[PATCH 1/3] sched: enable interrupts and drop rq-lock during newidlebalancing

[Gregory Haskins]

We do find_busiest_groups() et. al. without locks held for normal balancing,
so lets do it for newidle as well.  It will allow other cpus to make
forward progress (against our RQ) while we try to balance and allow 
some interrupts to occur.

Signed-off-by: Gregory Haskins <ghaskins@xxxxxxxxxx>
---

 kernel/sched.c |   44 ++++++++++++++++++++++++++++++++++++++------
 1 files changed, 38 insertions(+), 6 deletions(-)

diff --git a/kernel/sched.c b/kernel/sched.c
index 31f91d9..490e6bc 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -3333,6 +3333,16 @@ load_balance_newidle(int this_cpu, struct rq *this_rq, struct sched_domain *sd)
 	int sd_idle = 0;
 	int all_pinned = 0;
 	cpumask_t cpus = CPU_MASK_ALL;
+	int nr_running;
+
+	schedstat_inc(sd, lb_count[CPU_NEWLY_IDLE]);
+
+	/*
+	 * We are in a preempt-disabled section, so dropping the lock/irq
+	 * here simply means that other cores may acquire the lock,
+	 * and interrupts may occur.
+	 */
+	spin_unlock_irq(&this_rq->lock);
 
 	/*
 	 * When power savings policy is enabled for the parent domain, idle
@@ -3344,7 +3354,6 @@ load_balance_newidle(int this_cpu, struct rq *this_rq, struct sched_domain *sd)
 	    !test_sd_parent(sd, SD_POWERSAVINGS_BALANCE))
 		sd_idle = 1;
 
-	schedstat_inc(sd, lb_count[CPU_NEWLY_IDLE]);
 redo:
 	group = find_busiest_group(sd, this_cpu, &imbalance, CPU_NEWLY_IDLE,
 				   &sd_idle, &cpus, NULL);
@@ -3366,14 +3375,33 @@ redo:
 
 	ld_moved = 0;
 	if (busiest->nr_running > 1) {
-		/* Attempt to move tasks */
-		double_lock_balance(this_rq, busiest);
-		/* this_rq->clock is already updated */
-		update_rq_clock(busiest);
+		local_irq_disable();
+		double_rq_lock(this_rq, busiest);
+
+		BUG_ON(this_cpu != smp_processor_id());
+
+		/*
+		 * Checking rq->nr_running covers both the case where
+		 * newidle-balancing pulls a task, as well as if something
+		 * else issued a NEEDS_RESCHED (since we would only need
+		 * a reschedule if something was moved to us)
+		 */
+		if (this_rq->nr_running) {
+			double_rq_unlock(this_rq, busiest);
+			local_irq_enable();
+			goto out_balanced;
+		}
+
 		ld_moved = move_tasks(this_rq, this_cpu, busiest,
 					imbalance, sd, CPU_NEWLY_IDLE,
 					&all_pinned);
-		spin_unlock(&busiest->lock);
+
+		nr_running = this_rq->nr_running;
+		double_rq_unlock(this_rq, busiest);
+		local_irq_enable();
+
+		if (nr_running)
+			goto out_balanced;
 
 		if (unlikely(all_pinned)) {
 			cpu_clear(cpu_of(busiest), cpus);
@@ -3382,6 +3410,8 @@ redo:
 		}
 	}
 
+	spin_lock_irq(&this_rq->lock);
+
 	if (!ld_moved) {
 		schedstat_inc(sd, lb_failed[CPU_NEWLY_IDLE]);
 		if (!sd_idle && sd->flags & SD_SHARE_CPUPOWER &&
@@ -3393,6 +3423,8 @@ redo:
 	return ld_moved;
 
 out_balanced:
+	spin_lock_irq(&this_rq->lock);
+
 	schedstat_inc(sd, lb_balanced[CPU_NEWLY_IDLE]);
 	if (!sd_idle && sd->flags & SD_SHARE_CPUPOWER &&
 	    !test_sd_parent(sd, SD_POWERSAVINGS_BALANCE))

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