[PATCH 2/9] sched: Fix race in idle_balance()

From: Peter Zijlstra
Date: Tue Jan 21 2014 - 06:30:04 EST

From: Daniel Lezcano <daniel.lezcano@xxxxxxxxxx>

The scheduler main function 'schedule()' checks if there are no more tasks
on the runqueue. Then it checks if a task should be pulled in the current
runqueue in idle_balance() assuming it will go to idle otherwise.

But the idle_balance() releases the rq->lock in order to lookup in the sched
domains and takes the lock again right after. That opens a window where
another cpu may put a task in our runqueue, so we won't go to idle but
we have filled the idle_stamp, thinking we will.

This patch closes the window by checking if the runqueue has been modified
but without pulling a task after taking the lock again, so we won't go to idle
right after in the __schedule() function.

Cc: alex.shi@xxxxxxxxxx
Cc: peterz@xxxxxxxxxxxxx
Cc: mingo@xxxxxxxxxx
Signed-off-by: Daniel Lezcano <daniel.lezcano@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/1389949444-14821-2-git-send-email-daniel.lezcano@xxxxxxxxxx
kernel/sched/fair.c | 7 +++++++
1 file changed, 7 insertions(+)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6417,6 +6417,13 @@ void idle_balance(struct rq *this_rq)


+ /*
+ * While browsing the domains, we released the rq lock.
+ * A task could have be enqueued in the meantime
+ */
+ if (this_rq->nr_running && !pulled_task)
+ return;
if (pulled_task || time_after(jiffies, this_rq->next_balance)) {
* We are going idle. next_balance may be set based on

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