tracing: Fix hwlat kthread migration

Linus,

It was reported to me that the thread created by the hwlat tracer does not migrate after the first instance. I found that there was a small bug in the logic, and fixed it. It's minor, but should be fixed regardless. There's not much impact outside the hwlat tracer.

Please pull the latest trace-4.10-rc2 tree, which can be found at:

git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-trace.git
trace-4.10-rc2

Tag SHA1: c3330f7b334f77118861ed8e864d82d217aa5efd Head SHA1: 79c6f448c8b79c321e4a1f31f98194e4f6b6cae7

Steven Rostedt (VMware) (1):
tracing: Fix hwlat kthread migration

----

 kernel/trace/trace_hwlat.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)
---------------------------
commit 79c6f448c8b79c321e4a1f31f98194e4f6b6cae7
Author: Steven Rostedt (VMware) <[email protected]>
Date:   Mon Jan 30 19:27:10 2017 -0500

tracing: Fix hwlat kthread migration

The hwlat tracer creates a kernel thread at start of the tracer. It is pinned to a single CPU and will move to the next CPU after each period of running. If the user modifies the migration thread's affinity, it will not change after that happens.

The original code created the thread at the first instance it was called, but later was changed to destroy the thread after the tracer was finished, and would not be created until the next instance of the tracer was established. The code that initialized the affinity was only called on the initial instantiation of the tracer. After that, it was not initialized, and the previous affinity did not match the current newly created one, making it appear that the user modified the thread's affinity when it did not, and the thread failed to migrate again.

index 775569ec50d0..af344a1bf0d0 100644
@@ -266,7 +266,7 @@ static int get_sample(void)
 static struct cpumask save_cpumask;
 static bool disable_migrate;
 
-static void move_to_next_cpu(void)
+static void move_to_next_cpu(bool initmask)
 {
 	static struct cpumask *current_mask;
 	int next_cpu;

@@ -275,7 +275,7 @@ static void move_to_next_cpu(void)
 		return;
 
 	/* Just pick the first CPU on first iteration */
-	if (!current_mask) {
+	if (initmask) {
 		current_mask = &save_cpumask;
 		get_online_cpus();
 		cpumask_and(current_mask, cpu_online_mask, tracing_buffer_mask);

@@ -330,10 +330,12 @@ static void move_to_next_cpu(void)
 static int kthread_fn(void *data)
 {
 	u64 interval;
+	bool initmask = true;
 
 	while (!kthread_should_stop()) {
 
-		move_to_next_cpu();
+		move_to_next_cpu(initmask);
+		initmask = false;
 
 		local_irq_disable();
 		get_sample();

That tag doesn't exist. It looks like it's the "urgent" branch you want me to pull, but no tag for me there...

Forgot to push?

Linus

Crap. Yeah, I forgot to push the tag. It's the same as trace/ftrace/urgent that I push to have the zero day bot test it. But that's not signed.

I just pushed it now.

Sorry about that :-/

-- Steve