sched: Make scale_rt_power() deal with backward clocks

Mike reported that, while unlikely, its entirely possible for
scale_rt_power() to see the time go backwards. This yields rather
'interesting' results.

So like all other sites that deal with clocks; make this one ignore
backward clock movement too.

Reported-by: Mike Galbraith <bitbucket@online.de>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20140227094035.GZ9987@twins.programming.kicks-ass.net
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 7570dd9..5e157f1 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5564,6 +5564,7 @@ static unsigned long scale_rt_power(int cpu)
 {
        struct rq *rq = cpu_rq(cpu);
        u64 total, available, age_stamp, avg;
+       s64 delta;
 
        /*
         * Since we're reading these variables without serialization make sure
@@ -5572,7 +5573,11 @@ static unsigned long scale_rt_power(int cpu)
        age_stamp = ACCESS_ONCE(rq->age_stamp);
        avg = ACCESS_ONCE(rq->rt_avg);
 
-       total = sched_avg_period() + (rq_clock(rq) - age_stamp);
+       delta = rq_clock(rq) - age_stamp;
+       if (unlikely(delta < 0))
+               delta = 0;
+
+       total = sched_avg_period() + delta;
 
        if (unlikely(total < avg)) {
                /* Ensures that power won't end up being negative */