sched/nohz: Rewrite and fix load-avg computation -- again

Thanks to Charles Wang for spotting the defects in the current code:

 - If we go idle during the sample window -- after sampling, we get a
   negative bias because we can negate our own sample.

 - If we wake up during the sample window we get a positive bias
   because we push the sample to a known active period.

So rewrite the entire nohz load-avg muck once again, now adding
copious documentation to the code.

Reported-and-tested-by: Doug Smythies <dsmythies@telus.net>
Reported-and-tested-by: Charles Wang <muming.wq@gmail.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: stable@kernel.org
Link: http://lkml.kernel.org/r/1340373782.18025.74.camel@twins
[ minor edits ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 2 insertions, 0 deletions

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 869997833928..4a08472c3ca7 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -406,6 +406,7 @@ static void tick_nohz_stop_sched_tick(struct tick_sched *ts)
                  */
                 if (!ts->tick_stopped) {
                         select_nohz_load_balancer(1);
+                        calc_load_enter_idle();
 
                         ts->idle_tick = hrtimer_get_expires(&ts->sched_timer);
                         ts->tick_stopped = 1;
@@ -597,6 +598,7 @@ void tick_nohz_idle_exit(void)
                 account_idle_ticks(ticks);
 #endif
 
+        calc_load_exit_idle();
         touch_softlockup_watchdog();
         /*
          * Cancel the scheduled timer and restore the tick