rcu: Only do rcu_read_unlock_special() wakeups if expedited

Currently, rcu_read_unlock_special() will do wakeups whenever it is safe
to do so.  However, wakeups are expensive, and they are only really
needed when the just-ended RCU read-side critical section is blocking
an expedited grace period (in which case speed is of the essence)
or on a nohz_full CPU (where it might be a good long time before an
interrupt arrives).  This commit therefore checks for these conditions,
and does the expensive wakeups only if doing so would be useful.

Note it can be rather expensive to determine whether or not the current
task (as opposed to the current CPU) is blocking the current expedited
grace period.  Doing so requires traversing the ->blkd_tasks list, which
can be quite long.  This commit therefore cheats:  If the current task
is on a given ->blkd_tasks list, and some task on that list is blocking
the current expedited grace period, the code assumes that the current
task is blocking that expedited grace period.

Reported-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>

1 files changed, 10 insertions, 2 deletions

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 75110ea75d01..d15cdab6aeb4 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -606,20 +606,28 @@ static void rcu_read_unlock_special(struct task_struct *t)
 	local_irq_save(flags);
 	irqs_were_disabled = irqs_disabled_flags(flags);
 	if (preempt_bh_were_disabled || irqs_were_disabled) {
+		bool exp;
+		struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
+		struct rcu_node *rnp = rdp->mynode;
+
 		t->rcu_read_unlock_special.b.exp_hint = false;
+		exp = (t->rcu_blocked_node && t->rcu_blocked_node->exp_tasks) ||
+		      (rdp->grpmask & rnp->expmask) ||
+		      tick_nohz_full_cpu(rdp->cpu);
 		// Need to defer quiescent state until everything is enabled.
-		if (irqs_were_disabled && use_softirq &&
+		if (exp && irqs_were_disabled && use_softirq &&
 		    (in_irq() || !t->rcu_read_unlock_special.b.deferred_qs)) {
 			// Using softirq, safe to awaken, and we get
 			// no help from enabling irqs, unlike bh/preempt.
 			raise_softirq_irqoff(RCU_SOFTIRQ);
-		} else if (irqs_were_disabled && !use_softirq &&
+		} else if (exp && irqs_were_disabled && !use_softirq &&
 			   !t->rcu_read_unlock_special.b.deferred_qs) {
 			// Safe to awaken and we get no help from enabling
 			// irqs, unlike bh/preempt.
 			invoke_rcu_core();
 		} else {
 			// Enabling BH or preempt does reschedule, so...
+			// Also if no expediting or NO_HZ_FULL, slow is OK.
 			set_tsk_need_resched(current);
 			set_preempt_need_resched();
 		}