[PATCH v2 RFC 13/13] sched/pelt: Always allow load updates

[Qais Yousef <qyousef@layalina.io>]

1024us period can cause a problem at dequeue if the last udpate (due to
tick) has happened less than this period. Running a periodic task I can
see the dequeued util_avg changing by 15-20 points due to this variation
- which on HMP system with small cores can mean a big jump in freqs.

Before

                                         periodic-2977 util_avg
     ┌┬─────────┬─────────┬──────────┬─────────┬─────────┬─────────┬──────────┬─────────┬─────────┬┐
140.0┼┼─▐▀▛▜─▄▄▄▖─────────┼──────────┼─────────┼────────▄▄──▗▄▄▄──▗▄▄▄────────┼─────────┼─────────┼┤
     ││ ▐ ▌▐ ▌▐ ▌  ▛▜▄   ▄▟▜▄▖  ▐▜▛▌ │▗▄       │        ▌▐  ▐ ▌▐  ▐│▌▐ ▛▙▄    ▗▄    ▐▜  │ ▗▄      ││
     │▀▙▟ ▌▐ ▌▐ ▌  ▌▐▐ ▐▜▌█▐▌█▜▛█▐▌▌ │▐▐       │  ▛▜▀▛▜▀▌▐▀▙▟ ▌▐  ▐│▌▐ ▌█▐ ▗▄▛█▐▛█▜▄▟▐  │ ▐▐▀▌▗▄▖ ││
     ││▌▐ ▌▐▄▌▐ ▌  ▌▐▐ ▐▐▌█▐▌█▐▌█▐▌▌ │▐▐  ▐▀▙▄ ▛▜▄▌▐ ▌▐ ▌▐ ▌▐ ▌▐▄▖▐│▌▐ ▌█▐ ▐▐▌█▐▌█▐▌█▐▛▙▄ ▐▐ ▌▐ ▛▜││
129.5┼┼▌▐─▌▐─▌▐─▌──▌▐▐─▐▐▌█▐▌█▐▌█▐▌▙▄▄▟▐──▐─▌▐─▌▐─▌▐─▌▐─▌▐─▌▐─▌▐─▛▜┼▌▐─▌█▐─▐▐▌█▐▌█▐▌█▐▌▌▐─▐▐─▌▐─▌▐┼┤
     ││▌▐ ▌▐ ▌▐ ▛▜▀▌▐▐ ▐▐▌█▐▌█▐▌█▐▌█▐│▛▐  ▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐│▌▐▄▌█▐ ▐▐▌█▐▌█▐▌█▐▌▌▐▀▛▐ ▌▐ ▌▐││
     ││▌▐ ▌▐ ▌▐ ▌▐ ▌▐▐ ▐▐▌█▐▌█▐▌█▐▌█▐│▌▐▀▙▟ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐│▌▐ ▌█▐ ▐▐▌█▐▌█▐▌█▐▌▌▐ ▌▐ ▙▟ ▌▐││
119.0┼┼▌▐─▌▐─▌▐─▌▐─▌▐▐▛█▐▌█▐▌█▐▌█▐▌█▐┼▌▐─▌▐─▌▐─▌▐─▌▐─▌▐─▌▐─▌▐─▌▐─▌▐┼▌▐─▌█▐▄▟▐▌█▐▌█▐▌█▐▌▌▐─▌▐─▌▐─▌▐┼┤
     ││▌▐ ▌▐ ▌▐ ▌▐ ▌▐▐▌█▐▌█▐▌█▐▌█▐▌█▐│▌▐ ▌▐ ▌▐▄▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐│▌▐ ▌█▐▌█▐▌█▐▌█▐▌█▐▌▌▐ ▌▐ ▌▐ ▌▐▀│
     ││▌▐ ▌▐ ▌▐ ▌▐ ▌▐▐▌█▐▌█▐▌▛▐▘█▐▌█▐│▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐│▌▐ ▌█▐▌█▐▌█▐▌▛▐▘█▐▌▌▐ ▌▐ ▌▐ ▌▐││
     ││▌▐ ▌▐ ▌▐ ▌▐ ▌▐▐▌█▐▌█▐▌▌▐ █▐▘█▐│▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐│▌▐ ▌█▐▌█▐▌█▐▌▌▐ █▐▘▌▐ ▌▐ ▌▐ ▌▐││
108.5┼┼▌▐─▌▐─▌▐─▌▐─▌▐▐▌█▐▌█▐▌▌▐─█▐─▌▐┼▌▐─▌▐─▌▐─▌▐─▌▐─▌▐─▌▐─▌▐─▌▐─▌▐┼▌▐─▌█▐▌█▐▌█▐▌▌▐─█▐─▌▐─▌▐─▌▐─▌▐┼┤
     ││▌▐ ▌▐ ▌▐ ▌▐ ▌▐▐▌█▐▌█▐▌▌▐ █▐ ▌▐│▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐│▌▐ ▌█▐▌█▐▌█▐▌▌▐ █▐ ▌▐ ▌▐ ▌▐ ▌▐││
     ││▌▐ ▌▐ ▌▐ ▌▐ ▌▐▐▌█▐▌█▐▌▌▐ █▐ ▌▐│▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐│▌▐ ▌█▐▌█▐▌█▐▌▌▐ █▐ ▌▐ ▌▐ ▌▐ ▌▐││
     ││         │   ▝ ▘█▐▌█▝▘▘▝ ▀▝ ▘▝│▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▘▝  │         │    ▝▐▌█▐▌█▝▘▘▝ ▀▝ ▘▝ ▘▐ ▌▐ ▌▐││
 98.0┼┼─────────┼───────▝▘┼──────────┼───▘▝─▌▐─▌▐─▌──────┼─────────┼───────▀▝▘┼─────────┼────▘▝─▘▝┼┤
     └┼─────────┼─────────┼──────────┼─────────┼─────────┼─────────┼──────────┼─────────┼─────────┼┘
    2.00      2.11      2.22       2.33      2.44      2.56      2.67       2.78      2.89     3.00

After

                                         periodic-2968 util_avg
     ┌┬─────────┬─────────┬──────────┬─────────┬─────────┬─────────┬──────────┬─────────┬─────────┬┐
139.0┼▄▖──▄▄▄▄▄▄▖─────────┼──────────┼─────────┼──────▗▄▄▟▀▛▜▀▛▜▄▖─┼──────────┼─────────┼─────────┼┤
     ││▛▜▀▌▐ ▌▐ ▛▜▀▙▄    ▄▟▀▛▜▄      │         │   ▐▀▛▜ ▌▐ ▌▐ ▌▐ ▛▜▀▙▄        │         │         ││
     ││▌▐ ▌▐ ▌▐ ▌▐ ▌▐▀▙▟▀▌▐ ▌▐▐▛█▜▛█▜▛█▜▛▙▄    │ ▄▞▜ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐│▌▐▀▛▜    ▄▄▄▄▟▜▛█▜▛█▜▛█▜▛█▜▄▄▄││
     ││▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐▐▌█▐▌█▐▌█▐▌█▐▛█▜▛█▜▌▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐│▌▐ ▌▐▀▛▜▀▌▐ ▌█▐▌█▐▌█▐▌█▐▌█▐▌█▐▛│
128.2┼┼▌▐─▌▐─▌▐─▌▐─▌▐─▌▐─▌▐─▌▐▐▌█▐▌█▐▌█▐▌█▐▌█▐▌█▐▌▌▐─▌▐─▌▐─▌▐─▌▐─▌▐┼▌▐─▌▐─▌▐─▌▐─▌█▐▌█▐▌█▐▌█▐▌█▐▌█▐▌┤
     ││▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐▐▌█▐▌█▐▌█▐▌█▐▌█▐▌█▐▌▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐│▌▐ ▌▐ ▌▐ ▌▐ ▌█▐▌█▐▌█▐▌█▐▌█▐▌█▐▌│
     ││▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐▐▌█▐▌█▐▌█▐▌█▐▌█▐▌█▐▌▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐│▌▐ ▌▐ ▌▐ ▌▐ ▌█▐▌█▐▌█▐▌█▐▌█▐▌█▐▌│
117.5┼┼▌▐─▌▐─▌▐─▌▐─▌▐─▌▐─▌▐─▌▐▐▌█▐▌█▐▌█▐▌█▐▌█▐▌█▐▌▌▐─▌▐─▌▐─▌▐─▌▐─▌▐┼▌▐─▌▐─▌▐─▌▐─▌█▐▌█▐▌█▐▌█▐▌▛▐▘█▐▌┤
     ││▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐▐▌█▐▌█▐▌█▐▌█▐ ▛▐▌█▐▌▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐│▌▐ ▌▐ ▌▐ ▌▐ ▌█▐▌█▐▌█▐▌█▐▌▌▐ ▛▐▌│
     ││▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐▐▌█▐▌█▐▌█▐▌█▐ ▌▐ ▛▐▌▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐│▌▐ ▌▐ ▌▐ ▌▐ ▌█▐▌█▐▌█▐▌█▐▌▌▐ ▌▐││
     ││▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐▐▌█▐▌█▐▌█▐▌█▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐│▌▐ ▌▐ ▌▐ ▌▐ ▌█▐▌█▐▌█▐▌█▐▌▌▐ ▌▐││
106.8┼┼▌▐─▌▐─▌▐─▌▐─▌▐─▌▐─▌▐─▌▐▐▌█▐▌▛▐▌█▐▌█▐─▌▐─▌▐─▌▐─▌▐─▌▐─▌▐─▌▐─▌▐┼▌▐─▌▐─▌▐─▌▐─▌█▐▌█▐▌▛▐▌█▐▌▌▐─▌▐┼┤
     ││▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐▐▌█▐▌▌▐│▌▐▌█▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐│▌▐ ▌▐ ▌▐ ▌▐ ▌█▐▌█▐▌▌▐ ▌▐▘▌▐ ▌▐││
     ││▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐▐▌█▐▌▌▐│▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▌▐ ▘▝ ▘   ▝ ▌▐│▌▐ ▌▐ ▌▐ ▌▐ ▌█▐▌█▐▌▌▐ ▌▐ ▌▐ ▌▐││
     ││         │   ▝ ▘▐ ▘│    ▘▀▝▘▘▝│▌▐ ▌▐ ▌▐ ▌▐ ▌▝     │         │ ▝ ▘▝ ▌▐ ▌▐ ▌▜▐▌█▐▌▘▝ ▘▝ ▘▐ ▌▐││
 96.0┼┼─────────┼──────▝──┼──────────┼────▝─▘▝─▘▐─▘──────┼─────────┼──────▘▐─▘▝─▘───────┼───────▘─┼┤
     └┼─────────┼─────────┼──────────┼─────────┼─────────┼─────────┼──────────┼─────────┼─────────┼┘
    2.00      2.11      2.22       2.33      2.44      2.56      2.67       2.78      2.89     3.00

Also the new util_est periodic detection logic can be thrown off by this
variation. With this fix it now stabilizes pretty well.

Before

                                 periodic-2977 util_est.enqueued running
     ┌─────────────────────────────────────────────────────────────────────────────────────────────┐
157.0┤               ▙▄ ▗▄  ▗▄▄▄ ▗▄  ▗▄▄▄▗▄▄  ▗▄▄▖ ▄   ▄▄▄   ▄  ▄▖▖  ▄▄▄▄▄▖▖▝▙▄▄▄▄▄▄▖ ▗▄           │
119.5┤             ▗▄▌▘▀▀ ▀▀▀ ▝▀▀▘▝▀▀▀ ▝▀▘ ▝▀▀▘ ▀▝▀▘▀▀▀▘▝▀▀▀▀▀▀▀▘▝▝▀▀ ▀   ▝▝▀  ▀   ▀▀▀▀            │
 82.0┤             ▟                                                                               │
     │             ▌                                                                               │
 44.5┤             ▌                                                                               │
  7.0┤      ▗   ▗▖ ▌                                                                               │
     └┬─────────┬─────────┬──────────┬─────────┬─────────┬─────────┬──────────┬─────────┬─────────┬┘
    0.00      0.65      1.30       1.95      2.60      3.25      3.90       4.56      5.21     5.86

After

                                 periodic-2968 util_est.enqueued running
     ┌─────────────────────────────────────────────────────────────────────────────────────────────┐
139.0┤              ▗▟▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀    │
106.5┤             ▐▛                                                                              │
 74.0┤             ▟                                                                               │
     │             ▌                                                                               │
 41.5┤             ▌                                                                               │
  9.0┤        ▗▖  ▗▌                                                                               │
     └┬─────────┬─────────┬──────────┬─────────┬─────────┬─────────┬──────────┬─────────┬─────────┬┘
    0.00      0.65      1.30       1.95      2.60      3.25      3.90       4.55      5.20     5.85

Signed-off-by: Qais Yousef <qyousef@layalina.io>
---

I tried to do the update every 256us intead of every period, but this didn't
help to flatten util_est.

If doing the update always is too much, AND, I didn't miss something else that
could be contributing to this problem, would another sched feature to allow
those who want accuracy vs those who want minimal overhead take their pick?

 kernel/sched/pelt.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/kernel/sched/pelt.c b/kernel/sched/pelt.c
index dbd450798b03..64f9e60023a9 100644
--- a/kernel/sched/pelt.c
+++ b/kernel/sched/pelt.c
@@ -224,8 +224,7 @@ ___update_load_sum(u64 now, struct sched_avg *sa,
 	 * Step 1: accumulate *_sum since last_update_time. If we haven't
 	 * crossed period boundaries, finish.
 	 */
-	if (!accumulate_sum(delta, sa, load, runnable, running))
-		return 0;
+	accumulate_sum(delta, sa, load, runnable, running);
 
 	return 1;
 }
-- 
2.34.1