rcu: Omit self-awaken when setting up expedited grace period

When setting up an expedited grace period, if there were no readers, the
task will awaken itself.  This commit removes this useless self-awakening.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

3 files changed, 14 insertions, 7 deletions

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 69b6cdd4f944..8afb2e89745b 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -1320,7 +1320,7 @@ static void __rcu_offline_cpu(int cpu, struct rcu_state *rsp)
         else
                 raw_spin_unlock_irqrestore(&rnp->lock, flags);
         if (need_report & RCU_OFL_TASKS_EXP_GP)
-                rcu_report_exp_rnp(rsp, rnp);
+                rcu_report_exp_rnp(rsp, rnp, true);
         rcu_node_kthread_setaffinity(rnp, -1);
 }
 
diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index 0963fa1541ac..fd2f87db2ab1 100644
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -444,7 +444,8 @@ static void rcu_preempt_check_callbacks(int cpu);
 static void rcu_preempt_process_callbacks(void);
 void call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu));
 #if defined(CONFIG_HOTPLUG_CPU) || defined(CONFIG_TREE_PREEMPT_RCU)
-static void rcu_report_exp_rnp(struct rcu_state *rsp, struct rcu_node *rnp);
+static void rcu_report_exp_rnp(struct rcu_state *rsp, struct rcu_node *rnp,
+                               bool wake);
 #endif /* #if defined(CONFIG_HOTPLUG_CPU) || defined(CONFIG_TREE_PREEMPT_RCU) */
 static int rcu_preempt_pending(int cpu);
 static int rcu_preempt_needs_cpu(int cpu);
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index 708dc579634d..0f095d1cc16d 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -410,7 +410,7 @@ static noinline void rcu_read_unlock_special(struct task_struct *t)
                  * then we need to report up the rcu_node hierarchy.
                  */
                 if (!empty_exp && empty_exp_now)
-                        rcu_report_exp_rnp(&rcu_preempt_state, rnp);
+                        rcu_report_exp_rnp(&rcu_preempt_state, rnp, true);
         } else {
                 local_irq_restore(flags);
         }
@@ -732,9 +732,13 @@ static int sync_rcu_preempt_exp_done(struct rcu_node *rnp)
  * recursively up the tree.  (Calm down, calm down, we do the recursion
  * iteratively!)
  *
+ * Most callers will set the "wake" flag, but the task initiating the
+ * expedited grace period need not wake itself.
+ *
  * Caller must hold sync_rcu_preempt_exp_mutex.
  */
-static void rcu_report_exp_rnp(struct rcu_state *rsp, struct rcu_node *rnp)
+static void rcu_report_exp_rnp(struct rcu_state *rsp, struct rcu_node *rnp,
+                               bool wake)
 {
         unsigned long flags;
         unsigned long mask;
@@ -747,7 +751,8 @@ static void rcu_report_exp_rnp(struct rcu_state *rsp, struct rcu_node *rnp)
                 }
                 if (rnp->parent == NULL) {
                         raw_spin_unlock_irqrestore(&rnp->lock, flags);
-                        wake_up(&sync_rcu_preempt_exp_wq);
+                        if (wake)
+                                wake_up(&sync_rcu_preempt_exp_wq);
                         break;
                 }
                 mask = rnp->grpmask;
@@ -780,7 +785,7 @@ sync_rcu_preempt_exp_init(struct rcu_state *rsp, struct rcu_node *rnp)
                 must_wait = 1;
         }
         if (!must_wait)
-                rcu_report_exp_rnp(rsp, rnp);
+                rcu_report_exp_rnp(rsp, rnp, false); /* Don't wake self. */
 }
 
 /*
@@ -1072,7 +1077,8 @@ EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);
  * report on tasks preempted in RCU read-side critical sections during
  * expedited RCU grace periods.
  */
-static void rcu_report_exp_rnp(struct rcu_state *rsp, struct rcu_node *rnp)
+static void rcu_report_exp_rnp(struct rcu_state *rsp, struct rcu_node *rnp,
+                               bool wake)
 {
         return;
 }