[PATCH 20/24] sched/fair: Avoid re-setting virtual deadline on migrations

Peter Zijlstra
2024-07-27T11:02:23Z

During OSPM24 Youssef noted that migrations are re-setting the virtual
deadline. Notably everything that does a dequeue-enqueue, like setting
nice, changing preferred numa-node, and a myriad of other random crap,
will cause this to happen.

This shouldn't be. Preserve the relative virtual deadline across such
dequeue/enqueue cycles.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
include/linux/sched.h | 6 ++++--
kernel/sched/fair.c | 23 ++++++++++++++++++-----
kernel/sched/features.h | 4 ++++
3 files changed, 26 insertions(+), 7 deletions(-)

--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -542,8 +542,10 @@ struct sched_entity {
u64 min_vruntime;

struct list_head group_node;
- unsigned int on_rq;
- unsigned int sched_delayed;
+ unsigned char on_rq;
+ unsigned char sched_delayed;
+ unsigned char rel_deadline;
+ /* hole */

u64 exec_start;
u64 sum_exec_runtime;
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5270,6 +5270,12 @@ place_entity(struct cfs_rq *cfs_rq, stru

se->vruntime = vruntime - lag;

+ if (sched_feat(PLACE_REL_DEADLINE) && se->rel_deadline) {
+ se->deadline += se->vruntime;
+ se->rel_deadline = 0;
+ return;
+ }
+
/*
* When joining the competition; the existing tasks will be,
* on average, halfway through their slice, as such start tasks
@@ -5382,6 +5388,8 @@ static __always_inline void return_cfs_r
static bool
dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
{
+ bool sleep = flags & DEQUEUE_SLEEP;
+
if (flags & DEQUEUE_DELAYED) {
/*
* DEQUEUE_DELAYED is typically called from pick_next_entity()
@@ -5391,19 +5399,18 @@ dequeue_entity(struct cfs_rq *cfs_rq, st
SCHED_WARN_ON(!se->sched_delayed);
se->sched_delayed = 0;
} else {
- bool sleep = flags & DEQUEUE_SLEEP;
-
+ bool delay = sleep;
/*
* DELAY_DEQUEUE relies on spurious wakeups, special task
* states must not suffer spurious wakeups, excempt them.
*/
if (flags & DEQUEUE_SPECIAL)
- sleep = false;
+ delay = false;

- SCHED_WARN_ON(sleep && se->sched_delayed);
+ SCHED_WARN_ON(delay && se->sched_delayed);
update_curr(cfs_rq);

- if (sched_feat(DELAY_DEQUEUE) && sleep &&
+ if (sched_feat(DELAY_DEQUEUE) && delay &&
!entity_eligible(cfs_rq, se)) {
if (cfs_rq->next == se)
cfs_rq->next = NULL;
@@ -5434,6 +5441,11 @@ dequeue_entity(struct cfs_rq *cfs_rq, st
clear_buddies(cfs_rq, se);

update_entity_lag(cfs_rq, se);
+ if (sched_feat(PLACE_REL_DEADLINE) && !sleep) {
+ se->deadline -= se->vruntime;
+ se->rel_deadline = 1;
+ }
+
if (se != cfs_rq->curr)
__dequeue_entity(cfs_rq, se);
se->on_rq = 0;
@@ -13024,6 +13036,7 @@ static void switched_from_fair(struct rq
* over the excursion into the new class.
*/
p->se.vlag = 0;
+ p->se.rel_deadline = 0;
}

static void switched_to_fair(struct rq *rq, struct task_struct *p)
--- a/kernel/sched/features.h
+++ b/kernel/sched/features.h
@@ -10,6 +10,10 @@ SCHED_FEAT(PLACE_LAG, true)
*/
SCHED_FEAT(PLACE_DEADLINE_INITIAL, true)
/*
+ * Preserve relative virtual deadline on 'migration'.
+ */
+SCHED_FEAT(PLACE_REL_DEADLINE, true)
+/*
* Inhibit (wakeup) preemption until the current task has either matched the
* 0-lag point or until is has exhausted it's slice.
*/