sched: Allow newidle balancing to bail out of load_balance

While doing newidle load balancing, it is possible for new tasks to
arrive, such as with pending wakeups. newidle_balance() already accounts
for this by exiting the sched_domain load_balance() iteration if it
detects these cases. This is very important for minimizing wakeup
latency.

However, if we are already in load_balance(), we may stay there for a
while before returning back to newidle_balance(). This is most
exacerbated if we enter a 'goto redo' loop in the LBF_ALL_PINNED case. A
very straightforward workaround to this is to adjust should_we_balance()
to bail out if we're doing a CPU_NEWLY_IDLE balance and new tasks are
detected.

This was tested with the following reproduction:
- two threads that take turns sleeping and waking each other up are
  affined to two cores
- a large number of threads with 100% utilization are pinned to all
  other cores

Without this patch, wakeup latency was ~120us for the pair of threads,
almost entirely spent in load_balance(). With this patch, wakeup latency
is ~6us.
Signed-off-by: Josh Don <joshdon@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20220609025515.2086253-1-joshdon@google.com

kernel/sched/fair.c

@@ -9824,9 +9824,15 @@ static int should_we_balance(struct lb_env *env)
 	/*
 	 * In the newly idle case, we will allow all the CPUs
 	 * to do the newly idle load balance.
+	 *
+	 * However, we bail out if we already have tasks or a wakeup pending,
+	 * to optimize wakeup latency.
 	 */
-	if (env->idle == CPU_NEWLY_IDLE)
+	if (env->idle == CPU_NEWLY_IDLE) {
+		if (env->dst_rq->nr_running > 0 || env->dst_rq->ttwu_pending)
+			return 0;
 		return 1;
+	}
 
 	/* Try to find first idle CPU */
 	for_each_cpu_and(cpu, group_balance_mask(sg), env->cpus) {