rcu/nocb: Conditionally wake up rcuo if not already waiting on GP

A callback enqueuer currently wakes up the rcuo kthread if it is adding
the first non-done callback of a CPU, whether the kthread is waiting on
a grace period or not (unless the CPU is offline).

This looks like a desired behaviour because then the rcuo kthread
doesn't wait for the end of the current grace period to handle the
callback. It is accelerated right away and assigned to the next grace
period. The GP kthread is notified about that fact and iterates with
the upcoming GP without sleeping in-between.

However this best-case scenario is contradicted by a few details,
depending on the situation:

1) If the callback is a non-bypass one queued with IRQs enabled, the
   wake up only occurs if no other pending callbacks are on the list.
   Therefore the theoretical "optimization" actually applies on rare
   occasions.

2) If the callback is a non-bypass one queued with IRQs disabled, the
   situation is similar with even more uncertainty due to the deferred
   wake up.

3) If the callback is lazy, a few jiffies don't make any difference.

4) If the callback is bypass, the wake up timer is programmed 2 jiffies
   ahead by rcuo in case the regular pending queue has been handled
   in the meantime. The rare storm of callbacks can otherwise wait for
   the currently elapsing grace period to be flushed and handled.

For all those reasons, the optimization is only theoretical and
occasional. Therefore it is reasonable that callbacks enqueuers only
wake up the rcuo kthread when it is not already waiting on a grace
period to complete.

Signed-off-by: Frederic Weisbecker <[email protected]>
Signed-off-by: Neeraj Upadhyay <[email protected]>

Author: Frederic Weisbecker

kernel/rcu/tree_nocb.h

@@ -216,10 +216,7 @@ static bool __wake_nocb_gp(struct rcu_data *rdp_gp,
 	raw_spin_unlock_irqrestore(&rdp_gp->nocb_gp_lock, flags);
 	if (needwake) {
 		trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("DoWake"));
-		if (cpu_is_offline(raw_smp_processor_id()))
-			swake_up_one_online(&rdp_gp->nocb_gp_wq);
-		else
-			wake_up_process(rdp_gp->nocb_gp_kthread);
+		swake_up_one_online(&rdp_gp->nocb_gp_wq);
 	}
 
 	return needwake;