rcu/nocb: Avoid synchronous wakeup in __call_rcu_nocb_wake()

When callbacks are in full flow, the common case is waiting for a
grace period, and this grace period will normally take a few jiffies to
complete.  It therefore isn't all that helpful for __call_rcu_nocb_wake()
to do a synchronous wakeup in this case.  This commit therefore turns this
into a timer-based deferred wakeup of the no-CBs grace-period kthread.

Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>

1 files changed, 5 insertions, 14 deletions

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 25a53742ca68..4b59ef1cbc8b 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -1900,22 +1900,13 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone,
         } else if (len > rdp->qlen_last_fqs_check + qhimark) {
                 /* ... or if many callbacks queued. */
                 rdp->qlen_last_fqs_check = len;
-                if (!rdp->nocb_cb_sleep &&
-                    rcu_segcblist_ready_cbs(&rdp->cblist)) {
-                        // Already going full tilt, so don't try to rewake.
-                        rcu_nocb_unlock_irqrestore(rdp, flags);
-                } else {
+                if (rdp->nocb_cb_sleep ||
+                    !rcu_segcblist_ready_cbs(&rdp->cblist)) {
                         rcu_advance_cbs_nowake(rdp->mynode, rdp);
-                        if (!irqs_disabled_flags(flags)) {
-                                wake_nocb_gp(rdp, false, flags);
-                                trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
-                                                    TPS("WakeOvf"));
-                        } else {
-                                wake_nocb_gp_defer(rdp, RCU_NOCB_WAKE_FORCE,
-                                                   TPS("WakeOvfIsDeferred"));
-                                rcu_nocb_unlock_irqrestore(rdp, flags);
-                        }
+                        wake_nocb_gp_defer(rdp, RCU_NOCB_WAKE_FORCE,
+                                           TPS("WakeOvfIsDeferred"));
                 }
+                rcu_nocb_unlock_irqrestore(rdp, flags);
         } else {
                 trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("WakeNot"));
                 rcu_nocb_unlock_irqrestore(rdp, flags);