[PATCH v3] sched/fair: Preserve PLACE_DEADLINE_INITIAL deadline

[Daniel Jordan]

An entity is supposed to get an earlier deadline with
PLACE_DEADLINE_INITIAL when it's forked, but the deadline gets
overwritten soon after in enqueue_entity() the first time a forked
entity is woken so that PLACE_DEADLINE_INITIAL is effectively a no-op.

Placing in task_fork_fair() seems unnecessary since none of the values
that get set (slice, vruntime, deadline) are used before they're set
again at enqueue time, so get rid of that (and with it all of
task_fork_fair()) and just pass ENQUEUE_INITIAL to enqueue_entity() via
wake_up_new_task().

Fixes: e8f331bcc270 ("sched/smp: Use lag to simplify cross-runqueue placement")
Signed-off-by: Daniel Jordan <daniel.m.jordan@xxxxxxxxxx>
Reviewed-by: Chen Yu <yu.c.chen@xxxxxxxxx>
Tested-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
---

v3
 - Rebased on tip/sched/core and retested.  The "none of the values that
   get set (slice, vruntime, deadline) are used" part was checked
   without this patch by poisoning slice, vruntime, and deadline when
   placing from task_fork_fair() and checking for the poisoned values
   everywhere they're read until the entity is placed again in enqueue_entity().
 - Added Chen Yu's R-b and Prateek's T-b

 kernel/sched/core.c |  2 +-
 kernel/sched/fair.c | 24 ------------------------
 2 files changed, 1 insertion(+), 25 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 264c2eb380d7f..bdfdd9598137c 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4856,7 +4856,7 @@ void wake_up_new_task(struct task_struct *p)
 	update_rq_clock(rq);
 	post_init_entity_util_avg(p);
 
-	activate_task(rq, p, ENQUEUE_NOCLOCK);
+	activate_task(rq, p, ENQUEUE_INITIAL | ENQUEUE_NOCLOCK);
 	trace_sched_wakeup_new(p);
 	wakeup_preempt(rq, p, WF_FORK);
 #ifdef CONFIG_SMP
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 4b70b0d146987..544de3b43e146 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -12417,29 +12417,6 @@ static void task_tick_fair(struct rq *rq, struct task_struct *curr, int queued)
 	task_tick_core(rq, curr);
 }
 
-/*
- * called on fork with the child task as argument from the parent's context
- *  - child not yet on the tasklist
- *  - preemption disabled
- */
-static void task_fork_fair(struct task_struct *p)
-{
-	struct sched_entity *se = &p->se, *curr;
-	struct cfs_rq *cfs_rq;
-	struct rq *rq = this_rq();
-	struct rq_flags rf;
-
-	rq_lock(rq, &rf);
-	update_rq_clock(rq);
-
-	cfs_rq = task_cfs_rq(current);
-	curr = cfs_rq->curr;
-	if (curr)
-		update_curr(cfs_rq);
-	place_entity(cfs_rq, se, ENQUEUE_INITIAL);
-	rq_unlock(rq, &rf);
-}
-
 /*
  * Priority of the task has changed. Check to see if we preempt
  * the current task.
@@ -12943,7 +12920,6 @@ DEFINE_SCHED_CLASS(fair) = {
 #endif
 
 	.task_tick		= task_tick_fair,
-	.task_fork		= task_fork_fair,
 
 	.prio_changed		= prio_changed_fair,
 	.switched_from		= switched_from_fair,
-- 
2.42.0