Subject: Re: [BUG almost bisected] Splat in dequeue_rt_stack() and build error

From: Peter Zijlstra <[email-masked]>
Date: Fri, 23 Aug 2024 09:47:05 +0200

On Wed, Aug 21, 2024 at 02:57:16PM -0700, Paul E. McKenney wrote:

> 2e0199df252a ("sched/fair: Prepare exit/cleanup paths for delayed_dequeue")
> 
> The preceding commit is very reliable.
> 
> Only instead of (or maybe as well as?) introducing the dequeue_rt_stack()
> bug, the 2e0199df252a commit introduced a build bug:
> 
> ------------------------------------------------------------------------
> 
> In file included from kernel/sched/fair.c:54:
> kernel/sched/fair.c: In function ‘switched_from_fair’:
> kernel/sched/sched.h:2154:58: error: ‘__SCHED_FEAT_DELAY_ZERO’ undeclared (first use in this function); did you mean ‘__SCHED_FEAT_LATENCY_WARN’?
>  2154 | #define sched_feat(x) !!(sysctl_sched_features & (1UL << __SCHED_FEAT_##x))
>       |                                                          ^~~~~~~~~~~~~
> kernel/sched/fair.c:12878:21: note: in expansion of macro ‘sched_feat’
> 12878 |                 if (sched_feat(DELAY_ZERO) && p->se.vlag > 0)
>       |                     ^~~~~~~~~~
> kernel/sched/sched.h:2154:58: note: each undeclared identifier is reported only once for each function it appears in
>  2154 | #define sched_feat(x) !!(sysctl_sched_features & (1UL << __SCHED_FEAT_##x))
>       |                                                          ^~~~~~~~~~~~~
> kernel/sched/fair.c:12878:21: note: in expansion of macro ‘sched_feat’
> 12878 |                 if (sched_feat(DELAY_ZERO) && p->se.vlag > 0)
>       |                     ^~~~~~~~~~
> 

Oh gawd, last minute back-merges :/

Does the below help any? That's more or less what it was before Valentin
asked me why it was weird like that :-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 6be618110885..5757dd50b02f 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -13107,7 +13107,6 @@ static void switched_from_fair(struct rq *rq, struct task_struct *p)
 	 * and we cannot use DEQUEUE_DELAYED.
 	 */
 	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)