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>
Tested-by: Josh Boyer <jwboyer@redhat.com>
Tested-by: Christian Hoffmann <email@christianhoffmann.info>

1 files changed, 8 insertions, 3 deletions

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index ade788320dd6..c95fa89ffef9 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -1911,8 +1911,6 @@ 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_quiesce = 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;
@@ -1937,8 +1935,15 @@ 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_quiesce = 0;
+                        rdp->qs_pending = 0;
                         rdp->passed_quiesce_gpnum = rnp->gpnum - 1;
                         trace_rcu_grace_period(rsp->name, rdp->gpnum, "cpuonl");
                 }