[PATCH] rcuwait: do not enter RCU protection unless a wakeup is needed

[Paolo Bonzini]

In some cases, rcuwait_wake_up can be called even if the actual likelihood
of a wakeup is very low.  If CONFIG_PREEMPT_RCU is active, the resulting
rcu_read_lock/rcu_read_unlock pair can be relatively expensive, and in
fact it is unnecessary when there is no w->task to keep alive: the
memory barrier before the read is what matters in order to avoid missed
wakeups.

Therefore, do an early check of w->task right after the barrier, and skip
rcu_read_lock/rcu_read_unlock unless there is someone waiting for a wakeup.

Running kvm-unit-test/vmexit.flat with APICv disabled, most interrupt
injection tests (tscdeadline*, self_ipi*, x2apic_self_ipi*) improve
by around 600 cpu cycles.

Cc: Davidlohr Bueso <dave@xxxxxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Reported-by: Wanpeng Li <wanpengli@xxxxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
---
 kernel/exit.c | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index 91a43e57a32e..a38a08dbf85e 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -234,8 +234,6 @@ int rcuwait_wake_up(struct rcuwait *w)
 	int ret = 0;
 	struct task_struct *task;
 
-	rcu_read_lock();
-
 	/*
 	 * Order condition vs @task, such that everything prior to the load
 	 * of @task is visible. This is the condition as to why the user called
@@ -245,10 +243,22 @@ int rcuwait_wake_up(struct rcuwait *w)
 	 *    WAIT                WAKE
 	 *    [S] tsk = current	  [S] cond = true
 	 *        MB (A)	      MB (B)
-	 *    [L] cond		  [L] tsk
+	 *    [L] cond		  [L] rcuwait_active(w)
+	 *                            task = rcu_dereference(w->task)
 	 */
 	smp_mb(); /* (B) */
 
+#ifdef CONFIG_PREEMPT_RCU
+	/*
+	 * The cost of rcu_read_lock() dominates for preemptible RCU,
+	 * avoid it if possible.
+	 */
+	if (!rcuwait_active(w))
+		return ret;
+#endif
+
+	rcu_read_lock();
+
 	task = rcu_dereference(w->task);
 	if (task)
 		ret = wake_up_process(task);
-- 
2.27.0