Patch "x86/cpu/AMD: Fix cpu_llc_id for AMD Fam17h systems" has been added to the 4.8-stable tree

This is a note to let you know that I've just added the patch titled

x86/cpu/AMD: Fix cpu_llc_id for AMD Fam17h systems

to the 4.8-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:

     x86-cpu-amd-fix-cpu_llc_id-for-amd-fam17h-systems.patch

and it can be found in the queue-4.8 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Yazen Ghannam <[email protected]>
Date: Tue, 8 Nov 2016 09:35:06 +0100
Subject: x86/cpu/AMD: Fix cpu_llc_id for AMD Fam17h systems

From: Yazen Ghannam <[email protected]>

commit b0b6e86846093c5f8820386bc01515f857dd8faa upstream.

cpu_llc_id (Last Level Cache ID) derivation on AMD Fam17h has an underflow bug when extracting the socket_id value. It starts from 0 so subtracting 1 from it will result in an invalid value. This breaks scheduling topology later on since the cpu_llc_id will be incorrect.

For example, the the cpu_llc_id of the *other* CPU in the loops in set_cpu_sibling_map() underflows and we're generating the funniest thread_siblings masks and then when I run 8 threads of nbench, they get spread around the LLC domains in a very strange pattern which doesn't give you the normal scheduling spread one would expect for performance.

Other things like EDAC use cpu_llc_id so they will be b0rked too.

So, the APIC ID is preset in APICx020 for bits 3 and above: they contain the core complex, node and socket IDs.

The LLC is at the core complex level so we can find a unique cpu_llc_id by right shifting the APICID by 3 because then the least significant bit will be the Core Complex ID.

---
 arch/x86/kernel/cpu/amd.c |    6 +-----
 1 file changed, 1 insertion(+), 5 deletions(-)

@@ -347,7 +347,6 @@ static void amd_detect_cmp(struct cpuinf
 #ifdef CONFIG_SMP
 	unsigned bits;
 	int cpu = smp_processor_id();
-	unsigned int socket_id, core_complex_id;
 
 	bits = c->x86_coreid_bits;
 	/* Low order bits define the core id (index of core in socket) */

@@ -365,10 +364,7 @@ static void amd_detect_cmp(struct cpuinf
 	 if (c->x86 != 0x17 || !cpuid_edx(0x80000006))
 		return;
 
-	socket_id	= (c->apicid >> bits) - 1;
-	core_complex_id	= (c->apicid & ((1 << bits) - 1)) >> 3;
-
-	per_cpu(cpu_llc_id, cpu) = (socket_id << 3) | core_complex_id;
+	per_cpu(cpu_llc_id, cpu) = c->apicid >> 3;
 #endif
 }
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.8/x86-cpu-amd-fix-cpu_llc_id-for-amd-fam17h-systems.patch