powerpc/perf/hv-24x7: Add cpu hotplug support

Patch here adds cpu hotplug functions to hv_24x7 pmu.
A new cpuhp_state "CPUHP_AP_PERF_POWERPC_HV_24x7_ONLINE" enum
is added.

The online callback function updates the cpumask only if its
empty. As the primary intention of adding hotplug support
is to designate a CPU to make HCALL to collect the
counter data.

The offline function test and clear corresponding cpu in a cpumask
and update cpumask to any other active cpu.
Signed-off-by: Kajol Jain <kjain@linux.ibm.com>
Reviewed-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200709051836.723765-2-kjain@linux.ibm.com

arch/powerpc/perf/hv-24x7.c

@@ -31,6 +31,8 @@ static int interface_version;
 /* Whether we have to aggregate result data for some domains. */
 static bool aggregate_result_elements;
 
+static cpumask_t hv_24x7_cpumask;
+
 static bool domain_is_valid(unsigned domain)
 {
 	switch (domain) {
@@ -1641,6 +1643,45 @@ static struct pmu h_24x7_pmu = {
 	.capabilities = PERF_PMU_CAP_NO_EXCLUDE,
 };
 
+static int ppc_hv_24x7_cpu_online(unsigned int cpu)
+{
+	if (cpumask_empty(&hv_24x7_cpumask))
+		cpumask_set_cpu(cpu, &hv_24x7_cpumask);
+
+	return 0;
+}
+
+static int ppc_hv_24x7_cpu_offline(unsigned int cpu)
+{
+	int target;
+
+	/* Check if exiting cpu is used for collecting 24x7 events */
+	if (!cpumask_test_and_clear_cpu(cpu, &hv_24x7_cpumask))
+		return 0;
+
+	/* Find a new cpu to collect 24x7 events */
+	target = cpumask_last(cpu_active_mask);
+
+	if (target < 0 || target >= nr_cpu_ids) {
+		pr_err("hv_24x7: CPU hotplug init failed\n");
+		return -1;
+	}
+
+	/* Migrate 24x7 events to the new target */
+	cpumask_set_cpu(target, &hv_24x7_cpumask);
+	perf_pmu_migrate_context(&h_24x7_pmu, cpu, target);
+
+	return 0;
+}
+
+static int hv_24x7_cpu_hotplug_init(void)
+{
+	return cpuhp_setup_state(CPUHP_AP_PERF_POWERPC_HV_24x7_ONLINE,
+			  "perf/powerpc/hv_24x7:online",
+			  ppc_hv_24x7_cpu_online,
+			  ppc_hv_24x7_cpu_offline);
+}
+
 static int hv_24x7_init(void)
 {
 	int r;
@@ -1685,6 +1726,11 @@ static int hv_24x7_init(void)
 	if (r)
 		return r;
 
+	/* init cpuhotplug */
+	r = hv_24x7_cpu_hotplug_init();
+	if (r)
+		return r;
+
 	r = perf_pmu_register(&h_24x7_pmu, h_24x7_pmu.name, -1);
 	if (r)
 		return r;

include/linux/cpuhotplug.h

@@ -181,6 +181,7 @@ enum cpuhp_state {
 	CPUHP_AP_PERF_POWERPC_CORE_IMC_ONLINE,
 	CPUHP_AP_PERF_POWERPC_THREAD_IMC_ONLINE,
 	CPUHP_AP_PERF_POWERPC_TRACE_IMC_ONLINE,
+	CPUHP_AP_PERF_POWERPC_HV_24x7_ONLINE,
 	CPUHP_AP_WATCHDOG_ONLINE,
 	CPUHP_AP_WORKQUEUE_ONLINE,
 	CPUHP_AP_RCUTREE_ONLINE,