rcu: Simplify curing of load woes

Make the functions creating the kthreads wake them up.  Leverage the
fact that the per-node and boost kthreads can run anywhere, thus
dispensing with the need to wake them up once the incoming CPU has
gone fully online.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Daniel J Blueman <daniel.blueman@gmail.com>

2 files changed, 22 insertions, 54 deletions

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 89419ff92e99..0a8ec5b2e208 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -1635,6 +1635,20 @@ static int rcu_cpu_kthread(void *arg)
  * to manipulate rcu_cpu_kthread_task.  There might be another CPU
  * attempting to access it during boot, but the locking in kthread_bind()
  * will enforce sufficient ordering.
+ *
+ * Please note that we cannot simply refuse to wake up the per-CPU
+ * kthread because kthreads are created in TASK_UNINTERRUPTIBLE state,
+ * which can result in softlockup complaints if the task ends up being
+ * idle for more than a couple of minutes.
+ *
+ * However, please note also that we cannot bind the per-CPU kthread to its
+ * CPU until that CPU is fully online.  We also cannot wait until the
+ * CPU is fully online before we create its per-CPU kthread, as this would
+ * deadlock the system when CPU notifiers tried waiting for grace
+ * periods.  So we bind the per-CPU kthread to its CPU only if the CPU
+ * is online.  If its CPU is not yet fully online, then the code in
+ * rcu_cpu_kthread() will wait until it is fully online, and then do
+ * the binding.
  */
 static int __cpuinit rcu_spawn_one_cpu_kthread(int cpu)
 {
@@ -1647,12 +1661,14 @@ static int __cpuinit rcu_spawn_one_cpu_kthread(int cpu)
         t = kthread_create(rcu_cpu_kthread, (void *)(long)cpu, "rcuc%d", cpu);
         if (IS_ERR(t))
                 return PTR_ERR(t);
-        kthread_bind(t, cpu);
+        if (cpu_online(cpu))
+                kthread_bind(t, cpu);
         per_cpu(rcu_cpu_kthread_cpu, cpu) = cpu;
         WARN_ON_ONCE(per_cpu(rcu_cpu_kthread_task, cpu) != NULL);
-        per_cpu(rcu_cpu_kthread_task, cpu) = t;
         sp.sched_priority = RCU_KTHREAD_PRIO;
         sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
+        per_cpu(rcu_cpu_kthread_task, cpu) = t;
+        wake_up_process(t); /* Get to TASK_INTERRUPTIBLE quickly. */
         return 0;
 }
 
@@ -1759,12 +1775,11 @@ static int __cpuinit rcu_spawn_one_node_kthread(struct rcu_state *rsp,
                 raw_spin_unlock_irqrestore(&rnp->lock, flags);
                 sp.sched_priority = 99;
                 sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
+                wake_up_process(t); /* get to TASK_INTERRUPTIBLE quickly. */
         }
         return rcu_spawn_one_boost_kthread(rsp, rnp, rnp_index);
 }
 
-static void rcu_wake_one_boost_kthread(struct rcu_node *rnp);
-
 /*
  * Spawn all kthreads -- called as soon as the scheduler is running.
  */
@@ -1772,30 +1787,18 @@ static int __init rcu_spawn_kthreads(void)
 {
         int cpu;
         struct rcu_node *rnp;
-        struct task_struct *t;
 
         rcu_kthreads_spawnable = 1;
         for_each_possible_cpu(cpu) {
                 per_cpu(rcu_cpu_has_work, cpu) = 0;
-                if (cpu_online(cpu)) {
+                if (cpu_online(cpu))
                         (void)rcu_spawn_one_cpu_kthread(cpu);
-                        t = per_cpu(rcu_cpu_kthread_task, cpu);
-                        if (t)
-                                wake_up_process(t);
-                }
         }
         rnp = rcu_get_root(rcu_state);
         (void)rcu_spawn_one_node_kthread(rcu_state, rnp);
-        if (rnp->node_kthread_task)
-                wake_up_process(rnp->node_kthread_task);
         if (NUM_RCU_NODES > 1) {
-                rcu_for_each_leaf_node(rcu_state, rnp) {
+                rcu_for_each_leaf_node(rcu_state, rnp)
                         (void)rcu_spawn_one_node_kthread(rcu_state, rnp);
-                        t = rnp->node_kthread_task;
-                        if (t)
-                                wake_up_process(t);
-                        rcu_wake_one_boost_kthread(rnp);
-                }
         }
         return 0;
 }
@@ -2221,31 +2224,6 @@ static void __cpuinit rcu_prepare_kthreads(int cpu)
 }
 
 /*
- * kthread_create() creates threads in TASK_UNINTERRUPTIBLE state,
- * but the RCU threads are woken on demand, and if demand is low this
- * could be a while triggering the hung task watchdog.
- *
- * In order to avoid this, poke all tasks once the CPU is fully
- * up and running.
- */
-static void __cpuinit rcu_online_kthreads(int cpu)
-{
-        struct rcu_data *rdp = per_cpu_ptr(rcu_state->rda, cpu);
-        struct rcu_node *rnp = rdp->mynode;
-        struct task_struct *t;
-
-        t = per_cpu(rcu_cpu_kthread_task, cpu);
-        if (t)
-                wake_up_process(t);
-
-        t = rnp->node_kthread_task;
-        if (t)
-                wake_up_process(t);
-
-        rcu_wake_one_boost_kthread(rnp);
-}
-
-/*
  * Handle CPU online/offline notification events.
  */
 static int __cpuinit rcu_cpu_notify(struct notifier_block *self,
@@ -2262,7 +2240,6 @@ static int __cpuinit rcu_cpu_notify(struct notifier_block *self,
                 rcu_prepare_kthreads(cpu);
                 break;
         case CPU_ONLINE:
-                rcu_online_kthreads(cpu);
         case CPU_DOWN_FAILED:
                 rcu_node_kthread_setaffinity(rnp, -1);
                 rcu_cpu_kthread_setrt(cpu, 1);
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index c8bff3099a89..ea2e2fb79e81 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -1299,15 +1299,10 @@ static int __cpuinit rcu_spawn_one_boost_kthread(struct rcu_state *rsp,
         raw_spin_unlock_irqrestore(&rnp->lock, flags);
         sp.sched_priority = RCU_KTHREAD_PRIO;
         sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
+        wake_up_process(t); /* get to TASK_INTERRUPTIBLE quickly. */
         return 0;
 }
 
-static void __cpuinit rcu_wake_one_boost_kthread(struct rcu_node *rnp)
-{
-        if (rnp->boost_kthread_task)
-                wake_up_process(rnp->boost_kthread_task);
-}
-
 #else /* #ifdef CONFIG_RCU_BOOST */
 
 static void rcu_initiate_boost(struct rcu_node *rnp, unsigned long flags)
@@ -1331,10 +1326,6 @@ static int __cpuinit rcu_spawn_one_boost_kthread(struct rcu_state *rsp,
         return 0;
 }
 
-static void __cpuinit rcu_wake_one_boost_kthread(struct rcu_node *rnp)
-{
-}
-
 #endif /* #else #ifdef CONFIG_RCU_BOOST */
 
 #ifndef CONFIG_SMP