Re: [PATCH v25 1/9] sched: Make class_schedulers avoid pushing current, and get rid of proxy_tag_curr()

[Peter Zijlstra]

On Mon, Mar 16, 2026 at 11:04:28PM -0700, John Stultz wrote:
> On Mon, Mar 16, 2026 at 10:41 PM K Prateek Nayak <kprateek.nayak@xxxxxxx> wrote:
> > On 3/17/2026 10:19 AM, John Stultz wrote:
> > >
> > > I guess adding a new helper function to manually do the
> > > put_prev/set_next could be added to the top level __schedule() logic
> > > in the (prev != next) case, though we'll have to preserve the
> > > prev_donor on the stack probably.
> >
> > That seems like the best option to me too.
> >
> > Also, deadline, RT, fair, and idle don't really care about the "next"
> > argument of put_prev_task() and the only one that does care is
> > put_prev_task_scx() to call switch_class() callback so putting it as
> > either NULL or "rq->donor" should be safe.
> 
> Ack.
> Here's the change I'm testing tonight (against 6.18):
> https://github.com/johnstultz-work/linux-dev/commit/0cc72a4923143f496e33711cbcc1afdf6d861ca6
> 
> Feel free to suggest a better name for the helper function. It feels a
> little clunky (and sort of sad right after getting rid of the clunky
> proxy_tag_curr(), to re-add something so similar).

Does this capture it?

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7100,9 +7103,11 @@ static void __sched notrace __schedule(i
 pick_again:
 	assert_balance_callbacks_empty(rq);
 	next = pick_next_task(rq, rq->donor, &rf);
-	rq_set_donor(rq, next);
 	rq->next_class = next->sched_class;
 	if (sched_proxy_exec()) {
+		struct task_struct *prev_donor = rq->donor;
+
+		rq_set_donor(rq, next);
 		if (unlikely(next->blocked_on)) {
 			next = find_proxy_task(rq, next, &rf);
 			if (!next) {
@@ -7114,6 +7119,24 @@ static void __sched notrace __schedule(i
 				goto keep_resched;
 			}
 		}
+
+		/*
+		 * When transitioning like:
+		 *
+		 *	   prev		next
+		 * donor:    B		  B
+		 * curr:     A		  B
+		 *
+		 * then put_prev_set_next_task() will not have done anything,
+		 * since B == B. However, A might have missed a RT/DL balance
+		 * opportunity due to being on_cpu.
+		 */
+		if (next == rq->donor && next == prev_donor) {
+			next->sched_class->put_prev_task(rq, next, next);
+			next->sched_class->set_next_task(rq, next, true);
+		}
+	} else {
+		rq_set_donor(rq, next);
 	}
 picked:
 	clear_tsk_need_resched(prev);