Linux-3.1-rc2

Add better fix from Paul McKenney for rcu scheduling issue (rhbz 726877)

1 files changed, 47 insertions, 0 deletions

diff --git a/rcu-avoid-just-onlined-cpu-resched.patch b/rcu-avoid-just-onlined-cpu-resched.patch
new file mode 100644
index 000000000..517f37591
--- /dev/null
+++ b/rcu-avoid-just-onlined-cpu-resched.patch
@@ -0,0 +1,47 @@
+rcu: Avoid having just-onlined CPU resched itself when RCU is idle
+
+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.
+
+Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
+---
+ kernel/rcutree.c |   13 +++++++++----
+ 1 files changed, 9 insertions(+), 4 deletions(-)
+
+diff --git a/kernel/rcutree.c b/kernel/rcutree.c
+index ba06207..472d6b2 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... */
+-			rdp->completed = rnp->completed;
++			/*
++			 * 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. */
+-- 
+1.7.6
+