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Date:   Tue, 18 Dec 2018 04:42:52 -0800
From:   tip-bot for Hui Wang <tipbot@zytor.com>
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To:     linux-tip-commits@vger.kernel.org
Subject: [tip:x86/cpu] x86/topology: Use total_cpus for max logical packages
 calculation
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Commit-ID:  aa02ef099cff042c2a9109782ec2bf1bffc955d4
Gitweb:     https://git.kernel.org/tip/aa02ef099cff042c2a9109782ec2bf1bffc955d4
Author:     Hui Wang <john.wanghui@huawei.com>
AuthorDate: Wed, 7 Nov 2018 10:36:43 +0800
Committer:  Thomas Gleixner <tglx@linutronix.de>
CommitDate: Tue, 18 Dec 2018 13:38:37 +0100

x86/topology: Use total_cpus for max logical packages calculation

nr_cpu_ids can be limited on the command line via nr_cpus=. This can break the
logical package management because it results in a smaller number of packages
while in kdump kernel.

Check below case:
There is a two sockets system, each socket has 8 cores, which has 16 logical
cpus while HT was turn on.

 0  1  2  3  4  5  6  7     |    16 17 18 19 20 21 22 23
 cores on socket 0               threads on socket 0
 8  9 10 11 12 13 14 15     |    24 25 26 27 28 29 30 31
 cores on socket 1               threads on socket 1

While starting the kdump kernel with command line option nr_cpus=16 panic
was triggered on one of the cpus 24-31 eg. 26, then online cpu will be
1-15, 26(cpu 0 was disabled in kdump), ncpus will be 16 and
__max_logical_packages will be 1, but actually two packages were booted on.

This issue can reproduced by set kdump option nr_cpus=<real physical core
numbers>, and then trigger panic on last socket's thread, for example:

taskset -c 26 echo c > /proc/sysrq-trigger

Use total_cpus which will not be limited by nr_cpus command line to calculate
the value of __max_logical_packages.

Signed-off-by: Hui Wang <john.wanghui@huawei.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: <guijianfeng@huawei.com>
Cc: <wencongyang2@huawei.com>
Cc: <douliyang1@huawei.com>
Cc: <qiaonuohan@huawei.com>
Link: https://lkml.kernel.org/r/20181107023643.22174-1-john.wanghui@huawei.com

---
 arch/x86/kernel/smpboot.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index a9134d1910b9..ccd1f2a8e557 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -1347,7 +1347,7 @@ void __init calculate_max_logical_packages(void)
 	 * extrapolate the boot cpu's data to all packages.
 	 */
 	ncpus = cpu_data(0).booted_cores * topology_max_smt_threads();
-	__max_logical_packages = DIV_ROUND_UP(nr_cpu_ids, ncpus);
+	__max_logical_packages = DIV_ROUND_UP(total_cpus, ncpus);
 	pr_info("Max logical packages: %u\n", __max_logical_packages);
 }