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 <mi...@redhat.com>
Cc: Peter Zijlstra <pet...@infradead.org>
Cc: Thomas Gleixner <t...@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Joonwoo Park <joonw...@codeaurora.org>
---
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
