rcu/nocb: Reduce contention at no-CBs registry-time CB advancement (6608c3a0) · Commits · 戴 / test

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 <paulmck@linux.ibm.com>

kernel/rcu/tree.c

+3 −2

Original line number	Diff line number	Diff line
		@@ -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);
		}

		/*

kernel/rcu/tree_plugin.h

+1 −3

Original line number	Diff line number	Diff line
		@@ -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"));

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