From: Ingo Molnar <mingo@elte.hu>
Date: Thu, 1 Mar 2007 23:58:51 +0000 (-0500)
Subject: sched: fix SMT scheduler bug
X-Git-Tag: v2.6.20.2~30
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=d499ac7a3681e270074e880879d0e0a5ad0849fa;p=thirdparty%2Fkernel%2Fstable.git

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 a5a0b610865f0..62db30c73554d 100644
--- 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)