linux-kernel - [tip:x86/urgent] x86/smpboot: Do not use smp_num_siblings in __max_logical

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Date:   Thu, 7 Dec 2017 01:40:09 -0800
From:   tip-bot for Prarit Bhargava <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, williams@...hat.com, hpa@...or.com,
        mingo@...nel.org, tglx@...utronix.de, prarit@...hat.com,
        kubakici@...pl
Subject: [tip:x86/urgent] x86/smpboot: Do not use smp_num_siblings in
 __max_logical_packages calculation

Commit-ID:  947134d9b00f342415af7eddd42a5fce7262a1b9
Gitweb:     https://git.kernel.org/tip/947134d9b00f342415af7eddd42a5fce7262a1b9
Author:     Prarit Bhargava <prarit@...hat.com>
AuthorDate: Mon, 4 Dec 2017 11:45:21 -0500
Committer:  Thomas Gleixner <tglx@...utronix.de>
CommitDate: Thu, 7 Dec 2017 10:28:22 +0100

x86/smpboot: Do not use smp_num_siblings in __max_logical_packages calculation

Documentation/x86/topology.txt defines smp_num_siblings as "The number of
threads in a core".  Since commit bbb65d2d365e ("x86: use cpuid vector 0xb
when available for detecting cpu topology") smp_num_siblings is the
maximum number of threads in a core.  If Simultaneous MultiThreading
(SMT) is disabled on a system, smp_num_siblings is 2 and not 1 as
expected.

Use topology_max_smt_threads(), which contains the active numer of threads,
in the __max_logical_packages calculation.

On a single socket, single core, single thread system __max_smt_threads has
not been updated when the __max_logical_packages calculation happens, so its
zero which makes the package estimate fail. Initialize it to one, which is
the minimum number of threads on a core.

[ tglx: Folded the __max_smt_threads fix in ]

Fixes: b4c0a7326f5d ("x86/smpboot: Fix __max_logical_packages estimate")
Reported-by: Jakub Kicinski <kubakici@...pl>
Signed-off-by: Prarit Bhargava <prarit@...hat.com
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Tested-by: Jakub Kicinski <kubakici@...pl>
Cc: netdev@...r.kernel.org
Cc: "netdev@...r.kernel.org"
Cc: Clark Williams <williams@...hat.com>
Link: https://lkml.kernel.org/r/20171204164521.17870-1-prarit@redhat.com
---
 arch/x86/kernel/smpboot.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index 05a97d5..35cb20994 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -106,7 +106,7 @@ EXPORT_SYMBOL(__max_logical_packages);
 static unsigned int logical_packages __read_mostly;
 
 /* Maximum number of SMT threads on any online core */
-int __max_smt_threads __read_mostly;
+int __read_mostly __max_smt_threads = 1;
 
 /* Flag to indicate if a complete sched domain rebuild is required */
 bool x86_topology_update;
@@ -1304,7 +1304,7 @@ void __init native_smp_cpus_done(unsigned int max_cpus)
 	 * Today neither Intel nor AMD support heterogenous systems so
 	 * extrapolate the boot cpu's data to all packages.
 	 */
-	ncpus = cpu_data(0).booted_cores * smp_num_siblings;
+	ncpus = cpu_data(0).booted_cores * topology_max_smt_threads();
 	__max_logical_packages = DIV_ROUND_UP(nr_cpu_ids, ncpus);
 	pr_info("Max logical packages: %u\n", __max_logical_packages);