[PATCH] trace/hwlat: Do not restart per-cpu threads if they are already running

[Tero Kristo]

Check if the hwlatd thread for the cpu is already running, before
starting a new one. This avoids running multiple instances of the same
CPU thread on the system. Also, do not wipe the contents of the
per-cpu kthread data when starting the tracer, as this can completely
forget about already running instances and start new additional per-cpu
threads. Fixes issues where fiddling with either the mode of the hwlat
tracer or doing cpu-hotplugs messes up the internal book-keeping
resulting in stale hwlatd threads.

Signed-off-by: Tero Kristo <tero.kristo@xxxxxxxxxxxxxxx>
---
 kernel/trace/trace_hwlat.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/kernel/trace/trace_hwlat.c b/kernel/trace/trace_hwlat.c
index d440ddd5fd8b..c4945f8adc11 100644
--- a/kernel/trace/trace_hwlat.c
+++ b/kernel/trace/trace_hwlat.c
@@ -492,6 +492,10 @@ static int start_cpu_kthread(unsigned int cpu)
 {
 	struct task_struct *kthread;
 
+	/* Do not start a new hwlatd thread if it is already running */
+	if (per_cpu(hwlat_per_cpu_data, cpu).kthread)
+		return 0;
+
 	kthread = kthread_run_on_cpu(kthread_fn, NULL, cpu, "hwlatd/%u");
 	if (IS_ERR(kthread)) {
 		pr_err(BANNER "could not start sampling thread\n");
@@ -584,9 +588,6 @@ static int start_per_cpu_kthreads(struct trace_array *tr)
 	 */
 	cpumask_and(current_mask, cpu_online_mask, tr->tracing_cpumask);
 
-	for_each_online_cpu(cpu)
-		per_cpu(hwlat_per_cpu_data, cpu).kthread = NULL;
-
 	for_each_cpu(cpu, current_mask) {
 		retval = start_cpu_kthread(cpu);
 		if (retval)
-- 
2.25.1