From 6608c3a027bcc0b34cc02bc764ea9f52b9dce46f Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Sat, 1 Jun 2019 06:16:38 -0700 Subject: rcu/nocb: Reduce contention at no-CBs registry-time CB advancement Currently, __call_rcu_nocb_wake() conditionally acquires the leaf rcu_node structure's ->lock, and only afterwards does rcu_advance_cbs_nowake() check to see if it is possible to advance callbacks without potentially needing to awaken the grace-period kthread. Given that the no-awaken check can be done locklessly, this commit reverses the order, so that rcu_advance_cbs_nowake() is invoked without holding the leaf rcu_node structure's ->lock and rcu_advance_cbs_nowake() checks the grace-period state before conditionally acquiring that lock, thus reducing the number of needless acquistions of the leaf rcu_node structure's ->lock. Signed-off-by: Paul E. McKenney --- kernel/rcu/tree.c | 5 +++-- kernel/rcu/tree_plugin.h | 4 +--- 2 files changed, 4 insertions(+), 5 deletions(-) (limited to 'kernel/rcu') diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index a6ddfae6978d..ec320658aeef 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -1341,10 +1341,11 @@ static bool rcu_advance_cbs(struct rcu_node *rnp, struct rcu_data *rdp) static void __maybe_unused rcu_advance_cbs_nowake(struct rcu_node *rnp, struct rcu_data *rdp) { - raw_lockdep_assert_held_rcu_node(rnp); - if (!rcu_seq_state(rcu_seq_current(&rnp->gp_seq))) + if (!rcu_seq_state(rcu_seq_current(&rnp->gp_seq)) || + !raw_spin_trylock_rcu_node(rnp)) return; WARN_ON_ONCE(rcu_advance_cbs(rnp, rdp)); + raw_spin_unlock_rcu_node(rnp); } /* diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h index 12212764ecd8..09c15f619e78 100644 --- a/kernel/rcu/tree_plugin.h +++ b/kernel/rcu/tree_plugin.h @@ -1668,10 +1668,8 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone, if (!rdp->nocb_cb_sleep && rcu_segcblist_ready_cbs(&rdp->cblist)) { // Already going full tilt, so don't try to rewake. - } else if (rcu_segcblist_pend_cbs(&rdp->cblist) && - raw_spin_trylock_rcu_node(rdp->mynode)) { + } else if (rcu_segcblist_pend_cbs(&rdp->cblist)) { rcu_advance_cbs_nowake(rdp->mynode, rdp); - raw_spin_unlock_rcu_node(rdp->mynode); } else { wake_nocb_gp_defer(rdp, RCU_NOCB_WAKE_FORCE, TPS("WakeOvfIsDeferred")); -- cgit v1.2.3