sched: Fix signed unsigned comparison in check_preempt_tick()

Signed unsigned comparison may lead to superfluous resched if leftmost
is right of the current task, wasting a few cycles, and inadvertently
_lengthening_ the current task's slice.

Reported-by: Venkatesh Pallipadi <venki@google.com>
Signed-off-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1294202477.9384.5.camel@marge.simson.net>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 3 insertions, 0 deletions

diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index 414145cf5344..77e9166d7bbf 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -1062,6 +1062,9 @@ check_preempt_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr)
                 struct sched_entity *se = __pick_next_entity(cfs_rq);
                 s64 delta = curr->vruntime - se->vruntime;
 
+                if (delta < 0)
+                        return;
+
                 if (delta > ideal_runtime)
                         resched_task(rq_of(cfs_rq)->curr);
         }