sched: fix SMT scheduler bug

The SMT scheduler incorrectly skips kernel threads even if they are
runnable (but they are preempted by a higher-prio user-space task which got
SMT-delayed by an even higher-priority task running on a sibling CPU).

Fix this for now by only doing the SMT-nice optimization if the
to-be-delayed task is the only runnable task.  (This should cover most of
the real-life cases anyway.)

This bug has been in the SMT scheduler since 2.6.17 or so, but has only
been noticed now by the active check in the dynticks code.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Cc: Michal Piotrowski <michal.k.k.piotrowski@gmail.com>
Cc: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Chuck Ebbert <cebbert@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/kernel/sched.c b/kernel/sched.c
index a5a0b610865f08d455ae23e1c3b279682eee36dc..62db30c73554ddfd7dedd40c9b38ed9aec7cf31e 100644 (file)
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -3528,7 +3528,7 @@ need_resched_nonpreemptible:
                }
        }
        next->sleep_type = SLEEP_NORMAL;
-       if (dependent_sleeper(cpu, rq, next))
+       if (rq->nr_running == 1 && dependent_sleeper(cpu, rq, next))
                next = rq->idle;
 switch_tasks:
        if (next == rq->idle)