sched/core: Fix pick_next_task() for RT,DL

Pavan noticed that the following commit:

  49ee576809d8 ("sched/core: Optimize pick_next_task() for idle_sched_class")

... broke RT,DL balancing by robbing them of the opportinty to do new-'idle'
balancing when their last runnable task (on that runqueue) goes away.

Reported-by: Pavan Kondeti <pkondeti@codeaurora.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Fixes: 49ee576809d8 ("sched/core: Optimize pick_next_task() for idle_sched_class")
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index bbfb917a9b4998f8254b4cae92f2dce129761bbb..6699d43a88430a27abeda42e65e6f7d93cddc359 100644 (file)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3273,10 +3273,15 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
        struct task_struct *p;
 
        /*
-        * Optimization: we know that if all tasks are in
-        * the fair class we can call that function directly:
+        * Optimization: we know that if all tasks are in the fair class we can
+        * call that function directly, but only if the @prev task wasn't of a
+        * higher scheduling class, because otherwise those loose the
+        * opportunity to pull in more work from other CPUs.
         */
-       if (likely(rq->nr_running == rq->cfs.h_nr_running)) {
+       if (likely((prev->sched_class == &idle_sched_class ||
+                   prev->sched_class == &fair_sched_class) &&
+                  rq->nr_running == rq->cfs.h_nr_running)) {
+
                p = fair_sched_class.pick_next_task(rq, prev, rf);
                if (unlikely(p == RETRY_TASK))
                        goto again;