core level).
> 
> Currently the ID provided is the offset of the Processor
> Hierarchy Nodes Structure within PPTT.  Whilst this is unique
> it is not terribly elegant so alternative suggestions welcome.
> 
> Note that arm64 / ACPI does not provide any means of identifying
> a die level in the topology but that may be unrelate to the cluster
> level.
> 
> RFC questions:
> 1) Naming
> 2) Related to naming, do we want to represent all potential levels,
>    or this enough?  On Kunpeng920, the next level up from cluster happens
>    to be covered by llc cache sharing, but in theory more than one
>    level of cluster description might be needed by some future system.
> 3) Do we need DT code in place? I'm not sure any DT based ARM64
>    systems would have enough complexity for this to be useful.
> 4) Other architectures?  Is this useful on x86 for example?
> 
> [1] ACPI Specification 6.3 - section 5.2.29.1 processor hierarchy node
>     structure (Type 0)
> [2] https://github.com/hisilicon/hwloc/tree/linux-cluster
> 
> Signed-off-by: Jonathan Cameron <Jonathan.Cameron at huawei.com>
> ---
> 
>  Documentation/admin-guide/cputopology.rst | 26 ++++++++--

You are adding new sysfs files here, but not adding Documentation/ABI/
entries as well?  This cputopology document is nice, but no one knows to
look there for sysfs stuff :)

thanks,

greg k-h