[PATCH V4 05/12] trace/hwlat: Support hotplug operations

[Daniel Bristot de Oliveira <bristot@redhat.com>]

Enable and disable hwlat thread during cpu hotplug online
and offline operations, respectivelly.

Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Alexandre Chartre <alexandre.chartre@oracle.com>
Cc: Clark Willaims <williams@redhat.com>
Cc: John Kacur <jkacur@redhat.com>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: x86@kernel.org
Cc: linux-doc@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Daniel Bristot de Oliveira <bristot@redhat.com>
---
 kernel/trace/trace.c       |  6 ++++++
 kernel/trace/trace_hwlat.c | 34 ++++++++++++++++++++++++++++++++++
 2 files changed, 40 insertions(+)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 9299057feb56..c094865e2f71 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -5064,7 +5064,13 @@ int tracing_set_cpumask(struct trace_array *tr,
 	arch_spin_unlock(&tr->max_lock);
 	local_irq_enable();
 
+	/*
+	 * tracing_cpumask is read by tracers that support CPU
+	 * hotplug.
+	 */
+	get_online_cpus();
 	cpumask_copy(tr->tracing_cpumask, tracing_cpumask_new);
+	put_online_cpus();
 
 	return 0;
 }
diff --git a/kernel/trace/trace_hwlat.c b/kernel/trace/trace_hwlat.c
index 6c6918e86087..9fcfd588c4f6 100644
--- a/kernel/trace/trace_hwlat.c
+++ b/kernel/trace/trace_hwlat.c
@@ -490,6 +490,35 @@ static int start_cpu_kthread(unsigned int cpu)
 	return 0;
 }
 
+/*
+ * hwlat_cpu_init - CPU hotplug online callback function
+ */
+static int hwlat_cpu_init(unsigned int cpu)
+{
+	struct trace_array *tr = hwlat_trace;
+
+	if (!hwlat_busy)
+		return 0;
+
+	if (!cpumask_test_cpu(cpu, tr->tracing_cpumask))
+		return 0;
+
+	return start_cpu_kthread(cpu);
+}
+
+/*
+ * hwlat_cpu_die - CPU hotplug offline callback function
+ */
+static int hwlat_cpu_die(unsigned int cpu)
+{
+	if (!hwlat_busy)
+		return 0;
+
+	stop_cpu_kthread(cpu);
+
+	return 0;
+}
+
 /*
  * start_per_cpu_kthreads - Kick off the hardware latency sampling/detector kthreads
  *
@@ -903,6 +932,11 @@ __init static int init_hwlat_tracer(void)
 	if (ret)
 		return ret;
 
+	ret = cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "trace/hwlat:online",
+				hwlat_cpu_init, hwlat_cpu_die);
+	if (ret < 0)
+		pr_warn(BANNER "Error to init cpu hotplug support\n");
+
 	init_tracefs();
 
 	return 0;
-- 
2.31.1