Re: [PATCH 2/3] sched: Unloop sched avg decaying

[Peter Zijlstra]

On Thu, Jun 30, 2016 at 02:52:26PM +0200, Frederic Weisbecker wrote:
> On Tue, Jun 14, 2016 at 05:58:42PM +0200, Peter Zijlstra wrote:

> > Why not add the division to the nohz exit path only?
> 
> It would be worse I think because we may exit much more often from nohz
> than we reach a sched_avg_period().
> 
> So the only safe optimization I can do for now is:

How about something like this then?

---

 kernel/sched/core.c | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 3387e4f14fc9..fd1ae4c4105f 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -665,9 +665,23 @@ bool sched_can_stop_tick(struct rq *rq)
 
 void sched_avg_update(struct rq *rq)
 {
-       s64 period = sched_avg_period();
+       s64 delta, period = sched_avg_period();
 
-       while ((s64)(rq_clock(rq) - rq->age_stamp) > period) {
+       delta = (s64)(rq_clock(rq) - rq->age_stamp);
+       if (likely(delta < period))
+               return;
+
+       if (unlikely(delta > 3*period)) {
+               int pending;
+               u64 rem;
+
+               pending = div64_u64_rem(delta, period, &rem);
+               rq->age_stamp += delta - rem;
+               rq->rt_avg >>= pending;
+               return;
+       }
+
+       while (delta > period) {
                /*
                 * Inline assembly required to prevent the compiler
                 * optimising this loop into a divmod call.
@@ -675,6 +689,7 @@ void sched_avg_update(struct rq *rq)
                 */
                asm("" : "+rm" (rq->age_stamp));
                rq->age_stamp += period;
+               delta -= period;
                rq->rt_avg /= 2;
        }
 }