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SubjectRe: [PATCH v3] x86,sched: allow topologies where NUMA nodes share an LLC
On Wed, 28 Mar 2018, Alison Schofield wrote:
> From: Alison Schofield <alison.schofield@intel.com>
>
> Intel's Skylake Server CPUs have a different LLC topology than previous
> generations. When in Sub-NUMA-Clustering (SNC) mode, the package is
> divided into two "slices", each containing half the cores, half the LLC,
> and one memory controller and each slice is enumerated to Linux as a
> NUMA node. This is similar to how the cores and LLC were arranged
> for the Cluster-On-Die (CoD) feature.
>
> CoD allowed the same cache line to be present in each half of the LLC.
> But, with SNC, each line is only ever present in *one* slice. This
> means that the portion of the LLC *available* to a CPU depends on the
> data being accessed:
>
> Remote socket: entire package LLC is shared
> Local socket->local slice: data goes into local slice LLC
> Local socket->remote slice: data goes into remote-slice LLC. Slightly
> higher latency than local slice LLC.
>
> The biggest implication from this is that a process accessing all
> NUMA-local memory only sees half the LLC capacity.
>
> The CPU describes its cache hierarchy with the CPUID instruction. One
> of the CPUID leaves enumerates the "logical processors sharing this
> cache". This information is used for scheduling decisions so that tasks
> move more freely between CPUs sharing the cache.
>
> But, the CPUID for the SNC configuration discussed above enumerates
> the LLC as being shared by the entire package. This is not 100%
> precise because the entire cache is not usable by all accesses. But,
> it *is* the way the hardware enumerates itself, and this is not likely
> to change.
>
> This breaks the sane_topology() check in the smpboot.c code because
> this topology is considered not-sane. To fix this, add a vendor and
> model specific check to never call topology_sane() for these systems.
> Also, just like "Cluster-on-Die" we throw out the "coregroup"
> sched_domain_topology_level and use NUMA information from the SRAT
> alone.
>
> This is OK at least on the hardware we are immediately concerned about
> because the LLC sharing happens at both the slice and at the package
> level, which are also NUMA boundaries.

So that addresses the scheduler interaction, but it still leaves the
information in the sysfs files unchanged. See cpu/intel_cacheinfo.c. There
are applications which use that information so it should be correct.

Thanks,

tglx

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