rcu/nocb: Enable re-awakening under high callback load

The __call_rcu_nocb_wake() function and its predecessors set
->qlen_last_fqs_check to zero for the first callback and to LONG_MAX / 2
for forced reawakenings.  The former can result in a too-quick reawakening
when there are many callbacks ready to invoke and the latter prevents a
second reawakening.  This commit therefore sets ->qlen_last_fqs_check
to the current number of callbacks in both cases.  While in the area,
this commit also moves both assignments under ->nocb_lock.

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

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index ae92771..7077ef7 100644 (file)
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -1628,6 +1628,7 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone,
        // Need to actually to a wakeup.
        len = rcu_segcblist_n_cbs(&rdp->cblist);
        if (was_alldone) {
+               rdp->qlen_last_fqs_check = len;
                if (!irqs_disabled_flags(flags)) {
                        /* ... if queue was empty ... */
                        wake_nocb_gp(rdp, false, flags);
@@ -1638,9 +1639,9 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone,
                                           TPS("WakeEmptyIsDeferred"));
                        rcu_nocb_unlock_irqrestore(rdp, flags);
                }
-               rdp->qlen_last_fqs_check = 0;
        } else if (len > rdp->qlen_last_fqs_check + qhimark) {
                /* ... or if many callbacks queued. */
+               rdp->qlen_last_fqs_check = len;
                if (!irqs_disabled_flags(flags)) {
                        wake_nocb_gp(rdp, true, flags);
                        trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
@@ -1650,7 +1651,6 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone,
                                           TPS("WakeOvfIsDeferred"));
                        rcu_nocb_unlock_irqrestore(rdp, flags);
                }
-               rdp->qlen_last_fqs_check = LONG_MAX / 2;
        } else {
                trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("WakeNot"));
                rcu_nocb_unlock_irqrestore(rdp, flags);