[PATCH v6 1/2] smpboot: allow excluding cpus from the smpboot threads

[<cmetcalf@xxxxxxxxxx>]

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>
---
v6: change from an "exclude" data pointer to a more generic
    valid_cpu() callback [Frederic]

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

diff --git a/include/linux/smpboot.h b/include/linux/smpboot.h
index d600afb21926..4648a4576ae4 100644
--- a/include/linux/smpboot.h
+++ b/include/linux/smpboot.h
@@ -27,6 +27,8 @@ 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!
+ * @valid_cpu:		Optional function, called when creating the threads,
+ *			to limit the set of cpus on which threads are created.
  * @selfparking:	Thread is not parked by the park function.
  * @thread_comm:	The base name of the thread
  */
@@ -41,6 +43,7 @@ struct smp_hotplug_thread {
 	void				(*park)(unsigned int cpu);
 	void				(*unpark)(unsigned int cpu);
 	void				(*pre_unpark)(unsigned int cpu);
+	int				(*valid_cpu)(unsigned int cpu);
 	bool				selfparking;
 	const char			*thread_comm;
 };
diff --git a/kernel/smpboot.c b/kernel/smpboot.c
index c697f73d82d6..6ffc2dacb94a 100644
--- a/kernel/smpboot.c
+++ b/kernel/smpboot.c
@@ -173,6 +173,9 @@ __smpboot_create_thread(struct smp_hotplug_thread *ht, unsigned int cpu)
 	if (tsk)
 		return 0;
 
+	if (ht->valid_cpu && !ht->valid_cpu(cpu))
+		return 0;
+
 	td = kzalloc_node(sizeof(*td), GFP_KERNEL, cpu_to_node(cpu));
 	if (!td)
 		return -ENOMEM;
@@ -221,9 +224,11 @@ 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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