The information in /sys/devices/system/cpu/cpuX/topology
directory is useful for userspace monitoring applications and in-tree
utilities like cpupower & turbostat.

When down'ing a thread the /sys/devices/system/cpu/cpuX/topology directory is
removed during the CPU_DEAD hotplug callback in the kernel.  The problem
with this model is that the thread's core has not been physically removed
and the data in the topology directory is still valid and the core's
location is now lost to userspace.

This patch adds CONFIG_PERMANENT_CPU_TOPOLOGY, and is Y by default for
x86, an N for all other arches.  When enabled the kernel is modified so
that the topology directory is added to the core cpu sysfs files so that
the topology directory exists while the CPU is physically present.  When
disabled, the behavior of the current kernel is maintained (that is, the
topology directory is removed on a soft down and added on an soft up of a
thread).

Adding CONFIG_PERMANENT_CPU_TOPOLOGY may require additional architecture
so that the cpumask data the CPU's topology is not cleared during a CPU
down.

Before patch:

[root@hp-z620-01 ~]# grep ^ /sys/devices/system/cpu/cpu10/topology/*
/sys/devices/system/cpu/cpu10/topology/core_id:3
/sys/devices/system/cpu/cpu10/topology/core_siblings:ffff
/sys/devices/system/cpu/cpu10/topology/core_siblings_list:0-15
/sys/devices/system/cpu/cpu10/topology/physical_package_id:0
/sys/devices/system/cpu/cpu10/topology/thread_siblings:0404
/sys/devices/system/cpu/cpu10/topology/thread_siblings_list:2,10

Down a cpu

[root@hp-z620-01 ~]# echo 0 > /sys/devices/system/cpu/cpu10/online

[root@hp-z620-01 ~]# ls /sys/devices/system/cpu/cpu10/topology
ls: cannot access topology: No such file or directory

After patch:

[root@hp-z620-01 ~]# grep ^ /sys/devices/system/cpu/cpu10/topology/*
/sys/devices/system/cpu/cpu10/topology/core_id:3
/sys/devices/system/cpu/cpu10/topology/core_siblings:ffff
/sys/devices/system/cpu/cpu10/topology/core_siblings_list:0-15
/sys/devices/system/cpu/cpu10/topology/physical_package_id:0
/sys/devices/system/cpu/cpu10/topology/thread_siblings:0404
/sys/devices/system/cpu/cpu10/topology/thread_siblings_list:2,10

Down a cpu

[root@hp-z620-01 ~]# echo 0 > /sys/devices/system/cpu/cpu10/online

[root@hp-z620-01 ~]# grep ^ /sys/devices/system/cpu/cpu10/topology/*
/sys/devices/system/cpu/cpu10/topology/core_id:3
/sys/devices/system/cpu/cpu10/topology/core_siblings:0000
/sys/devices/system/cpu/cpu10/topology/core_siblings_list:
/sys/devices/system/cpu/cpu10/topology/physical_package_id:0
/sys/devices/system/cpu/cpu10/topology/thread_siblings:0000
/sys/devices/system/cpu/cpu10/topology/thread_siblings_list:

I did some testing with and without BOOTPARAM_HOTPLUG_CPU0 enabled,
and up'd and down'd threads in sequence, randomly, by thread group, by
socket group and didn't see any issues.

core_siblings and thread_siblings are "numa siblings that are online"
and "thread siblings that are online" and are used as such within the kernel.
They must be zero'd out when the thread is offline.

CONFIG_PERMANENT_CPU_TOPOLOGY=y changes the lifetime of the topology
directory from existing when a thread is online to when a thread is
created and destroyed.

Signed-off-by: Prarit Bhargava <pra...@redhat.com>
Cc: Thomas Gleixner <t...@linutronix.de>
Cc: Ingo Molnar <mi...@redhat.com>
Cc: "H. Peter Anvin" <h...@zytor.com>
Cc: x...@kernel.org
Cc: Greg Kroah-Hartman <gre...@linuxfoundation.org>
Cc: Peter Zijlstra <pet...@infradead.org>
Cc: Len Brown <len.br...@intel.com>
Cc: Borislav Petkov <b...@suse.de>
Cc: Andi Kleen <a...@linux.intel.com>
Cc: Jiri Olsa <jo...@redhat.com>
Cc: Juergen Gross <jgr...@suse.com>
---
 arch/x86/kernel/smpboot.c |    3 ---
 drivers/base/Kconfig      |   12 ++++++++++++
 drivers/base/cpu.c        |    8 ++++++++
 3 files changed, 20 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index 4296beb8fdd3..ae82f8f45b61 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -1472,7 +1472,6 @@ static void recompute_smt_state(void)
 static void remove_siblinginfo(int cpu)
 {
        int sibling;
-       struct cpuinfo_x86 *c = &cpu_data(cpu);
 
        for_each_cpu(sibling, topology_core_cpumask(cpu)) {
                cpumask_clear_cpu(cpu, topology_core_cpumask(sibling));
@@ -1490,8 +1489,6 @@ static void remove_siblinginfo(int cpu)
        cpumask_clear(cpu_llc_shared_mask(cpu));
        cpumask_clear(topology_sibling_cpumask(cpu));
        cpumask_clear(topology_core_cpumask(cpu));
-       c->phys_proc_id = 0;
-       c->cpu_core_id = 0;
        cpumask_clear_cpu(cpu, cpu_sibling_setup_mask);
        recompute_smt_state();
 }
diff --git a/drivers/base/Kconfig b/drivers/base/Kconfig
index 98504ec99c7d..b3935a272c3c 100644
--- a/drivers/base/Kconfig
+++ b/drivers/base/Kconfig
@@ -324,4 +324,16 @@ config CMA_ALIGNMENT
 
 endif
 
+config PERMANENT_CPU_TOPOLOGY
+       bool "Permanent CPU Topology"
+       depends on HOTPLUG_CPU
+       def_bool y if X86_64
+       help
+         This option configures CPU topology to be permanent for the lifetime
+         of the CPU (until it is physically removed).  Selecting Y here
+         results in the kernel reporting the physical location for offlined
+         CPUs.
+
+         If unsure, leave the default value as is.
+
 endmenu
diff --git a/drivers/base/cpu.c b/drivers/base/cpu.c
index ce722fdf48c6..891afc94c149 100644
--- a/drivers/base/cpu.c
+++ b/drivers/base/cpu.c
@@ -263,6 +263,7 @@ static struct attribute_group topology_attr_group = {
        .name = "topology"
 };
 
+#ifndef CONFIG_PERMANENT_CPU_TOPOLOGY
 /* Add/Remove cpu_topology interface for CPU device */
 static int topology_add_dev(unsigned int cpu)
 {
@@ -319,11 +320,15 @@ out:
 }
 
 device_initcall(topology_sysfs_init);
+#endif
 
 static const struct attribute_group *common_cpu_attr_groups[] = {
 #ifdef CONFIG_KEXEC
        &crash_note_cpu_attr_group,
 #endif
+#ifdef CONFIG_PERMANENT_CPU_TOPOLOGY
+       &topology_attr_group,
+#endif
        NULL
 };
 
@@ -331,6 +336,9 @@ static const struct attribute_group 
*hotplugable_cpu_attr_groups[] = {
 #ifdef CONFIG_KEXEC
        &crash_note_cpu_attr_group,
 #endif
+#ifdef CONFIG_PERMANENT_CPU_TOPOLOGY
+       &topology_attr_group,
+#endif
        NULL
 };
 
-- 
1.7.9.3

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