linux-kernel - Re: [RFC][PATCH] x86, sched: allow topolgies where NUMA nodes share an LLC

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Date:   Tue, 7 Nov 2017 09:30:19 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     linux-kernel@...r.kernel.org, tony.luck@...el.com,
        tim.c.chen@...ux.intel.com, hpa@...ux.intel.com, bp@...en8.de,
        rientjes@...gle.com, imammedo@...hat.com, prarit@...hat.com,
        toshi.kani@...com, brice.goglin@...il.com, mingo@...nel.org
Subject: Re: [RFC][PATCH] x86, sched: allow topolgies where NUMA nodes share
 an LLC

On Mon, Nov 06, 2017 at 02:15:00PM -0800, Dave Hansen wrote:

> 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.

So CPUID and SRAT will remain inconsistent; even in future products?
That would absolutely blow chunks.

If that is the case, we'd best use a fake feature like
X86_BUG_TOPOLOGY_BROKEN and use that instead of an ever growing list of
models in this code.

> +/*
> + * Set if a package/die has multiple NUMA nodes inside.
> + * AMD Magny-Cours, Intel Cluster-on-Die, and Intel
> + * Sub-NUMA Clustering have this.
> + */
> +static bool x86_has_numa_in_package;
> +
>  static bool match_llc(struct cpuinfo_x86 *c, struct cpuinfo_x86 *o)
>  {
>  	int cpu1 = c->cpu_index, cpu2 = o->cpu_index;
>  
> +	/* Do not match if we do not have a valid APICID for cpu: */
> +	if (per_cpu(cpu_llc_id, cpu1) == BAD_APICID)
> +		return false;
> +
> +	/* Do not match if LLC id does not match: */
> +	if (per_cpu(cpu_llc_id, cpu1) != per_cpu(cpu_llc_id, cpu2))
> +		return false;
>  
> +	/*
> +	 * Some Intel CPUs enumerate an LLC that is shared by
> +	 * multiple NUMA nodes.  The LLC on these systems is
> +	 * shared for off-package data acccess but private to the
> +	 * NUMA node (half of the package) for on-package access.
> +	 *
> +	 * CPUID can only enumerate the cache as being shared *or*
> +	 * unshared, but not this particular configuration.  The
> +	 * CPU in this case enumerates the cache to be shared
> +	 * across the entire package (spanning both NUMA nodes).
> +	 */
> +	if (!topology_same_node(c, o) &&
> +	    (c->x86_model == INTEL_FAM6_SKYLAKE_X)) {

This needs a c->x86_vendor test; imagine the fun when AMD releases a
part with model == SKX ...

> +		/* Use NUMA instead of coregroups for scheduling: */
> +		x86_has_numa_in_package = true;
> +
> +		/*
> +		 * Now, tell the truth, that the LLC matches. But,
> +		 * note that throwing away coregroups for
> +		 * scheduling means this will have no actual effect.
> +		 */
> +		return true;

What are the ramifications here? Is anybody else using that cpumask
outside of the scheduler topology setup?

> +	}
> +
> +	return topology_sane(c, o, "llc");
>  }