To: Borislav Petkov <bp@suse.de>, LKML <linux-kernel@vger.kernel.org>
From: Brice Goglin <Brice.Goglin@inria.fr>
Subject: AMD Bulldozer topology regression since 4.6
Message-ID: <583D2833.4030508@inria.fr>
Date: Tue, 29 Nov 2016 08:03:15 +0100
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Hello

Since Linux 4.6 (and still in 4.9-rc5 at least), both AMD Bulldozer
cores of a single dual-core compute unit report the same core_id:

$ cat /sys/devices/system/cpu/cpu{?,??}/topology/core_id
0
0
1
1
2
2
3
0
3
[...]


Before 4.5 (and for a very long time), the kernel reported different
core_ids:

0
1
2
3
4
5
6
7
0
[...]


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?

commit 8196dab4fc159943df6baaac04973bb1accb7100
Author: Borislav Petkov <bp@suse.de>
Date:   Fri Mar 25 15:52:36 2016 +0100

    x86/cpu: Get rid of compute_unit_id
    
    It is cpu_core_id anyway.
    
    Signed-off-by: Borislav Petkov <bp@suse.de>
    Link: http://lkml.kernel.org/r/1458917557-8757-3-git-send-email-bp@alien8.de
    Signed-off-by: Thomas Gleixner <tglx@linutronix.de>


Thanks
Brice