[PATCH RFC tip/core/urgent] rcu: Avoid having just-onlined CPUresched itself when RCU is idle

[Paul E. McKenney]

CPUs set rdp->qs_pending when coming online to resolve races with
grace-period start.  However, this means that if RCU is idle, the
just-onlined CPU might needlessly send itself resched IPIs.  Adjust the
online-CPU initialization to avoid this, and also to correctly cause
the CPU to respond to the current grace period if needed.

This patch is believed to fix or otherwise suppress problems in
https://bugzilla.redhat.com/show_bug.cgi?id=726877, however, the
relationship is not apparent to this patch's author.  That said, it is
hard to argue with test results.

This will be available from -rcu as soon as kernel.org's mirrors update:

git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-2.6-rcu.git rcu/urgent

Reported-by: Nicolas Mailhot <nicolas.mailhot@xxxxxxxxxxx>
Reported-by: Michal Jaegermann <michal@xxxxxxxxxxxx>
Reported-by: Josh Boyer <jwboyer@xxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Tested-by: Josh Boyer <jwboyer@xxxxxxxxxx>

 rcutree.c |   11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index ba06207..39cc360 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -1865,8 +1865,6 @@ rcu_init_percpu_data(int cpu, struct rcu_state *rsp, int preemptible)
 
 	/* Set up local state, ensuring consistent view of global state. */
 	raw_spin_lock_irqsave(&rnp->lock, flags);
-	rdp->passed_quiesc = 0;  /* We could be racing with new GP, */
-	rdp->qs_pending = 1;	 /*  so set up to respond to current GP. */
 	rdp->beenonline = 1;	 /* We have now been online. */
 	rdp->preemptible = preemptible;
 	rdp->qlen_last_fqs_check = 0;
@@ -1891,8 +1889,15 @@ rcu_init_percpu_data(int cpu, struct rcu_state *rsp, int preemptible)
 		rnp->qsmaskinit |= mask;
 		mask = rnp->grpmask;
 		if (rnp == rdp->mynode) {
-			rdp->gpnum = rnp->completed; /* if GP in progress... */
+			/*
+			 * If there is a grace period in progress, we will
+			 * set up to wait for it next time we run the
+			 * RCU core code.
+			 */
+			rdp->gpnum = rnp->completed;
 			rdp->completed = rnp->completed;
+			rdp->passed_quiesc = 0;
+			rdp->qs_pending = 0;
 			rdp->passed_quiesc_completed = rnp->completed - 1;
 		}
 		raw_spin_unlock(&rnp->lock); /* irqs already disabled. */
--
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