[PATCH 21/24] sched/eevdf: Allow shorter slices to wakeup-preempt

Peter Zijlstra
2024-07-27T11:02:24Z

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 | 64 ++++++++++++++++++++++++++++++++++++++++++------
kernel/sched/features.h | 5 +++
2 files changed, 61 insertions(+), 8 deletions(-)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -973,10 +973,10 @@ static void clear_buddies(struct cfs_rq
* XXX: strictly: vd_i += N*r_i/w_i such that: vd_i > ve_i
* this is probably good enough.
*/
-static void update_deadline(struct cfs_rq *cfs_rq, struct sched_entity *se)
+static bool update_deadline(struct cfs_rq *cfs_rq, struct sched_entity *se)
{
if ((s64)(se->vruntime - se->deadline) < 0)
- return;
+ return false;

/*
* For EEVDF the virtual time slope is determined by w_i (iow.
@@ -993,10 +993,7 @@ static void update_deadline(struct cfs_r
/*
* The task has consumed its request, reschedule.
*/
- if (cfs_rq->nr_running > 1) {
- resched_curr(rq_of(cfs_rq));
- clear_buddies(cfs_rq, se);
- }
+ return true;
}

#include "pelt.h"
@@ -1134,6 +1131,38 @@ static inline void update_curr_task(stru
dl_server_update(p->dl_server, delta_exec);
}

+static inline bool did_preempt_short(struct cfs_rq *cfs_rq, struct sched_entity *curr)
+{
+ if (!sched_feat(PREEMPT_SHORT))
+ return false;
+
+ if (curr->vlag == curr->deadline)
+ return false;
+
+ return !entity_eligible(cfs_rq, curr);
+}
+
+static inline bool do_preempt_short(struct cfs_rq *cfs_rq,
+ struct sched_entity *pse, struct sched_entity *se)
+{
+ if (!sched_feat(PREEMPT_SHORT))
+ return false;
+
+ if (pse->slice >= se->slice)
+ return false;
+
+ if (!entity_eligible(cfs_rq, pse))
+ return false;
+
+ if (entity_before(pse, se))
+ return true;
+
+ if (!entity_eligible(cfs_rq, se))
+ return true;
+
+ return false;
+}
+
/*
* Used by other classes to account runtime.
*/
@@ -1157,6 +1186,7 @@ static void update_curr(struct cfs_rq *c
struct sched_entity *curr = cfs_rq->curr;
struct rq *rq = rq_of(cfs_rq);
s64 delta_exec;
+ bool resched;

if (unlikely(!curr))
return;
@@ -1166,7 +1196,7 @@ static void update_curr(struct cfs_rq *c
return;

curr->vruntime += calc_delta_fair(delta_exec, curr);
- update_deadline(cfs_rq, curr);
+ resched = update_deadline(cfs_rq, curr);
update_min_vruntime(cfs_rq);

if (entity_is_task(curr)) {
@@ -1184,6 +1214,14 @@ static void update_curr(struct cfs_rq *c
}

account_cfs_rq_runtime(cfs_rq, delta_exec);
+
+ if (rq->nr_running == 1)
+ return;
+
+ if (resched || did_preempt_short(cfs_rq, curr)) {
+ resched_curr(rq);
+ clear_buddies(cfs_rq, curr);
+ }
}

static void update_curr_fair(struct rq *rq)
@@ -8611,7 +8649,17 @@ static void check_preempt_wakeup_fair(st
cfs_rq = cfs_rq_of(se);
update_curr(cfs_rq);
/*
- * XXX pick_eevdf(cfs_rq) != se ?
+ * If @p has a shorter slice than current and @p is eligible, override
+ * current's slice protection in order to allow preemption.
+ *
+ * Note that even if @p does not turn out to be the most eligible
+ * task at this moment, current's slice protection will be lost.
+ */
+ if (do_preempt_short(cfs_rq, pse, se) && se->vlag == se->deadline)
+ se->vlag = se->deadline + 1;
+
+ /*
+ * If @p has become the most eligible task, force preemption.
*/
if (pick_eevdf(cfs_rq) == pse)
goto preempt;
--- a/kernel/sched/features.h
+++ b/kernel/sched/features.h
@@ -18,6 +18,11 @@ SCHED_FEAT(PLACE_REL_DEADLINE, true)
* 0-lag point or until is has exhausted it's slice.
*/
SCHED_FEAT(RUN_TO_PARITY, true)
+/*
+ * Allow wakeup of tasks with a shorter slice to cancel RESPECT_SLICE for
+ * current.
+ */
+SCHED_FEAT(PREEMPT_SHORT, true)

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