sched: Fix signed unsigned comparison in check_preempt_tick()

commit d7d8294415f0ce4254827d4a2a5ee88b00be52a8 upstream.

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>
Signed-off-by: Willy Tarreau <w@1wt.eu>

diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index cd9a40b5d50e08cdb61e417fbec5485497f73c16..fd6e5f2a160e663957423a02ac8d0910694a2e9d 100644 (file)
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -862,6 +862,9 @@ 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);
        }