kthread: Implement park/unpark facility

To avoid the full teardown/setup of per cpu kthreads in the case of
cpu hot(un)plug, provide a facility which allows to put the kthread
into a park position and unpark it when the cpu comes online again.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Reviewed-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Link: http://lkml.kernel.org/r/20120716103948.236618824@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 166 insertions, 19 deletions

diff --git a/kernel/kthread.c b/kernel/kthread.c
index b579af57ea10..146a6fa96825 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -37,11 +37,20 @@ struct kthread_create_info
 };
 
 struct kthread {
-        int should_stop;
+        unsigned long flags;
+        unsigned int cpu;
         void *data;
+        struct completion parked;
         struct completion exited;
 };
 
+enum KTHREAD_BITS {
+        KTHREAD_IS_PER_CPU = 0,
+        KTHREAD_SHOULD_STOP,
+        KTHREAD_SHOULD_PARK,
+        KTHREAD_IS_PARKED,
+};
+
 #define to_kthread(tsk) \
         container_of((tsk)->vfork_done, struct kthread, exited)
 
@@ -52,13 +61,29 @@ struct kthread {
  * and this will return true.  You should then return, and your return
  * value will be passed through to kthread_stop().
  */
-int kthread_should_stop(void)
+bool kthread_should_stop(void)
 {
-        return to_kthread(current)->should_stop;
+        return test_bit(KTHREAD_SHOULD_STOP, &to_kthread(current)->flags);
 }
 EXPORT_SYMBOL(kthread_should_stop);
 
 /**
+ * kthread_should_park - should this kthread park now?
+ *
+ * When someone calls kthread_park() on your kthread, it will be woken
+ * and this will return true.  You should then do the necessary
+ * cleanup and call kthread_parkme()
+ *
+ * Similar to kthread_should_stop(), but this keeps the thread alive
+ * and in a park position. kthread_unpark() "restarts" the thread and
+ * calls the thread function again.
+ */
+bool kthread_should_park(void)
+{
+        return test_bit(KTHREAD_SHOULD_PARK, &to_kthread(current)->flags);
+}
+
+/**
  * kthread_freezable_should_stop - should this freezable kthread return now?
  * @was_frozen: optional out parameter, indicates whether %current was frozen
  *
@@ -96,6 +121,24 @@ void *kthread_data(struct task_struct *task)
         return to_kthread(task)->data;
 }
 
+static void __kthread_parkme(struct kthread *self)
+{
+        __set_current_state(TASK_INTERRUPTIBLE);
+        while (test_bit(KTHREAD_SHOULD_PARK, &self->flags)) {
+                if (!test_and_set_bit(KTHREAD_IS_PARKED, &self->flags))
+                        complete(&self->parked);
+                schedule();
+                __set_current_state(TASK_INTERRUPTIBLE);
+        }
+        clear_bit(KTHREAD_IS_PARKED, &self->flags);
+        __set_current_state(TASK_RUNNING);
+}
+
+void kthread_parkme(void)
+{
+        __kthread_parkme(to_kthread(current));
+}
+
 static int kthread(void *_create)
 {
         /* Copy data: it's on kthread's stack */
@@ -105,9 +148,10 @@ static int kthread(void *_create)
         struct kthread self;
         int ret;
 
-        self.should_stop = 0;
+        self.flags = 0;
         self.data = data;
         init_completion(&self.exited);
+        init_completion(&self.parked);
         current->vfork_done = &self.exited;
 
         /* OK, tell user we're spawned, wait for stop or wakeup */
@@ -117,9 +161,11 @@ static int kthread(void *_create)
         schedule();
 
         ret = -EINTR;
-        if (!self.should_stop)
-                ret = threadfn(data);
 
+        if (!test_bit(KTHREAD_SHOULD_STOP, &self.flags)) {
+                __kthread_parkme(&self);
+                ret = threadfn(data);
+        }
         /* we can't just return, we must preserve "self" on stack */
         do_exit(ret);
 }
@@ -172,8 +218,7 @@ static void create_kthread(struct kthread_create_info *create)
  * Returns a task_struct or ERR_PTR(-ENOMEM).
  */
 struct task_struct *kthread_create_on_node(int (*threadfn)(void *data),
-                                           void *data,
-                                           int node,
+                                           void *data, int node,
                                            const char namefmt[],
                                            ...)
 {
@@ -210,6 +255,13 @@ struct task_struct *kthread_create_on_node(int (*threadfn)(void *data),
 }
 EXPORT_SYMBOL(kthread_create_on_node);
 
+static void __kthread_bind(struct task_struct *p, unsigned int cpu)
+{
+        /* It's safe because the task is inactive. */
+        do_set_cpus_allowed(p, cpumask_of(cpu));
+        p->flags |= PF_THREAD_BOUND;
+}
+
 /**
  * kthread_bind - bind a just-created kthread to a cpu.
  * @p: thread created by kthread_create().
@@ -226,14 +278,112 @@ void kthread_bind(struct task_struct *p, unsigned int cpu)
                 WARN_ON(1);
                 return;
         }
-
-        /* It's safe because the task is inactive. */
-        do_set_cpus_allowed(p, cpumask_of(cpu));
-        p->flags |= PF_THREAD_BOUND;
+        __kthread_bind(p, cpu);
 }
 EXPORT_SYMBOL(kthread_bind);
 
 /**
+ * kthread_create_on_cpu - Create a cpu bound kthread
+ * @threadfn: the function to run until signal_pending(current).
+ * @data: data ptr for @threadfn.
+ * @cpu: The cpu on which the thread should be bound,
+ * @namefmt: printf-style name for the thread. Format is restricted
+ *           to "name.*%u". Code fills in cpu number.
+ *
+ * Description: This helper function creates and names a kernel thread
+ * The thread will be woken and put into park mode.
+ */
+struct task_struct *kthread_create_on_cpu(int (*threadfn)(void *data),
+                                          void *data, unsigned int cpu,
+                                          const char *namefmt)
+{
+        struct task_struct *p;
+
+        p = kthread_create_on_node(threadfn, data, cpu_to_node(cpu), namefmt,
+                                   cpu);
+        if (IS_ERR(p))
+                return p;
+        set_bit(KTHREAD_IS_PER_CPU, &to_kthread(p)->flags);
+        to_kthread(p)->cpu = cpu;
+        /* Park the thread to get it out of TASK_UNINTERRUPTIBLE state */
+        kthread_park(p);
+        return p;
+}
+
+static struct kthread *task_get_live_kthread(struct task_struct *k)
+{
+        struct kthread *kthread;
+
+        get_task_struct(k);
+        kthread = to_kthread(k);
+        /* It might have exited */
+        barrier();
+        if (k->vfork_done != NULL)
+                return kthread;
+        return NULL;
+}
+
+/**
+ * kthread_unpark - unpark a thread created by kthread_create().
+ * @k:          thread created by kthread_create().
+ *
+ * Sets kthread_should_park() for @k to return false, wakes it, and
+ * waits for it to return. If the thread is marked percpu then its
+ * bound to the cpu again.
+ */
+void kthread_unpark(struct task_struct *k)
+{
+        struct kthread *kthread = task_get_live_kthread(k);
+
+        if (kthread) {
+                clear_bit(KTHREAD_SHOULD_PARK, &kthread->flags);
+                /*
+                 * We clear the IS_PARKED bit here as we don't wait
+                 * until the task has left the park code. So if we'd
+                 * park before that happens we'd see the IS_PARKED bit
+                 * which might be about to be cleared.
+                 */
+                if (test_and_clear_bit(KTHREAD_IS_PARKED, &kthread->flags)) {
+                        if (test_bit(KTHREAD_IS_PER_CPU, &kthread->flags))
+                                __kthread_bind(k, kthread->cpu);
+                        wake_up_process(k);
+                }
+        }
+        put_task_struct(k);
+}
+
+/**
+ * kthread_park - park a thread created by kthread_create().
+ * @k: thread created by kthread_create().
+ *
+ * Sets kthread_should_park() for @k to return true, wakes it, and
+ * waits for it to return. This can also be called after kthread_create()
+ * instead of calling wake_up_process(): the thread will park without
+ * calling threadfn().
+ *
+ * Returns 0 if the thread is parked, -ENOSYS if the thread exited.
+ * If called by the kthread itself just the park bit is set.
+ */
+int kthread_park(struct task_struct *k)
+{
+        struct kthread *kthread = task_get_live_kthread(k);
+        int ret = -ENOSYS;
+
+        if (kthread) {
+                if (!test_bit(KTHREAD_IS_PARKED, &kthread->flags)) {
+                        set_bit(KTHREAD_SHOULD_PARK, &kthread->flags);
+                        if (k != current) {
+                                wake_up_process(k);
+                                wait_for_completion(&kthread->parked);
+                        }
+                }
+                ret = 0;
+        }
+        put_task_struct(k);
+        return ret;
+}
+
+/**
  * kthread_stop - stop a thread created by kthread_create().
  * @k: thread created by kthread_create().
  *
@@ -250,16 +400,13 @@ EXPORT_SYMBOL(kthread_bind);
  */
 int kthread_stop(struct task_struct *k)
 {
-        struct kthread *kthread;
+        struct kthread *kthread = task_get_live_kthread(k);
         int ret;
 
         trace_sched_kthread_stop(k);
-        get_task_struct(k);
-
-        kthread = to_kthread(k);
-        barrier(); /* it might have exited */
-        if (k->vfork_done != NULL) {
-                kthread->should_stop = 1;
+        if (kthread) {
+                set_bit(KTHREAD_SHOULD_STOP, &kthread->flags);
+                clear_bit(KTHREAD_SHOULD_PARK, &kthread->flags);
                 wake_up_process(k);
                 wait_for_completion(&kthread->exited);
         }