[PATCH 1/2] sched/fair: rearm hrtick timer when it's senseful

[Joonwoo Park]

When a new cfs task enqueued, fair scheduler rearms current cfs task's
hrtick expiration time with decreased slice.  But the slice stops
decreasing eventually when it reached sched_min_granularity.
Consequently cfs scheduler also stops rearming of hrtick timer because
hrtick expiration time for the current cfs task doesn't change.  This
is a legitimate optimization but there is a subtle error in the 'if'
condition so at present cfs scheduler stops rearming of hrtick timer
earlier than ideal that will cause a subtle unfairness.

 When sched_latency = 6ms, sched_min_granularity = 0.75ms,
 sched_nr_latency = 8 :

   nr_run  slice    period
   1       6.00 ms  6 ms
   2       3.00 ms  6 ms
   3       4.50 ms  6 ms
   4       1.50 ms  6 ms
   5       1.20 ms  6 ms
   6       1.00 ms  6 ms
   7       0.85 ms  6 ms
   8       0.75 ms  6 ms
   9+      0.75 ms  6.75ms

The first time when sched_slice becomes equal to sched_min_granularity
is when cfs_rq's nr_running becomes equal to sched_nr_latency.  Fix
the condition to rearm the hrtick nr_running up to sched_nr_latency.

Cc: Ingo Molnar <mingo@xxxxxxxxxx> 
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: linux-kernel@xxxxxxxxxxxxxxx
Signed-off-by: Joonwoo Park <joonwoop@xxxxxxxxxxxxxx>
---
 kernel/sched/fair.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 71c08a8..f465448 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4507,7 +4507,7 @@ static void hrtick_update(struct rq *rq)
 	if (!hrtick_enabled(rq) || curr->sched_class != &fair_sched_class)
 		return;
 
-	if (cfs_rq_of(&curr->se)->nr_running < sched_nr_latency)
+	if (cfs_rq_of(&curr->se)->nr_running <= sched_nr_latency)
 		hrtick_start_fair(rq, curr);
 }
 #else /* !CONFIG_SCHED_HRTICK */
-- 
2.9.3