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Subject[PATCH v9 1/3] smpboot: allow excluding cpus from the smpboot threads
Date
This change allows some cores to be excluded from running the
smp_hotplug_thread tasks. The following commit to update
kernel/watchdog.c to use this functionality is the motivating
example, and more information on the motivation is provided there.

A new smp_hotplug_thread field is introduced, "cpumask", which
is cpumask field managed by the smpboot subsystem that indicates whether
or not the given smp_hotplug_thread should run on that core; the
cpumask is checked when deciding whether to unpark the thread.

To limit the cpumask to less than cpu_possible, you must call
smpboot_update_cpumask_percpu_thread() after registering.

Signed-off-by: Chris Metcalf <cmetcalf@ezchip.com>
---
v9: move cpumask into smpboot_hotplug_thread and don't let the
client initialize it either [Frederic]
use alloc_cpumask_var, not a locked static cpumask [Frederic]

v8: make cpumask only updated by smpboot subsystem [Frederic]

v7: change from valid_cpu() callback to optional cpumask field
park smpboot threads rather than just not creating them

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

v5: switch from watchdog_exclude to watchdog_cpumask [Frederic]
simplify the smp_hotplug_thread API to watchdog [Frederic]

include/linux/smpboot.h | 6 ++++++
kernel/smpboot.c | 57 +++++++++++++++++++++++++++++++++++++++++++++++--
2 files changed, 61 insertions(+), 2 deletions(-)

include/linux/smpboot.h | 5 +++++
kernel/smpboot.c | 55 ++++++++++++++++++++++++++++++++++++++++++++++++-
2 files changed, 59 insertions(+), 1 deletion(-)

diff --git a/include/linux/smpboot.h b/include/linux/smpboot.h
index d600afb21926..7c42153edfac 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!
+ * @cpumask: Internal state. To update which threads are unparked,
+ * call smpboot_update_cpumask_percpu_thread().
* @selfparking: Thread is not parked by the park function.
* @thread_comm: The base name of the thread
*/
@@ -41,11 +43,14 @@ struct smp_hotplug_thread {
void (*park)(unsigned int cpu);
void (*unpark)(unsigned int cpu);
void (*pre_unpark)(unsigned int cpu);
+ struct cpumask cpumask;
bool selfparking;
const char *thread_comm;
};

int smpboot_register_percpu_thread(struct smp_hotplug_thread *plug_thread);
void smpboot_unregister_percpu_thread(struct smp_hotplug_thread *plug_thread);
+int smpboot_update_cpumask_percpu_thread(struct smp_hotplug_thread *plug_thread,
+ const struct cpumask *);

#endif
diff --git a/kernel/smpboot.c b/kernel/smpboot.c
index c697f73d82d6..0d131daf3e7f 100644
--- a/kernel/smpboot.c
+++ b/kernel/smpboot.c
@@ -232,7 +232,8 @@ void smpboot_unpark_threads(unsigned int cpu)

mutex_lock(&smpboot_threads_lock);
list_for_each_entry(cur, &hotplug_threads, list)
- smpboot_unpark_thread(cur, cpu);
+ if (cpumask_test_cpu(cpu, &cur->cpumask))
+ smpboot_unpark_thread(cur, cpu);
mutex_unlock(&smpboot_threads_lock);
}

@@ -258,6 +259,15 @@ static void smpboot_destroy_threads(struct smp_hotplug_thread *ht)
{
unsigned int cpu;

+ /* Unpark any threads that were voluntarily parked. */
+ for_each_cpu_not(cpu, &ht->cpumask) {
+ if (cpu_online(cpu)) {
+ struct task_struct *tsk = *per_cpu_ptr(ht->store, cpu);
+ if (tsk)
+ kthread_unpark(tsk);
+ }
+ }
+
/* We need to destroy also the parked threads of offline cpus */
for_each_possible_cpu(cpu) {
struct task_struct *tsk = *per_cpu_ptr(ht->store, cpu);
@@ -281,6 +291,7 @@ int smpboot_register_percpu_thread(struct smp_hotplug_thread *plug_thread)
unsigned int cpu;
int ret = 0;

+ cpumask_copy(&plug_thread->cpumask, cpu_possible_mask);
get_online_cpus();
mutex_lock(&smpboot_threads_lock);
for_each_online_cpu(cpu) {
@@ -316,6 +327,48 @@ void smpboot_unregister_percpu_thread(struct smp_hotplug_thread *plug_thread)
}
EXPORT_SYMBOL_GPL(smpboot_unregister_percpu_thread);

+/**
+ * smpboot_update_cpumask_percpu_thread - Adjust which per_cpu hotplug threads stay parked
+ * @plug_thread: Hotplug thread descriptor
+ * @new: Revised mask to use
+ *
+ * The cpumask field in the smp_hotplug_thread must not be updated directly
+ * by the client, but only by calling this function.
+ */
+int smpboot_update_cpumask_percpu_thread(struct smp_hotplug_thread *plug_thread,
+ const struct cpumask *new)
+{
+ struct cpumask *old = &plug_thread->cpumask;
+ cpumask_var_t tmp;
+ unsigned int cpu;
+
+ if (!alloc_cpumask_var(&tmp, GFP_KERNEL))
+ return -ENOMEM;
+
+ get_online_cpus();
+ mutex_lock(&smpboot_threads_lock);
+
+ /* Park threads that were exclusively enabled on the old mask. */
+ cpumask_andnot(&tmp, old, new);
+ for_each_cpu_and(cpu, &tmp, cpu_online_mask)
+ smpboot_park_thread(plug_thread, cpu);
+
+ /* Unpark threads that are exclusively enabled on the new mask. */
+ cpumask_andnot(&tmp, new, old);
+ for_each_cpu_and(cpu, &tmp, cpu_online_mask)
+ smpboot_unpark_thread(plug_thread, cpu);
+
+ cpumask_copy(old, new);
+
+ mutex_unlock(&smpboot_threads_lock);
+ put_online_cpus();
+
+ free_cpumask_var(tmp);
+
+ return 0;
+}
+EXPORT_SYMBOL_GPL(smpboot_update_cpumask_percpu_thread);
+
static DEFINE_PER_CPU(atomic_t, cpu_hotplug_state) = ATOMIC_INIT(CPU_POST_DEAD);

/*
--
2.1.2


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