[PATCH v4 1/2] smpboot: allow excluding cpus from the smpboot threads

[cmetcalf]

From: Chris Metcalf <cmetcalf@xxxxxxxxxx>

This change allows some cores to be excluded from running the
smp_hotplug_thread tasks.  The motivating example for this is
the watchdog threads, which by default we don't want to run
on any enabled nohz_full cores.

Signed-off-by: Chris Metcalf <cmetcalf@xxxxxxxxxx>
---
Relative to the quick diff I emailed out yesterday (sorry missed
adding Thomas to that one), this change uses a notion of "exclude_mask"
for smp_hotplug_thread.  I think this is a better fit in general to
the idea that smp_hotplug_thread should run everywhere (including on
any newly-onlined cpus), EXCEPT for cores that have been specifically
tagged as reserved for some reason.  Obviously for nohz_full we have
the nohz_full boot argument to use for this at the moment.

Thomas, Peter, how does this look to you?

 include/linux/smpboot.h |  2 ++
 kernel/smpboot.c        | 11 ++++++++---
 2 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/include/linux/smpboot.h b/include/linux/smpboot.h
index 13e929679550..0631964525f7 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!
+ * @exclude_mask:	Optional cpumask, specifying cores to exclude.
  * @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			*exclude_mask;
 	bool				selfparking;
 	const char			*thread_comm;
 };
diff --git a/kernel/smpboot.c b/kernel/smpboot.c
index 40190f28db35..7df326ed80eb 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->exclude_mask && cpumask_test_cpu(cpu, ht->exclude_mask))
+		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)
-- 
2.1.2

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