rcu: Cure load woes

Commit cc3ce5176d83 (rcu: Start RCU kthreads in TASK_INTERRUPTIBLE
state) fudges a sleeping task' state, resulting in the scheduler seeing
a TASK_UNINTERRUPTIBLE task going to sleep, but a TASK_INTERRUPTIBLE
task waking up. The result is unbalanced load calculation.

The problem that patch tried to address is that the RCU threads could
stay in UNINTERRUPTIBLE state for quite a while and triggering the hung
task detector due to on-demand wake-ups.

Cure the problem differently by always giving the tasks at least one
wake-up once the CPU is fully up and running, this will kick them out of
the initial UNINTERRUPTIBLE state and into the regular INTERRUPTIBLE
wait state.

[ The alternative would be teaching kthread_create() to start threads as
  INTERRUPTIBLE but that needs a tad more thought. ]

Reported-by: Damien Wyart <damien.wyart@free.fr>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: Paul E. McKenney <paul.mckenney@linaro.org>
Link: http://lkml.kernel.org/r/1306755291.1200.2872.camel@twins
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 46 insertions, 8 deletions

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 77a7671dd147..89419ff92e99 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -1648,7 +1648,6 @@ static int __cpuinit rcu_spawn_one_cpu_kthread(int cpu)
         if (IS_ERR(t))
                 return PTR_ERR(t);
         kthread_bind(t, cpu);
-        set_task_state(t, TASK_INTERRUPTIBLE);
         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;
@@ -1756,7 +1755,6 @@ static int __cpuinit rcu_spawn_one_node_kthread(struct rcu_state *rsp,
                 if (IS_ERR(t))
                         return PTR_ERR(t);
                 raw_spin_lock_irqsave(&rnp->lock, flags);
-                set_task_state(t, TASK_INTERRUPTIBLE);
                 rnp->node_kthread_task = t;
                 raw_spin_unlock_irqrestore(&rnp->lock, flags);
                 sp.sched_priority = 99;
@@ -1765,6 +1763,8 @@ static int __cpuinit rcu_spawn_one_node_kthread(struct rcu_state *rsp,
         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.
  */
@@ -1772,18 +1772,30 @@ 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;
 }
@@ -2188,14 +2200,14 @@ rcu_init_percpu_data(int cpu, struct rcu_state *rsp, int preemptible)
         raw_spin_unlock_irqrestore(&rsp->onofflock, flags);
 }
 
-static void __cpuinit rcu_online_cpu(int cpu)
+static void __cpuinit rcu_prepare_cpu(int cpu)
 {
         rcu_init_percpu_data(cpu, &rcu_sched_state, 0);
         rcu_init_percpu_data(cpu, &rcu_bh_state, 0);
         rcu_preempt_init_percpu_data(cpu);
 }
 
-static void __cpuinit rcu_online_kthreads(int cpu)
+static void __cpuinit rcu_prepare_kthreads(int cpu)
 {
         struct rcu_data *rdp = per_cpu_ptr(rcu_state->rda, cpu);
         struct rcu_node *rnp = rdp->mynode;
@@ -2209,6 +2221,31 @@ static void __cpuinit rcu_online_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,
@@ -2221,10 +2258,11 @@ static int __cpuinit rcu_cpu_notify(struct notifier_block *self,
         switch (action) {
         case CPU_UP_PREPARE:
         case CPU_UP_PREPARE_FROZEN:
-                rcu_online_cpu(cpu);
-                rcu_online_kthreads(cpu);
+                rcu_prepare_cpu(cpu);
+                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);