Re: [PATCH v3] watchdog: add watchdog_cpumask sysctl to assist nohz

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On 04/02/2015 02:33 PM, Peter Zijlstra wrote:
On Thu, Apr 02, 2015 at 02:16:09PM -0400, Chris Metcalf wrote:
On 04/02/2015 02:06 PM, Peter Zijlstra wrote:
On Thu, Apr 02, 2015 at 01:39:28PM -0400, cmetcalf@xxxxxxxxxx wrote:
@@ -431,6 +434,10 @@ static void watchdog_enable(unsigned int cpu)
  	hrtimer_init(hrtimer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
  	hrtimer->function = watchdog_timer_fn;
+	/* Exit if the cpu is not allowed for watchdog. */
+	if (!cpumask_test_cpu(cpu, watchdog_mask))
+		do_exit(0);
+
Ick, that doesn't look right for smpboot threads.
I didn't see a better way to make this happen without adding
a bunch of infrastructure to the smpboot thread mechanism
to use a cpumask other than for_each_online_cpu().  The exit
seems benign in my testing, but I agree it's not the cleanest
way to express what we're trying to do here.

Perhaps something like an optional cpumask_t pointer in
struct smp_hotplug_thread, which if present specifies the
cpus to run on, and otherwise we stick with cpu_online_mask?
What's wrong with just leaving the thread be but making sure it'll never
actually do anything?

I think a common case for nohz_full systems is that you'll
have a whole lot of watchdog threads that never do anything.
Our TILEGx-72 systems are often run with one housekeeping
core and the rest doing userspace nohz_full driver work.  So
not creating the threads seems tidier - it keeps 71 threads out
of the "ps" listing :-)

Here's a quick sketch of the delta from my previous patch to
one with a new smp_hotplug_thread.cpumask field.  If folks
are OK with modifying the smpboot threads like this, I think
it probably is a cleaner approach:

diff --git a/include/linux/smpboot.h b/include/linux/smpboot.h
index 13e929679550..f28519612ee3 100644
--- a/include/linux/smpboot.h
+++ b/include/linux/smpboot.h
@@ -27,6 +27,7 @@ struct smpboot_thread_data;
  * @pre_unpark:		Optional unpark function, called before the thread is
  *			unparked (cpu online). This is not guaranteed to be
  *			called on the target cpu of the thread. Careful!
+ * @cpumask:		Optional cpumask, specifying what cores to run on.
  * @selfparking:	Thread is not parked by the park function.
  * @thread_comm:	The base name of the thread
  */
@@ -41,6 +42,7 @@ struct smp_hotplug_thread {
 	void				(*park)(unsigned int cpu);
 	void				(*unpark)(unsigned int cpu);
 	void				(*pre_unpark)(unsigned int cpu);
+	cpumask_t			*cpumask;
 	bool				selfparking;
 	const char			*thread_comm;
 };
diff --git a/kernel/smpboot.c b/kernel/smpboot.c
index 40190f28db35..be503c2ddb5f 100644
--- a/kernel/smpboot.c
+++ b/kernel/smpboot.c
@@ -172,6 +172,9 @@ __smpboot_create_thread(struct smp_hotplug_thread *ht, unsigned int cpu)
 	if (tsk)
 		return 0;
+ if (ht->cpumask && !cpumask_test_cpu(cpu, ht->cpumask))
+		return 0;
+
 	td = kzalloc_node(sizeof(*td), GFP_KERNEL, cpu_to_node(cpu));
 	if (!td)
 		return -ENOMEM;
@@ -220,9 +223,11 @@ static void smpboot_unpark_thread(struct smp_hotplug_thread *ht, unsigned int cp
 {
 	struct task_struct *tsk = *per_cpu_ptr(ht->store, cpu);
- if (ht->pre_unpark)
-		ht->pre_unpark(cpu);
-	kthread_unpark(tsk);
+	if (tsk) {
+		if (ht->pre_unpark)
+			ht->pre_unpark(cpu);
+		kthread_unpark(tsk);
+	}
 }
void smpboot_unpark_threads(unsigned int cpu)
diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index 2140c2d81dc9..681e5648e093 100644
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -434,10 +434,6 @@ static void watchdog_enable(unsigned int cpu)
 	hrtimer_init(hrtimer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
 	hrtimer->function = watchdog_timer_fn;
- /* Exit if the cpu is not allowed for watchdog. */
-	if (!cpumask_test_cpu(cpu, watchdog_mask))
-		do_exit(0);
-
 	/* Enable the perf event */
 	watchdog_nmi_enable(cpu);
@@ -588,6 +584,7 @@ static struct smp_hotplug_thread watchdog_threads = {
 	.cleanup		= watchdog_cleanup,
 	.park			= watchdog_disable,
 	.unpark			= watchdog_enable,
+	.cpumask		= watchdog_mask,
 };
static void restart_watchdog_hrtimer(void *info)

--
Chris Metcalf, EZChip Semiconductor
http://www.ezchip.com

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