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@xxxxxxxxxx> > --- > > 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
