[PATCH] sched: fix SCHED_FAIR wake-idle logic error

[Gregory Haskins]

Hi Ingo,

I found this while looking at -rt, but it would in theory be a problem in
mainline too given a sufficient number of real-time tasks present.
PREEMPT_RT just helps make the likelyhood of RT tasks being running go up ;)

This patch applies to sched-devel and it solves a case where CFS tends to
piggy back wakeups on a single core.  The issue is that the presence of RT
tasks can of course inflate rq->nr_running such that wake_idle() will skip
trying to move to an idle core, thinking that we are already balanced.  In
reality, all the other cores could be idle but we are running RT tasks on the
affined core.

Regards,
-Greg

--------------------------------------
sched: fix SCHED_FAIR wake-idle logic error

We currently use an optimization to skip the overhead of wake-idle
processing if more than one task is assigned to a run-queue.  The
assumption is that the system must already be load-balanced or we
wouldnt be overloaded to begin with.

The problem is that we are looking at rq->nr_running, which may include
RT tasks in addition to CFS tasks.  Since the presence of RT tasks
really has no bearing on the balance status of CFS tasks, this throws
the calculation off.

This patch changes the logic to only consider the number of CFS tasks
when making the decision to optimze the wake-idle.

Signed-off-by: Gregory Haskins <ghaskins@xxxxxxxxxx>
CC: Ingo Molnar <mingo@xxxxxxx>
CC: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
---

 kernel/sched_fair.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index 89fa32b..80b7891 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -1007,7 +1007,7 @@ static int wake_idle(int cpu, struct task_struct *p)
 	 * sibling runqueue info. This will avoid the checks and cache miss
 	 * penalities associated with that.
 	 */
-	if (idle_cpu(cpu) || cpu_rq(cpu)->nr_running > 1)
+	if (idle_cpu(cpu) || cpu_rq(cpu)->cfs.nr_running > 1)
 		return cpu;
 
 	for_each_domain(cpu, sd) {

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