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Subject[PATCH] Fix booted_cores calculation in set_cpu_sibling_map()
Date
git commit 316ad248307fba13be40f01e92a22b89457c32bc changed
set_cpu_sibling_map() to a single loop.

On a 2 socket system with 4 nodes, with 8 cores/socket top of tree
now shows

.> cat /proc/cpuinfo | grep cores
cpu cores : 2
cpu cores : 4
cpu cores : 5
cpu cores : 6
cpu cores : 7
cpu cores : 8
cpu cores : 9
cpu cores : 10
cpu cores : 2
cpu cores : 4
cpu cores : 5
cpu cores : 6
cpu cores : 7
cpu cores : 8
cpu cores : 9
cpu cores : 10
cpu cores : 15
cpu cores : 15
cpu cores : 13
cpu cores : 11
cpu cores : 9
cpu cores : 7
cpu cores : 5
cpu cores : 3
cpu cores : 15
cpu cores : 15
cpu cores : 13
cpu cores : 11
cpu cores : 9
cpu cores : 7
cpu cores : 5
cpu cores : 3
This is wrong. The kernel should show

cpu cores : 8

for each entry.

set_cpu_sibling_map() must be two separate loops, the first which
calculates the cpu's sibling_maps and the second which determines
how many cores have been booted. The code does a check on
(cpumask_weight(cpu_sibling_mask(cpu)). The problem is that
the sibling_mask may not be fully populated by the time this
value is checked resulting in incorrect booted_cores values.

I've found this broken and tested the fix on both AMD multi-socket and Intel
multi-socket systems (as well as single socket single node systems).
-----8<-----
The code in set_cpu_sibling_map() checks the value of
cpu_sibling_mask(cpu) before the mask is fully populated. This results in
an incorrect calculation of each cpu's booted_cores. Move back to a two
pass mechanism to properly calculate booted_cores.

Signed-off-by: Prarit Bhargava <prarit@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: x86@kernel.org
---
arch/x86/kernel/smpboot.c | 4 ++++
1 files changed, 4 insertions(+), 0 deletions(-)
diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index f56f96d..019248e 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -381,6 +381,10 @@ void __cpuinit set_cpu_sibling_map(int cpu)

if ((i == cpu) || (has_mc && match_llc(c, o)))
link_mask(llc_shared, cpu, i);
+ }
+
+ for_each_cpu(i, cpu_sibling_setup_mask) {
+ o = &cpu_data(i);

if ((i == cpu) || (has_mc && match_mc(c, o))) {
link_mask(core, cpu, i);
--
1.7.1


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