[RFC v2 4/4] powerpc/papr_scm: Add cpu hotplug support for nvdimm pmu device

[Kajol Jain <kjain@linux.ibm.com>]

Patch here adds cpu hotplug functions to nvdimm pmu.
It adds cpumask to designate a cpu to make HCALL to
collect the counter data for the nvdimm device and
update ABI documentation accordingly.

Result in power9 lpar system:
command:# cat /sys/devices/nmem0/cpumask
0

Signed-off-by: Kajol Jain <kjain@linux.ibm.com>
---
 Documentation/ABI/testing/sysfs-bus-papr-pmem |  6 ++
 arch/powerpc/platforms/pseries/papr_scm.c     | 61 +++++++++++++++++++
 include/linux/nd.h                            | 17 ++++--
 3 files changed, 79 insertions(+), 5 deletions(-)

diff --git a/Documentation/ABI/testing/sysfs-bus-papr-pmem b/Documentation/ABI/testing/sysfs-bus-papr-pmem
index 38c4daf65af2..986df1691914 100644
--- a/Documentation/ABI/testing/sysfs-bus-papr-pmem
+++ b/Documentation/ABI/testing/sysfs-bus-papr-pmem
@@ -76,6 +76,12 @@ Description:	(RO) Attribute group to describe the magic bits
 		For example::
 		    noopstat = "event=0x1"
 
+What:		/sys/devices/nmemX/cpumask
+Date:		May 2021
+Contact:	linuxppc-dev <linuxppc-dev@lists.ozlabs.org>, linux-nvdimm@lists.01.org,
+Description:	(RO) This sysfs file exposes the cpumask which is designated to make
+                HCALLs to retrieve nvdimm pmu event counter data.
+
 What:		/sys/devices/nmemX/events
 Date:		May 2021
 Contact:	linuxppc-dev <linuxppc-dev@lists.ozlabs.org>, linux-nvdimm@lists.01.org,
diff --git a/arch/powerpc/platforms/pseries/papr_scm.c b/arch/powerpc/platforms/pseries/papr_scm.c
index f2d57da98ff4..76121d876b7f 100644
--- a/arch/powerpc/platforms/pseries/papr_scm.c
+++ b/arch/powerpc/platforms/pseries/papr_scm.c
@@ -339,6 +339,28 @@ static ssize_t drc_pmem_query_stats(struct papr_scm_priv *p,
 	return 0;
 }
 
+static ssize_t cpumask_show(struct device *dev,
+			    struct device_attribute *attr, char *buf)
+{
+	struct pmu *pmu = dev_get_drvdata(dev);
+	struct nvdimm_pmu *nd_pmu;
+
+	nd_pmu = container_of(pmu, struct nvdimm_pmu, pmu);
+
+	return cpumap_print_to_pagebuf(true, buf, cpumask_of(nd_pmu->cpu));
+}
+
+static DEVICE_ATTR_RO(cpumask);
+
+static struct attribute *nvdimm_cpumask_attrs[] = {
+	&dev_attr_cpumask.attr,
+	NULL,
+};
+
+static const struct attribute_group nvdimm_pmu_cpumask_group = {
+	.attrs = nvdimm_cpumask_attrs,
+};
+
 PMU_FORMAT_ATTR(event, "config:0-4");
 
 static struct attribute *nvdimm_pmu_format_attr[] = {
@@ -459,6 +481,24 @@ static void papr_scm_pmu_del(struct perf_event *event, int flags)
 	papr_scm_pmu_read(event);
 }
 
+static int nvdimm_pmu_offline_cpu(unsigned int cpu, struct hlist_node *node)
+{
+	struct nvdimm_pmu *nd_pmu;
+	int target;
+
+	nd_pmu = hlist_entry_safe(node, struct nvdimm_pmu, node);
+
+	if (cpu != nd_pmu->cpu)
+		return 0;
+
+	target = cpumask_last(cpu_active_mask);
+	if (target < 0 || target >= nr_cpu_ids)
+		return -1;
+
+	nd_pmu->cpu = target;
+	return 0;
+}
+
 static ssize_t device_show_string(struct device *dev, struct device_attribute *attr,
 				  char *buf)
 {
@@ -603,6 +643,7 @@ static int papr_scm_pmu_check_events(struct papr_scm_priv *p, struct nvdimm_pmu
 	/* Fill attribute groups for the nvdimm pmu device */
 	nd_pmu->attr_groups[NVDIMM_PMU_FORMAT_ATTR] = &nvdimm_pmu_format_group;
 	nd_pmu->attr_groups[NVDIMM_PMU_EVENT_ATTR] = nvdimm_pmu_events_group;
+	nd_pmu->attr_groups[NVDIMM_PMU_CPUMASK_ATTR] = &nvdimm_pmu_cpumask_group;
 	nd_pmu->attr_groups[NVDIMM_PMU_NULL_ATTR] = NULL;
 
 	kfree(single_stats);
@@ -652,6 +693,20 @@ static void papr_scm_pmu_register(struct papr_scm_priv *p)
 	nd_pmu->read = papr_scm_pmu_read;
 	nd_pmu->add = papr_scm_pmu_add;
 	nd_pmu->del = papr_scm_pmu_del;
+	nd_pmu->cpu = raw_smp_processor_id();
+
+	rc = cpuhp_setup_state_multi(CPUHP_AP_ONLINE_DYN,
+				     "perf/nvdimm:online",
+			      NULL, nvdimm_pmu_offline_cpu);
+	if (rc < 0)
+		goto pmu_cpuhp_setup_err;
+
+	nd_pmu->cpuhp_state = rc;
+
+	/* Register the pmu instance for cpu hotplug */
+	rc = cpuhp_state_add_instance_nocalls(nd_pmu->cpuhp_state, &nd_pmu->node);
+	if (rc)
+		goto cpuhp_instance_err;
 
 	rc = register_nvdimm_pmu(nd_pmu, p->pdev);
 	if (rc)
@@ -665,6 +720,10 @@ static void papr_scm_pmu_register(struct papr_scm_priv *p)
 	return;
 
 pmu_register_err:
+	cpuhp_state_remove_instance_nocalls(nd_pmu->cpuhp_state, &nd_pmu->node);
+cpuhp_instance_err:
+	cpuhp_remove_multi_state(nd_pmu->cpuhp_state);
+pmu_cpuhp_setup_err:
 	nvdimm_pmu_mem_free(nd_pmu);
 	kfree(p->nvdimm_events_map);
 pmu_check_events_err:
@@ -675,6 +734,8 @@ static void papr_scm_pmu_register(struct papr_scm_priv *p)
 
 static void nvdimm_pmu_uinit(struct nvdimm_pmu *nd_pmu)
 {
+	cpuhp_state_remove_instance_nocalls(nd_pmu->cpuhp_state, &nd_pmu->node);
+	cpuhp_remove_multi_state(nd_pmu->cpuhp_state);
 	unregister_nvdimm_pmu(&nd_pmu->pmu);
 	nvdimm_pmu_mem_free(nd_pmu);
 	kfree(nd_pmu);
diff --git a/include/linux/nd.h b/include/linux/nd.h
index a0e0619256be..177795413ab3 100644
--- a/include/linux/nd.h
+++ b/include/linux/nd.h
@@ -28,7 +28,8 @@ enum nvdimm_claim_class {
 /* Event attribute array index */
 #define NVDIMM_PMU_FORMAT_ATTR		0
 #define NVDIMM_PMU_EVENT_ATTR		1
-#define NVDIMM_PMU_NULL_ATTR		2
+#define NVDIMM_PMU_CPUMASK_ATTR		2
+#define NVDIMM_PMU_NULL_ATTR		3
 
 /**
  * struct nvdimm_pmu - data structure for nvdimm perf driver
@@ -37,7 +38,10 @@ enum nvdimm_claim_class {
  * @pmu: pmu data structure for nvdimm performance stats.
  * @dev: nvdimm device pointer.
  * @functions(event_init/add/del/read): platform specific pmu functions.
- * @attr_groups: data structure for events and formats.
+ * @attr_groups: data structure for events, formats and cpumask
+ * @cpu: designated cpu for counter access.
+ * @node: node for cpu hotplug notifier link.
+ * @cpuhp_state: state for cpu hotplug notification.
  */
 struct nvdimm_pmu {
 	const char *name;
@@ -49,10 +53,13 @@ struct nvdimm_pmu {
 	void (*read)(struct perf_event *event);
 	/*
 	 * Attribute groups for the nvdimm pmu. Index 0 used for
-	 * format attribute, index 1 used for event attribute and
-	 * index 2 kept as NULL.
+	 * format attribute, index 1 used for event attribute,
+	 * index 2 used for cpusmask attribute and index 3 kept as NULL.
 	 */
-	const struct attribute_group *attr_groups[3];
+	const struct attribute_group *attr_groups[4];
+	int cpu;
+	struct hlist_node node;
+	enum cpuhp_state cpuhp_state;
 };
 
 int register_nvdimm_pmu(struct nvdimm_pmu *nvdimm, struct platform_device *pdev);
-- 
2.27.0