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[PATCH 0/2] cpu hotplug, keep topology directory after soft remove event

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  [PATCH 0/2] cpu hotplug, keep topology directory after soft remove event Prarit Bhargava <prarit@redhat.com> - 2016-09-07 13:50 +0200
    [PATCH 1/2] drivers/base: Combine topology.c and cpu.c Prarit Bhargava <prarit@redhat.com> - 2016-09-07 13:50 +0200
    [PATCH 2/2] cpu hotplug, add CONFIG_PERMANENT_CPU_TOPOLOGY Prarit Bhargava <prarit@redhat.com> - 2016-09-07 13:50 +0200
      Re: [PATCH 2/2] cpu hotplug, add CONFIG_PERMANENT_CPU_TOPOLOGY Peter Zijlstra <peterz@infradead.org> - 2016-09-08 09:50 +0200
        Re: [PATCH 2/2] cpu hotplug, add CONFIG_PERMANENT_CPU_TOPOLOGY Prarit Bhargava <prarit@redhat.com> - 2016-09-08 15:50 +0200

#1478223 — [PATCH 0/2] cpu hotplug, keep topology directory after soft remove event

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 CPU 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 CPU has not been physically removed and the
data in the topology directory is still valid.  IOW, the cpu is still
present but the kernel has removed the topology directory making it
very difficult to determine exactly where the cpu is located.

This patchset 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 for the lifetime of the CPU.  When
disabled, the behavior of the current kernel is maintained (that is, the
topology directory is removed on a down and added on an up).  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.

This patchset combines drivers/base/topology.c and drivers/base/cpu.c to
implement CONFIG_PERMANENT_CPU_TOPOLOGY and leaves all arches except
x86 with the current behavior.

Signed-off-by: Prarit Bhargava <prarit@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: x86@kernel.org
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Len Brown <len.brown@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Juergen Gross <jgross@suse.com>

Prarit Bhargava (2):
  drivers/base: Combine topology.c and cpu.c
  cpu hotplug, add CONFIG_PERMANENT_CPU_TOPOLOGY

 arch/x86/kernel/smpboot.c |    3 -
 drivers/base/Kconfig      |   12 ++++
 drivers/base/Makefile     |    2 +-
 drivers/base/cpu.c        |  146 +++++++++++++++++++++++++++++++++++++++
 drivers/base/topology.c   |  168 ---------------------------------------------
 5 files changed, 159 insertions(+), 172 deletions(-)
 delete mode 100644 drivers/base/topology.c

-- 
1.7.9.3

[toc] | [next] | [standalone]

#1478225 — [PATCH 1/2] drivers/base: Combine topology.c and cpu.c

The topology.c file contains sysfs files that describe a cpu's location
and its siblings.  There is no purpose that this file is separate from
the core cpu code.  This patch combines topology.c into cpu.c to make the
next set of changes easier to understand.

There are no functional changes with this patch.

Signed-off-by: Prarit Bhargava <prarit@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: x86@kernel.org
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Len Brown <len.brown@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Juergen Gross <jgross@suse.com>

---
 drivers/base/Makefile   |    2 +-
 drivers/base/cpu.c      |  138 ++++++++++++++++++++++++++++++++++++++
 drivers/base/topology.c |  168 -----------------------------------------------
 3 files changed, 139 insertions(+), 169 deletions(-)
 delete mode 100644 drivers/base/topology.c

diff --git a/drivers/base/Makefile b/drivers/base/Makefile
index 2609ba20b396..e50491bb3d60 100644
--- a/drivers/base/Makefile
+++ b/drivers/base/Makefile
@@ -4,7 +4,7 @@ obj-y			:= component.o core.o bus.o dd.o syscore.o \
 			   driver.o class.o platform.o \
 			   cpu.o firmware.o init.o map.o devres.o \
 			   attribute_container.o transport_class.o \
-			   topology.o container.o property.o cacheinfo.o
+			   container.o property.o cacheinfo.o
 obj-$(CONFIG_DEVTMPFS)	+= devtmpfs.o
 obj-$(CONFIG_DMA_CMA) += dma-contiguous.o
 obj-y			+= power/
diff --git a/drivers/base/cpu.c b/drivers/base/cpu.c
index 691eeea2f19a..e8c8c1155b13 100644
--- a/drivers/base/cpu.c
+++ b/drivers/base/cpu.c
@@ -175,11 +175,149 @@ static struct attribute *crash_note_cpu_attrs[] = {
 	NULL
 };
 
+#define define_id_show_func(name)				\
+static ssize_t name##_show(struct device *dev,			\
+		struct device_attribute *attr, char *buf)	\
+{								\
+	return sprintf(buf, "%d\n", topology_##name(dev->id));	\
+}
+
+#define define_siblings_show_map(name, mask)				\
+static ssize_t name##_show(struct device *dev,				\
+			   struct device_attribute *attr, char *buf)	\
+{									\
+	return cpumap_print_to_pagebuf(false, buf, topology_##mask(dev->id));\
+}
+
+#define define_siblings_show_list(name, mask)				\
+static ssize_t name##_list_show(struct device *dev,			\
+				struct device_attribute *attr,		\
+				char *buf)				\
+{									\
+	return cpumap_print_to_pagebuf(true, buf, topology_##mask(dev->id));\
+}
+
+#define define_siblings_show_func(name, mask)	\
+	define_siblings_show_map(name, mask);	\
+	define_siblings_show_list(name, mask)
+
+define_id_show_func(physical_package_id);
+static DEVICE_ATTR_RO(physical_package_id);
+
+define_id_show_func(core_id);
+static DEVICE_ATTR_RO(core_id);
+
+define_siblings_show_func(thread_siblings, sibling_cpumask);
+static DEVICE_ATTR_RO(thread_siblings);
+static DEVICE_ATTR_RO(thread_siblings_list);
+
+define_siblings_show_func(core_siblings, core_cpumask);
+static DEVICE_ATTR_RO(core_siblings);
+static DEVICE_ATTR_RO(core_siblings_list);
+
+#ifdef CONFIG_SCHED_BOOK
+define_id_show_func(book_id);
+static DEVICE_ATTR_RO(book_id);
+define_siblings_show_func(book_siblings, book_cpumask);
+static DEVICE_ATTR_RO(book_siblings);
+static DEVICE_ATTR_RO(book_siblings_list);
+#endif
+
+#ifdef CONFIG_SCHED_DRAWER
+define_id_show_func(drawer_id);
+static DEVICE_ATTR_RO(drawer_id);
+define_siblings_show_func(drawer_siblings, drawer_cpumask);
+static DEVICE_ATTR_RO(drawer_siblings);
+static DEVICE_ATTR_RO(drawer_siblings_list);
+#endif
+
+static struct attribute *topology_attrs[] = {
+	&dev_attr_physical_package_id.attr,
+	&dev_attr_core_id.attr,
+	&dev_attr_thread_siblings.attr,
+	&dev_attr_thread_siblings_list.attr,
+	&dev_attr_core_siblings.attr,
+	&dev_attr_core_siblings_list.attr,
+#ifdef CONFIG_SCHED_BOOK
+	&dev_attr_book_id.attr,
+	&dev_attr_book_siblings.attr,
+	&dev_attr_book_siblings_list.attr,
+#endif
+#ifdef CONFIG_SCHED_DRAWER
+	&dev_attr_drawer_id.attr,
+	&dev_attr_drawer_siblings.attr,
+	&dev_attr_drawer_siblings_list.attr,
+#endif
+	NULL
+};
+
 static struct attribute_group crash_note_cpu_attr_group = {
 	.attrs = crash_note_cpu_attrs,
 };
 #endif
 
+static struct attribute_group topology_attr_group = {
+	.attrs = topology_attrs,
+	.name = "topology"
+};
+
+/* Add/Remove cpu_topology interface for CPU device */
+static int topology_add_dev(unsigned int cpu)
+{
+	struct device *dev = get_cpu_device(cpu);
+
+	return sysfs_create_group(&dev->kobj, &topology_attr_group);
+}
+
+static void topology_remove_dev(unsigned int cpu)
+{
+	struct device *dev = get_cpu_device(cpu);
+
+	sysfs_remove_group(&dev->kobj, &topology_attr_group);
+}
+
+static int topology_cpu_callback(struct notifier_block *nfb,
+				 unsigned long action, void *hcpu)
+{
+	unsigned int cpu = (unsigned long)hcpu;
+	int rc = 0;
+
+	switch (action) {
+	case CPU_UP_PREPARE:
+	case CPU_UP_PREPARE_FROZEN:
+		rc = topology_add_dev(cpu);
+		break;
+	case CPU_UP_CANCELED:
+	case CPU_UP_CANCELED_FROZEN:
+	case CPU_DEAD:
+	case CPU_DEAD_FROZEN:
+		topology_remove_dev(cpu);
+		break;
+	}
+	return notifier_from_errno(rc);
+}
+
+static int topology_sysfs_init(void)
+{
+	int cpu;
+	int rc = 0;
+
+	cpu_notifier_register_begin();
+
+	for_each_online_cpu(cpu) {
+		rc = topology_add_dev(cpu);
+		if (rc)
+			goto out;
+	}
+	__hotcpu_notifier(topology_cpu_callback, 0);
+
+out:
+	cpu_notifier_register_done();
+	return rc;
+}
+
+device_initcall(topology_sysfs_init);
+
 static const struct attribute_group *common_cpu_attr_groups[] = {
 #ifdef CONFIG_KEXEC
 	&crash_note_cpu_attr_group,
diff --git a/drivers/base/topology.c b/drivers/base/topology.c
deleted file mode 100644
index df3c97cb4c99..000000000000
--- a/drivers/base/topology.c
+++ /dev/null
@@ -1,168 +0,0 @@
-/*
- * driver/base/topology.c - Populate sysfs with cpu topology information
- *
- * Written by: Zhang Yanmin, Intel Corporation
- *
- * Copyright (C) 2006, Intel Corp.
- *
- * All rights reserved.
- *
- * This program is free software; you can redistribute it and/or modify
- * it under the terms of the GNU General Public License as published by
- * the Free Software Foundation; either version 2 of the License, or
- * (at your option) any later version.
- *
- * This program is distributed in the hope that it will be useful, but
- * WITHOUT ANY WARRANTY; without even the implied warranty of
- * MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE, GOOD TITLE or
- * NON INFRINGEMENT.  See the GNU General Public License for more
- * details.
- *
- * You should have received a copy of the GNU General Public License
- * along with this program; if not, write to the Free Software
- * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
- *
- */
-#include <linux/mm.h>
-#include <linux/cpu.h>
-#include <linux/module.h>
-#include <linux/hardirq.h>
-#include <linux/topology.h>
-
-#define define_id_show_func(name)				\
-static ssize_t name##_show(struct device *dev,			\
-		struct device_attribute *attr, char *buf)	\
-{								\
-	return sprintf(buf, "%d\n", topology_##name(dev->id));	\
-}
-
-#define define_siblings_show_map(name, mask)				\
-static ssize_t name##_show(struct device *dev,				\
-			   struct device_attribute *attr, char *buf)	\
-{									\
-	return cpumap_print_to_pagebuf(false, buf, topology_##mask(dev->id));\
-}
-
-#define define_siblings_show_list(name, mask)				\
-static ssize_t name##_list_show(struct device *dev,			\
-				struct device_attribute *attr,		\
-				char *buf)				\
-{									\
-	return cpumap_print_to_pagebuf(true, buf, topology_##mask(dev->id));\
-}
-
-#define define_siblings_show_func(name, mask)	\
-	define_siblings_show_map(name, mask);	\
-	define_siblings_show_list(name, mask)
-
-define_id_show_func(physical_package_id);
-static DEVICE_ATTR_RO(physical_package_id);
-
-define_id_show_func(core_id);
-static DEVICE_ATTR_RO(core_id);
-
-define_siblings_show_func(thread_siblings, sibling_cpumask);
-static DEVICE_ATTR_RO(thread_siblings);
-static DEVICE_ATTR_RO(thread_siblings_list);
-
-define_siblings_show_func(core_siblings, core_cpumask);
-static DEVICE_ATTR_RO(core_siblings);
-static DEVICE_ATTR_RO(core_siblings_list);
-
-#ifdef CONFIG_SCHED_BOOK
-define_id_show_func(book_id);
-static DEVICE_ATTR_RO(book_id);
-define_siblings_show_func(book_siblings, book_cpumask);
-static DEVICE_ATTR_RO(book_siblings);
-static DEVICE_ATTR_RO(book_siblings_list);
-#endif
-
-#ifdef CONFIG_SCHED_DRAWER
-define_id_show_func(drawer_id);
-static DEVICE_ATTR_RO(drawer_id);
-define_siblings_show_func(drawer_siblings, drawer_cpumask);
-static DEVICE_ATTR_RO(drawer_siblings);
-static DEVICE_ATTR_RO(drawer_siblings_list);
-#endif
-
-static struct attribute *default_attrs[] = {
-	&dev_attr_physical_package_id.attr,
-	&dev_attr_core_id.attr,
-	&dev_attr_thread_siblings.attr,
-	&dev_attr_thread_siblings_list.attr,
-	&dev_attr_core_siblings.attr,
-	&dev_attr_core_siblings_list.attr,
-#ifdef CONFIG_SCHED_BOOK
-	&dev_attr_book_id.attr,
-	&dev_attr_book_siblings.attr,
-	&dev_attr_book_siblings_list.attr,
-#endif
-#ifdef CONFIG_SCHED_DRAWER
-	&dev_attr_drawer_id.attr,
-	&dev_attr_drawer_siblings.attr,
-	&dev_attr_drawer_siblings_list.attr,
-#endif
-	NULL
-};
-
-static struct attribute_group topology_attr_group = {
-	.attrs = default_attrs,
-	.name = "topology"
-};
-
-/* Add/Remove cpu_topology interface for CPU device */
-static int topology_add_dev(unsigned int cpu)
-{
-	struct device *dev = get_cpu_device(cpu);
-
-	return sysfs_create_group(&dev->kobj, &topology_attr_group);
-}
-
-static void topology_remove_dev(unsigned int cpu)
-{
-	struct device *dev = get_cpu_device(cpu);
-
-	sysfs_remove_group(&dev->kobj, &topology_attr_group);
-}
-
-static int topology_cpu_callback(struct notifier_block *nfb,
-				 unsigned long action, void *hcpu)
-{
-	unsigned int cpu = (unsigned long)hcpu;
-	int rc = 0;
-
-	switch (action) {
-	case CPU_UP_PREPARE:
-	case CPU_UP_PREPARE_FROZEN:
-		rc = topology_add_dev(cpu);
-		break;
-	case CPU_UP_CANCELED:
-	case CPU_UP_CANCELED_FROZEN:
-	case CPU_DEAD:
-	case CPU_DEAD_FROZEN:
-		topology_remove_dev(cpu);
-		break;
-	}
-	return notifier_from_errno(rc);
-}
-
-static int topology_sysfs_init(void)
-{
-	int cpu;
-	int rc = 0;
-
-	cpu_notifier_register_begin();
-
-	for_each_online_cpu(cpu) {
-		rc = topology_add_dev(cpu);
-		if (rc)
-			goto out;
-	}
-	__hotcpu_notifier(topology_cpu_callback, 0);
-
-out:
-	cpu_notifier_register_done();
-	return rc;
-}
-
-device_initcall(topology_sysfs_init);
-- 
1.7.9.3

[toc] | [prev] | [next] | [standalone]

#1478231 — [PATCH 2/2] cpu hotplug, add CONFIG_PERMANENT_CPU_TOPOLOGY

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 CPU 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 CPU has not been physically removed and the
data in the topology directory is still valid and the cpu'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).
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 CPUs in sequence, randomly, by thread group, by
socket group and didn't see any issues.

Note: 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 CPU is offline.

Signed-off-by: Prarit Bhargava <prarit@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: x86@kernel.org
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Len Brown <len.brown@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Juergen Gross <jgross@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 e8c8c1155b13..c84cbd1defd5 100644
--- a/drivers/base/cpu.c
+++ b/drivers/base/cpu.c
@@ -261,6 +261,7 @@ 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)
 {
@@ -317,11 +318,15 @@ 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
 };
 
@@ -329,6 +334,9 @@ 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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#1478868 — Re: [PATCH 2/2] cpu hotplug, add CONFIG_PERMANENT_CPU_TOPOLOGY

On Wed, Sep 07, 2016 at 07:44:17AM -0400, Prarit Bhargava wrote:
> 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 CPU 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 CPU has not been physically removed and the
> data in the topology directory is still valid and the cpu'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).
> 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.

So how do you physically remove things and how does the information then
get removed?

Also, why is that an x86 only feature?

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#1479241 — Re: [PATCH 2/2] cpu hotplug, add CONFIG_PERMANENT_CPU_TOPOLOGY

On 09/08/2016 03:45 AM, Peter Zijlstra wrote:
> On Wed, Sep 07, 2016 at 07:44:17AM -0400, Prarit Bhargava wrote:
>> 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 CPU 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 CPU has not been physically removed and the
>> data in the topology directory is still valid and the cpu'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).
>> 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.
> 
> So how do you physically remove things and how does the information then
> get removed?
> 

Physical hotremove was done with a toggle switch enabled test box.

The topology directory gets removed when either common_cpu_attr_groups
(!CONFIG_HOTPLUG_CPU) or hotplugable_cpu_attr_groups (CONFIG_HOTPLUG_CPU) are
destroyed.  The cpu device's attributes are set in register_cpu() and the device
is destroyed in unregister_cpu().

IOW, the patch expands the lifetime of the topology directories to match that of
the device.

> Also, why is that an x86 only feature?

The only arch that I could test on was x86 and I wasn't able to test to see if
topology fields are valid for other arches on a soft remove.  So x86 by default
for now and other arches can make their decisions.

P.

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