rcu: Avoid redundant grace-period kthread wakeups

When setting up an in-the-future "advanced" grace period, the code needs
to wake up the relevant grace-period kthread, which it currently does
unconditionally.  However, this results in needless wakeups in the case
where the advanced grace period is being set up by the grace-period
kthread itself, which is a non-uncommon situation.  This commit therefore
checks to see if the running thread is the grace-period kthread, and
avoids doing the irq_work_queue()-mediated wakeup in that case.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

1 files changed, 5 insertions, 3 deletions

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 91840566e294..c6a064abd6a0 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -1576,10 +1576,12 @@ rcu_start_gp_advanced(struct rcu_state *rsp, struct rcu_node *rnp,
 
         /*
          * We can't do wakeups while holding the rnp->lock, as that
-         * could cause possible deadlocks with the rq->lock. Deter
-         * the wakeup to interrupt context.
+         * could cause possible deadlocks with the rq->lock. Defer
+         * the wakeup to interrupt context.  And don't bother waking
+         * up the running kthread.
          */
-        irq_work_queue(&rsp->wakeup_work);
+        if (current != rsp->gp_kthread)
+                irq_work_queue(&rsp->wakeup_work);
 }
 
 /*