Re: [PATCH] x86, sched: Allow NUMA nodes to share an LLC on Intel platforms

[Peter Zijlstra <peterz@infradead.org>]

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_ :-(