[PATCH] 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 <[e-mail]>
Cc: Michal Piotrowski <[e-mail]>
Cc: Nick Piggin <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

kernel/sched.c

···
 		}
 	}
 	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)