rcu/nocb: Reduce __call_rcu_nocb_wake() leaf rcu_node ->lock contention (296181d7) · Commits · 戴 / test

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>

kernel/rcu/tree_plugin.h

+10 −3

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

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