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From:   Hui Wang <john.wanghui@huawei.com>
To:     <linux-kernel@vger.kernel.org>, <tglx@linutronix.de>
CC:     <guijianfeng@huawei.com>, <wencongyang2@huawei.com>,
        <douliyang1@huawei.com>, <qiaonuohan@huawei.com>
Subject: [PATCH] x86/topology: use total_cpus for max logical packages calculation
Date:   Wed, 7 Nov 2018 10:36:43 +0800
Message-ID: <20181107023643.22174-1-john.wanghui@huawei.com>
MIME-Version: 1.0
Content-Type: text/plain
Sender: linux-kernel-owner@vger.kernel.org
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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 start kdump kernel with option nr_cpus=16, and 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>
---
 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);
 }
 
-- 
2.17.0