linux-kernel - Re: AMD Bulldozer topology regression since 4.6

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Date:   Tue, 29 Nov 2016 20:39:23 +0100
From:   Borislav Petkov <bp@...e.de>
To:     Brice Goglin <Brice.Goglin@...ia.fr>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: AMD Bulldozer topology regression since 4.6

On Tue, Nov 29, 2016 at 08:03:15AM +0100, Brice Goglin wrote:
> This causes user-space tools (at least all that rely on hwloc for
> topology discovery) to think the processor has dual-threaded cores
> instead of dual-core compute-unit modules.
> 
> The cause is likely this patch, which seems to assume both cores of a
> same compute-unit have the same ID, which is wrong?

Does that fix it?

Patch is against latest tip/master because we have some more changes in
that area.

---
diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
index 4daad1e39352..a070d7b0a133 100644
--- a/arch/x86/kernel/cpu/amd.c
+++ b/arch/x86/kernel/cpu/amd.c
@@ -305,15 +305,7 @@ static void amd_get_topology(struct cpuinfo_x86 *c)
 
 	/* get information required for multi-node processors */
 	if (boot_cpu_has(X86_FEATURE_TOPOEXT)) {
-		u32 eax, ebx, ecx, edx;
-
-		cpuid(0x8000001e, &eax, &ebx, &ecx, &edx);
-		node_id = ecx & 7;
-
-		/* get compute unit information */
-		smp_num_siblings = ((ebx >> 8) & 3) + 1;
-		c->x86_max_cores /= smp_num_siblings;
-		c->cpu_core_id = ebx & 0xff;
+		node_id = cpuid_ecx(0x8000001e) & 7;
 
 		/*
 		 * We may have multiple LLCs if L3 caches exist, so check if we
---

Just in case, I have one against Linus' master too:

---
diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
index 1e81a37c034e..19f50a9d6b42 100644
--- a/arch/x86/kernel/cpu/amd.c
+++ b/arch/x86/kernel/cpu/amd.c
@@ -305,15 +305,7 @@ static void amd_get_topology(struct cpuinfo_x86 *c)
 
 	/* get information required for multi-node processors */
 	if (boot_cpu_has(X86_FEATURE_TOPOEXT)) {
-		u32 eax, ebx, ecx, edx;
-
-		cpuid(0x8000001e, &eax, &ebx, &ecx, &edx);
-		node_id = ecx & 7;
-
-		/* get compute unit information */
-		smp_num_siblings = ((ebx >> 8) & 3) + 1;
-		c->x86_max_cores /= smp_num_siblings;
-		c->cpu_core_id = ebx & 0xff;
+		node_id = cpuid_ecx(0x8000001e) & 7;
 	} else if (cpu_has(c, X86_FEATURE_NODEID_MSR)) {
 		u64 value;
---

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
--