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

Currently, nocb_cb_wait() advances callbacks on each pass through its
loop, 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.

Note that nocb_cb_wait() doesn't worry about callbacks that have not
yet been assigned a grace period.  The idea is that the only reason for
nocb_cb_wait() to advance callbacks is to allow it to continue invoking
callbacks.  Time will tell whether this is the correct choice.

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 4b59ef1..f6f23a1 100644 (file)
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -2079,6 +2079,7 @@ static int rcu_nocb_gp_kthread(void *arg)
  */
 static void nocb_cb_wait(struct rcu_data *rdp)
 {
+       unsigned long cur_gp_seq;
        unsigned long flags;
        bool needwake_gp = false;
        struct rcu_node *rnp = rdp->mynode;
@@ -2091,7 +2092,9 @@ static void nocb_cb_wait(struct rcu_data *rdp)
        local_bh_enable();
        lockdep_assert_irqs_enabled();
        rcu_nocb_lock_irqsave(rdp, flags);
-       if (raw_spin_trylock_rcu_node(rnp)) { /* irqs already disabled. */
+       if (rcu_segcblist_nextgp(&rdp->cblist, &cur_gp_seq) &&
+           rcu_seq_done(&rnp->gp_seq, cur_gp_seq) &&
+           raw_spin_trylock_rcu_node(rnp)) { /* irqs already disabled. */
                needwake_gp = rcu_advance_cbs(rdp->mynode, rdp);
                raw_spin_unlock_rcu_node(rnp); /* irqs remain disabled. */
        }