[tip: sched/core] sched/fair: Avoid re-setting virtual deadline on 'migrations'

tip-bot2 for Peter Zijlstra
2024-08-18T06:23:08Z

The following commit has been merged into the sched/core branch of tip:

Commit-ID: 82e9d0456e06cebe2c89f3c73cdbc9e3805e9437
Gitweb: https://git.kernel.org/tip/82e9d0456e06cebe2c89f3c73cdbc9e3805e9437
Author: Peter Zijlstra <peterz@infradead.org>
AuthorDate: Fri, 31 May 2024 15:49:40 +02:00
Committer: Peter Zijlstra <peterz@infradead.org>
CommitterDate: Sat, 17 Aug 2024 11:06:45 +02:00

sched/fair: Avoid re-setting virtual deadline on 'migrations'

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>
Reviewed-by: Valentin Schneider <vschneid@redhat.com>
Tested-by: Valentin Schneider <vschneid@redhat.com>
Link: https://lkml.kernel.org/r/20240727105030.625119246@infradead.org
---
include/linux/sched.h | 6 ++++--
kernel/sched/fair.c | 23 ++++++++++++++++++-----
kernel/sched/features.h | 4 ++++
3 files changed, 26 insertions(+), 7 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 8a3a389..d25e1cf 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -544,8 +544,10 @@ 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;
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 0eb1bbf..fef0e1f 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5270,6 +5270,12 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)

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
@@ -5382,23 +5388,24 @@ static __always_inline void return_cfs_rq_runtime(struct cfs_rq *cfs_rq);
static bool
dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
{
+ bool sleep = flags & DEQUEUE_SLEEP;
+
update_curr(cfs_rq);

if (flags & DEQUEUE_DELAYED) {
SCHED_WARN_ON(!se->sched_delayed);
} 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);

- 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;
@@ -5429,6 +5436,11 @@ dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
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;
@@ -12992,6 +13004,7 @@ static void switched_from_fair(struct rq *rq, struct task_struct *p)
if (p->se.sched_delayed) {
dequeue_task(rq, p, DEQUEUE_NOCLOCK | DEQUEUE_SLEEP);
p->se.sched_delayed = 0;
+ p->se.rel_deadline = 0;
if (sched_feat(DELAY_ZERO) && p->se.vlag > 0)
p->se.vlag = 0;
}
diff --git a/kernel/sched/features.h b/kernel/sched/features.h
index 7fdeb55..caa4d72 100644
--- 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.
*/