Re: [PATCHSET v14 sched_ext/for-7.4] sched: Make proxy execution compatible with sched_ext

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Sep 22, 2026 at 06:51:39PM +0200, Andrea Righi wrote:

> Andrea Righi (16):
>       sched/core: Drop mutex locks before proxy rescheduling
>       sched/core: Dequeue waking proxy donors before reset
>       sched/core: Mark wakeups completed through ttwu_runnable()
>       sched: Add helper to block retained proxy donors
>       sched: Add sched_ext hooks for proxy execution

Right, so these add:

WF_TTWU_RQ:

  Used like ENQUEUE_DELAYED; could be fixed by generalizing that to cover all
  of p->is_blocked.

scx_allow_proxy_exec():

  Hook to kill proxy exec for scx

scx_proxy_reenqueue_retry():

  Like put_prev_task(), but for current. Ensures current gets put back on a DSQ
  once its done running.

scx_proxy_donor_start():

  Delayed set_next_task(), confirms donor will be used.

sched_proxy_block_task():

  Almost like switching_to_scx(), except it needs to change ctx->queued in case
  of p->is_blocked. Hence a new callback ran before sched_change_begin().



Now, I have:

  https://patch.msgid.link/20260917-sched-fair-hrtick-restart-v4-1-4dd1414da81a@gentwo.org,

pending, would something like the below on top of both this work?

(although I'm not convinced SC_CONFIRM is actually making it better)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1876,7 +1876,7 @@ static inline void uclamp_rq_inc(struct
 	if (!uclamp_is_used())
 		return;
 
-	if (unlikely(!p->sched_class->uclamp_enabled))
+	if (unlikely(!(p->sched_class->flags & SC_UCLAMP)))
 		return;
 
 	/* Only inc the delayed task which being woken up. */
@@ -1904,7 +1904,7 @@ static inline void uclamp_rq_dec(struct
 	if (!uclamp_is_used())
 		return;
 
-	if (unlikely(!p->sched_class->uclamp_enabled))
+	if (unlikely(!(p->sched_class->flags & SC_UCLAMP)))
 		return;
 
 	if (p->se.sched_delayed)
@@ -7268,9 +7268,10 @@ static void __sched notrace __schedule(i
 	rq->next_class = next->sched_class;
 	if (sched_proxy_exec()) {
 		struct task_struct *prev_donor = rq->donor;
+		struct task_struct *donor = next;
 
-		rq_set_donor(rq, next);
-		next->blocked_donor = NULL;
+		rq_set_donor(rq, donor);
+		donor->blocked_donor = NULL;
 		if (unlikely(next->is_blocked)) {
 			next = find_proxy_task(rq, next, &rf);
 			if (!next) {
@@ -7283,8 +7284,7 @@ static void __sched notrace __schedule(i
 				goto keep_resched;
 			}
 		}
-		if (rq->donor == prev_donor && prev != next) {
-			struct task_struct *donor = rq->donor;
+		if (donor == prev_donor && prev != next) {
 			/*
 			 * When transitioning like:
 			 *
@@ -7298,9 +7298,10 @@ static void __sched notrace __schedule(i
 			 * on_cpu.
 			 */
 			donor->sched_class->put_prev_task(rq, donor, donor);
-			donor->sched_class->set_next_task(rq, donor, true);
+			donor->sched_class->set_next_task(rq, donor, SNT_PICK);
 		}
-		scx_proxy_donor_start(rq);
+		if (donor->sched_class->flags & SC_CONFIRM)
+			donor->sched_class->set_next_task(rq, donor, SNT_CONFIRM);
 		scx_proxy_reenqueue_retry(rq, next);
 	} else {
 		rq_set_donor(rq, next);
--- a/kernel/sched/ext/ext.c
+++ b/kernel/sched/ext/ext.c
@@ -5069,8 +5069,12 @@ DEFINE_SCHED_CLASS(ext) = {
 
 	.update_curr		= update_curr_scx,
 
+	.flags			= 0
 #ifdef CONFIG_UCLAMP_TASK
-	.uclamp_enabled		= 1,
+				| SC_UCLAMP
+#endif
+#ifdef CONFIG_SCHED_PROXY_EXEC
+				| SC_CONFIRM
 #endif
 };
 
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -15887,7 +15887,7 @@ DEFINE_SCHED_CLASS(fair) = {
 #endif
 
 #ifdef CONFIG_UCLAMP_TASK
-	.uclamp_enabled		= 1,
+	.flags			= SC_UCLAMP,
 #endif
 };
 
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -2636,7 +2636,7 @@ DEFINE_SCHED_CLASS(rt) = {
 #endif
 
 #ifdef CONFIG_UCLAMP_TASK
-	.uclamp_enabled		= 1,
+	.flags			= SC_UCLAMP,
 #endif
 };
 
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2641,13 +2641,17 @@ enum snt_e {
 	SNT_NORMAL,	/* set_next_task() */
 	SNT_PICK,	/* put_prev_set_next_task(): prev != next */
 	SNT_REPICK,	/* put_prev_set_next_task(): prev == next */
+	SNT_CONFIRM,	/* confirm pick */
+};
+
+enum {
+	SC_UCLAMP	= 1<<0,
+	SC_CONFIRM	= 1<<1,
 };
 
 struct sched_class {
 
-#ifdef CONFIG_UCLAMP_TASK
-	int uclamp_enabled;
-#endif
+	unsigned int flags;
 
 	/*
 	 * move_queued_task/activate_task/enqueue_task: rq->lock