[PATCH 3/3] sched, fair: Stop searching for tasks in newidle balance if there are runnable tasks

[Jason Low]

It was found that when running some workloads (such as AIM7) on large systems
with many cores, CPUs do not remain idle for long. Thus, tasks can
wake/get enqueued while doing idle balancing.

In this patch, while traversing the domains in idle balance, in addition to
checking for pulled_task, we add an extra check for this_rq->nr_running for
determining if we should stop searching for tasks to pull. If there are
runnable tasks on this rq, then we will stop traversing the domains. This
reduces the chance that idle balance delays a task from running.

This patch resulted in approximately a 6% performance improvement when
running a Java Server workload on an 8 socket machine.

Signed-off-by: Jason Low <jason.low2@xxxxxx>
---
 kernel/sched/fair.c |    8 ++++++--
 1 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 3e3ffb8..232518c 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6689,7 +6689,6 @@ static int idle_balance(struct rq *this_rq)
 		if (sd->flags & SD_BALANCE_NEWIDLE) {
 			t0 = sched_clock_cpu(this_cpu);
 
-			/* If we've pulled tasks over stop searching: */
 			pulled_task = load_balance(this_cpu, this_rq,
 						   sd, CPU_NEWLY_IDLE,
 						   &continue_balancing);
@@ -6704,7 +6703,12 @@ static int idle_balance(struct rq *this_rq)
 		interval = msecs_to_jiffies(sd->balance_interval);
 		if (time_after(next_balance, sd->last_balance + interval))
 			next_balance = sd->last_balance + interval;
-		if (pulled_task)
+
+		/*
+		 * Stop searching for tasks to pull if there are
+		 * now runnable tasks on this rq.
+		 */
+		if (pulled_task || this_rq->nr_running > 0)
 			break;
 	}
 	rcu_read_unlock();
-- 
1.7.1

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