[RFC PATCH v2 11/35] arch_topology: Make register_cpu_capacity_sysctl() tolerant to late CPUs

[Russell King (Oracle)]

On Thu, Sep 14, 2023 at 01:01:26PM +0100, Jonathan Cameron wrote:
> On Wed, 13 Sep 2023 16:37:59 +0000
> James Morse <james.morse at arm.com> wrote:
> 
> > register_cpu_capacity_sysctl() adds a property to sysfs that describes
> > the CPUs capacity. This is done from a subsys_initcall() that assumes
> > all possible CPUs are registered.
> > 
> > With CPU hotplug, possible CPUs aren't registered until they become
> > present, (or for arm64 enabled). This leads to messages during boot:
> > | register_cpu_capacity_sysctl: too early to get CPU1 device!
> > and once these CPUs are added to the system, the file is missing.
> > 
> > Move this to a cpuhp callback, so that the file is created once
> > CPUs are brought online. This covers CPUs that are added late by
> > mechanisms like hotplug.
> > One observable difference is the file is now missing for offline CPUs.
> > 
> > Signed-off-by: James Morse <james.morse at arm.com>
> > ---
> > If the offline CPUs thing is a problem for the tools that consume
> > this value, we'd need to move cpu_capacity to be part of cpu.c's
> > common_cpu_attr_groups.
> 
> I think we should do that anyway and then use an is_visible() if we want to
> change whether it is visible in offline cpus.
> 
> Dynamic sysfs file creation is horrible - particularly when done
> from an totally different file from where the rest of the attributes
> are registered.  I'm curious what the history behind that is.
> 
> Whilst here, why is there a common_cpu_attr_groups which is
> identical to the hotpluggable_cpu_attr_groups in base/cpu.c?

Looking into doing this, the easy bit is adding the attribute group
with an appropriate .is_visible dependent on cpu_present(), but we
need to be able to call sysfs_update_groups() when the state of the
.is_visible() changes.

Given the comment in sysfs_update_groups() about "if an error occurs",
rather than making this part of common_cpu_attr_groups, would it be
better that it's part of its own set of groups, thus limiting the
damage from a possible error? I suspect, however, that any error at
that point means that the system is rather fatally wounded.

This is what I have so far to implement your idea, less the necessary
sysfs_update_groups() call when we need to change the visibility of
the attributes.

diff --git a/drivers/base/arch_topology.c b/drivers/base/arch_topology.c
index 9ccb7daee78e..06c9fc6620d2 100644
--- a/drivers/base/arch_topology.c
+++ b/drivers/base/arch_topology.c
@@ -215,43 +215,24 @@ static ssize_t cpu_capacity_show(struct device *dev,
 	return sysfs_emit(buf, "%lu\n", topology_get_cpu_scale(cpu->dev.id));
 }
 
-static void update_topology_flags_workfn(struct work_struct *work);
-static DECLARE_WORK(update_topology_flags_work, update_topology_flags_workfn);
-
 static DEVICE_ATTR_RO(cpu_capacity);
 
-static int cpu_capacity_sysctl_add(unsigned int cpu)
-{
-	struct device *cpu_dev = get_cpu_device(cpu);
-
-	if (!cpu_dev)
-		return -ENOENT;
-
-	device_create_file(cpu_dev, &dev_attr_cpu_capacity);
-
-	return 0;
-}
-
-static int cpu_capacity_sysctl_remove(unsigned int cpu)
+static umode_t cpu_present_attrs_visible(struct kobject *kobi,
+					 struct attribute *attr, int index)
 {
-	struct device *cpu_dev = get_cpu_device(cpu);
-
-	if (!cpu_dev)
-		return -ENOENT;
-
-	device_remove_file(cpu_dev, &dev_attr_cpu_capacity);
+	struct device *dev = kobj_to_dev(kobj);
+	struct cpu *cpu = container_of(dev, struct cpu, dev);
 
-	return 0;
+	return cpu_present(cpu->dev.id) ? attr->mode : 0;
 }
 
-static int register_cpu_capacity_sysctl(void)
-{
-	cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "topology/cpu-capacity",
-			  cpu_capacity_sysctl_add, cpu_capacity_sysctl_remove);
+const struct attribute_group cpu_capacity_attr_group = {
+	.is_visible = cpu_present_attrs_visible,
+	.attrs = cpu_capacity_attrs
+};
 
-	return 0;
-}
-subsys_initcall(register_cpu_capacity_sysctl);
+static void update_topology_flags_workfn(struct work_struct *work);
+static DECLARE_WORK(update_topology_flags_work, update_topology_flags_workfn);
 
 static int update_topology;
 
diff --git a/drivers/base/cpu.c b/drivers/base/cpu.c
index a19a8be93102..954b045705c2 100644
--- a/drivers/base/cpu.c
+++ b/drivers/base/cpu.c
@@ -192,6 +192,9 @@ static const struct attribute_group crash_note_cpu_attr_group = {
 static const struct attribute_group *common_cpu_attr_groups[] = {
 #ifdef CONFIG_KEXEC
 	&crash_note_cpu_attr_group,
+#endif
+#ifdef CONFIG_GENERIC_ARCH_TOPOLOGY
+	&cpu_capacity_attr_group,
 #endif
 	NULL
 };
diff --git a/include/linux/cpu.h b/include/linux/cpu.h
index e117c06e0c6b..745ad21e3dc8 100644
--- a/include/linux/cpu.h
+++ b/include/linux/cpu.h
@@ -30,6 +30,8 @@ struct cpu {
 	struct device dev;
 };
 
+extern const struct attribute_group cpu_capacity_attr_group;
+
 extern void boot_cpu_init(void);
 extern void boot_cpu_hotplug_init(void);
 extern void cpu_init(void);

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