[RFC][PATCH 09/10] sched/eevdf: Allow shorter slices to wakeup-preempt

Peter Zijlstra
2024-04-05T11:04:05Z

Part of the reason to have shorter slices is to improve
responsiveness. Allow shorter slices to preempt longer slices on
wakeup.

Task | Runtime ms | Switches | Avg delay ms | Max delay ms | Sum delay ms |

100ms massive_intr 500us cyclictest NO_PREEMPT_SHORT

1 massive_intr:(5) | 846018.956 ms | 779188 | avg: 0.273 ms | max: 58.337 ms | sum:212545.245 ms |
2 massive_intr:(5) | 853450.693 ms | 792269 | avg: 0.275 ms | max: 71.193 ms | sum:218263.588 ms |
3 massive_intr:(5) | 843888.920 ms | 771456 | avg: 0.277 ms | max: 92.405 ms | sum:213353.221 ms |
1 chromium-browse:(8) | 53015.889 ms | 131766 | avg: 0.463 ms | max: 36.341 ms | sum:60959.230 ms |
2 chromium-browse:(8) | 53864.088 ms | 136962 | avg: 0.480 ms | max: 27.091 ms | sum:65687.681 ms |
3 chromium-browse:(9) | 53637.904 ms | 132637 | avg: 0.481 ms | max: 24.756 ms | sum:63781.673 ms |
1 cyclictest:(5) | 12615.604 ms | 639689 | avg: 0.471 ms | max: 32.272 ms | sum:301351.094 ms |
2 cyclictest:(5) | 12511.583 ms | 642578 | avg: 0.448 ms | max: 44.243 ms | sum:287632.830 ms |
3 cyclictest:(5) | 12545.867 ms | 635953 | avg: 0.475 ms | max: 25.530 ms | sum:302374.658 ms |

100ms massive_intr 500us cyclictest PREEMPT_SHORT

1 massive_intr:(5) | 839843.919 ms | 837384 | avg: 0.264 ms | max: 74.366 ms | sum:221476.885 ms |
2 massive_intr:(5) | 852449.913 ms | 845086 | avg: 0.252 ms | max: 68.162 ms | sum:212595.968 ms |
3 massive_intr:(5) | 839180.725 ms | 836883 | avg: 0.266 ms | max: 69.742 ms | sum:222812.038 ms |
1 chromium-browse:(11) | 54591.481 ms | 138388 | avg: 0.458 ms | max: 35.427 ms | sum:63401.508 ms |
2 chromium-browse:(8) | 52034.541 ms | 132276 | avg: 0.436 ms | max: 31.826 ms | sum:57732.958 ms |
3 chromium-browse:(8) | 55231.771 ms | 141892 | avg: 0.469 ms | max: 27.607 ms | sum:66538.697 ms |
1 cyclictest:(5) | 13156.391 ms | 667412 | avg: 0.373 ms | max: 38.247 ms | sum:249174.502 ms |
2 cyclictest:(5) | 12688.939 ms | 665144 | avg: 0.374 ms | max: 33.548 ms | sum:248509.392 ms |
3 cyclictest:(5) | 13475.623 ms | 669110 | avg: 0.370 ms | max: 37.819 ms | sum:247673.390 ms |

As per the numbers the, this makes cyclictest (short slice) it's
max-delay more consistent and consistency drops the sum-delay. The
trade-off is that the massive_intr (long slice) gets more context
switches and a slight increase in sum-delay.

[mike: numbers]
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Mike Galbraith <umgwanakikbuti@gmail.com>
---
kernel/sched/fair.c | 11 ++++++++---
kernel/sched/features.h | 4 ++++
2 files changed, 12 insertions(+), 3 deletions(-)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8538,9 +8538,14 @@ static void check_preempt_wakeup_fair(st
cfs_rq = cfs_rq_of(se);
update_curr(cfs_rq);

- /*
- * XXX pick_eevdf(cfs_rq) != se ?
- */
+ if (sched_feat(PREEMPT_SHORT) && pse->slice < se->slice &&
+ entity_eligible(cfs_rq, pse) &&
+ (s64)(pse->deadline - se->deadline) < 0 &&
+ se->vlag == se->deadline) {
+ /* negate RUN_TO_PARITY */
+ se->vlag = se->deadline - 1;
+ }
+
if (pick_eevdf(cfs_rq) == pse)
goto preempt;

--- a/kernel/sched/features.h
+++ b/kernel/sched/features.h
@@ -14,6 +14,10 @@ SCHED_FEAT(PLACE_DEADLINE_INITIAL, true)
* 0-lag point or until is has exhausted it's slice.
*/
SCHED_FEAT(RUN_TO_PARITY, true)
+/*
+ * Allow tasks with a shorter slice to disregard RUN_TO_PARITY
+ */
+SCHED_FEAT(PREEMPT_SHORT, true)

/*
* Prefer to schedule the task we woke last (assuming it failed