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SubjectRe: [PATCH] x86, sched: Allow NUMA nodes to share an LLC on Intel platforms
On Tue, Feb 09, 2021 at 02:39:43PM -0800, Alison Schofield wrote:

> Commit 1340ccfa9a9a ("x86,sched: Allow topologies where NUMA nodes
> share an LLC") added a vendor and model specific check to skip the
> topology_sane() check for Intel's Sky Lake Server CPUs where NUMA
> nodes shared an LLC.
>
> This topology is no longer a quirk for Intel CPUs as Ice Lake and
> Sapphire Rapids CPUs exhibit the same topology. Rather than maintain
> the quirk list, define a synthetic flag that directs the scheduler
> to allow this topology without warning for all Intel CPUs when NUMA
> is configured.

Hurmph, I still think it's daft.

> diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
> index 816fdbec795a..027348261080 100644
> --- a/arch/x86/kernel/cpu/intel.c
> +++ b/arch/x86/kernel/cpu/intel.c
> @@ -719,6 +719,21 @@ static void init_intel(struct cpuinfo_x86 *c)
> tsx_disable();
>
> split_lock_init();
> +
> + /*
> + * Set X86_BUG_NUMA_SHARES_LLC to allow topologies where NUMA
> + * nodes share an LLC. In Sub-NUMA Clustering mode Intel CPUs
> + * may enumerate an LLC as shared by multiple NUMA nodes. The
> + * LLC is shared for off-package data access but private to
> + * the NUMA node for on-package access. This topology first
> + * appeared in SKYLAKE_X. It was treated as a quirk and allowed.
> + * This topology reappeared in ICELAKE_X and SAPPHIRERAPIDS_X.
> + * Rather than maintain a list of quirk CPUS, allow this topology
> + * on all Intel CPUs with NUMA configured. When this X86_BUG is
> + * set, the scheduler accepts this topology without warning.
> + */
> + if (IS_ENABLED(CONFIG_NUMA))
> + set_cpu_bug(c, X86_BUG_NUMA_SHARES_LLC);
> }

This seens wrong too, it shouldn't be allowed pre SKX. And ideally only
be allowed when SNC is enabled.

Please make this more specific than: all Intel CPUs. Ofcourse, since you
all knew this was an issue, you could've made it discoverable
_somewhere_ :-(

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