rcu/nocb: Reduce __call_rcu_nocb_wake() leaf rcu_node ->lock contention

Currently, __call_rcu_nocb_wake() advances callbacks each time that it
detects excessive numbers of callbacks, though only if it succeeds in
conditionally acquiring its leaf rcu_node structure's ->lock.  Despite
the conditional acquisition of ->lock, this does increase contention.
This commit therefore avoids advancing callbacks unless there are
callbacks in ->cblist whose grace period has completed and advancing
has not yet been done during this jiffy.

Note that this decision does not take the presence of new callbacks
into account.  That is because on this code path, there will always be
at least one new callback, namely the one we just enqueued.

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 f6f23a16bd641240c0b97c63348b3a12cb673e5b..f56fb4e97a8e1f0917d551376deefcfc342f2808 100644 (file)
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -1872,6 +1872,8 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone,
                                 unsigned long flags)
                                 __releases(rdp->nocb_lock)
 {
+       unsigned long cur_gp_seq;
+       unsigned long j;
        long len;
        struct task_struct *t;
 
@@ -1900,12 +1902,17 @@ 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)) {
+               j = jiffies;
+               if (j != rdp->nocb_gp_adv_time &&
+                   rcu_segcblist_nextgp(&rdp->cblist, &cur_gp_seq) &&
+                   rcu_seq_done(&rdp->mynode->gp_seq, cur_gp_seq)) {
                        rcu_advance_cbs_nowake(rdp->mynode, rdp);
+                       rdp->nocb_gp_adv_time = j;
+               }
+               if (rdp->nocb_cb_sleep ||
+                   !rcu_segcblist_ready_cbs(&rdp->cblist))
                        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"));