Since idle_balance() will release rq-lock for a while, there is a chance that
RT/DL tasks will be enqueued and ask for the resched, the func used to be
invoked ahead of pick_next_task(), which will make sure we drop into the
bottom-half inside pick_next_task().
Now since idle_balance() was done inside pick_next_task_fair(), pick_next_task()
can no longer make sure the priority, the worst case is that we will going to
pick the pulled fair task while there is RT/DL on rq which actually should be
picked up.
This patch will prevent this happen by some rechecking after idle_balance(), it
utilize the resched-flag for the case when RT/DL task was enqueued but don't ask
for resched (will that ever happened?).
CC: Ingo Molnar <mi...@kernel.org>
Suggested-by: Peter Zijlstra <pet...@infradead.org>
Signed-off-by: Michael Wang <wang...@linux.vnet.ibm.com>
---
kernel/sched/fair.c | 23 ++++++++++++++++++++++-
1 file changed, 22 insertions(+), 1 deletion(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 235cfa7..ce67514 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4776,6 +4776,16 @@ simple:
idle:
#ifdef CONFIG_SMP
+ /*
+ * We came here only when there is no more tasks on rq (top-half of
+ * pick_next_task()), and we are now going to pull some fair entities.
+ *
+ * Since prev is still the current on rq, clear it's resched-flag so
+ * we would be able to know when we got a new resched-request during
+ * idle_balance(), check below for more details.
+ */
+ clear_tsk_need_resched(prev);
+
idle_enter_fair(rq);
/*
* We must set idle_stamp _before_ calling idle_balance(), such that we
@@ -4784,7 +4794,18 @@ idle:
rq->idle_stamp = rq_clock(rq);
if (idle_balance(rq)) { /* drops rq->lock */
rq->idle_stamp = 0;
- goto again;
+ /*
+ * Before we start to pick one of the pulled fair entities, take
+ * care if some RT/DL tasks has been enqueued during the time
+ * we release rq-lock inside idle_balance().
+ *
+ * In such cases, since clear_tsk_need_resched() was done
+ * already, need_resched() will imply the request to sched-in
+ * the enqueued RT/DL tasks, so don't 'goto again' to make sure
+ * the priority.
+ */
+ if (rq->nr_running == rq->cfs.h_nr_running || !need_resched())
+ goto again;
}
#endif
--
1.7.9.5
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