[PATCH] sched/fair: Propagate load for throttled cfs_rq

[Aaron Lu <ziqianlu@bytedance.com>]

On Fri, Sep 05, 2025 at 02:53:28PM +0200, Peter Zijlstra wrote:
> On Fri, Sep 05, 2025 at 07:37:19PM +0800, Aaron Lu wrote:
> > Hi Peter,
> > 
> > On Thu, Sep 04, 2025 at 03:04:07PM +0800, Aaron Lu wrote:
> > > On Thu, Sep 04, 2025 at 11:14:31AM +0530, K Prateek Nayak wrote:
> > > > On 9/4/2025 1:57 AM, Peter Zijlstra wrote:
> > > > > So this is mostly tasks leaving/joining the class/cgroup. And its
> > > > > purpose seems to be to remove/add the blocked load component.
> > > > > 
> > > > > Previously throttle/unthrottle would {de,en}queue the whole subtree from
> > > > > PELT, see how {en,de}queue would also stop at throttle.
> > > > > 
> > > > > But now none of that is done; PELT is fully managed by the tasks
> > > > > {de,en}queueing.
> > > > > 
> > > > > So I'm thinking that when a task joins fair (deboost from RT or
> > > > > whatever), we add the blocking load and fully propagate it. If the task
> > > > > is subject to throttling, that will then happen 'naturally' and it will
> > > > > dequeue itself again.
> > > > 
> > > > That seems like the correct thing to do yes. Those throttled_cfs_rq()
> > > > checks in propagate_entity_cfs_rq() can be removed then.
> > > >
> > > 
> > > Not sure if I understand correctly, I've come to the below code
> > > according to your discussion:
> > >
> > 
> > Does the below diff look sane to you? If so, shall I send a separate
> > patch on top or fold it in patch3 and send an updated patch3?
> 
> Yeah, I suppose that works. Please send a follow up patch. It would also
> be good to have a comment that explains why we need that list_add_leaf
> thing. I think I see, but I'm sure I'll have forgotten all next time I
> see this code.

Here it is.

From d88c2d2be31bb3970708f9b78e1725b0e25824be Mon Sep 17 00:00:00 2001
From: Aaron Lu <ziqianlu@bytedance.com>
Date: Fri, 5 Sep 2025 14:21:50 +0800
Subject: [PATCH] sched/fair: Propagate load for throttled cfs_rq

Before task based throttle model, propagating load will stop at a
throttled cfs_rq and that propagate will happen on unthrottle time by
update_load_avg().

Now that there is no update_load_avg() on unthrottle for throttled
cfs_rq and all load tracking is done by task related operations, let the
propagate happen immediately.

While at it, add a comment to explain why cfs_rqs that are not affected
by throttle have to be added to leaf cfs_rq list in
propagate_entity_cfs_rq() per my understanding of commit 0258bdfaff5b
("sched/fair: Fix unfairness caused by missing load decay").

Signed-off-by: Aaron Lu <ziqianlu@bytedance.com>
---
 kernel/sched/fair.c | 26 ++++++++++++++++++--------
 1 file changed, 18 insertions(+), 8 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index df8dc389af8e1..03f16f70bff8a 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5729,6 +5729,11 @@ static inline int cfs_rq_throttled(struct cfs_rq *cfs_rq)
 	return cfs_bandwidth_used() && cfs_rq->throttled;
 }
 
+static inline bool cfs_rq_pelt_clock_throttled(struct cfs_rq *cfs_rq)
+{
+	return cfs_bandwidth_used() && cfs_rq->pelt_clock_throttled;
+}
+
 /* check whether cfs_rq, or any parent, is throttled */
 static inline int throttled_hierarchy(struct cfs_rq *cfs_rq)
 {
@@ -6721,6 +6726,11 @@ static inline int cfs_rq_throttled(struct cfs_rq *cfs_rq)
 	return 0;
 }
 
++static inline bool cfs_rq_pelt_clock_throttled(struct cfs_rq *cfs_rq)
+{
+	return false;
+}
+
 static inline int throttled_hierarchy(struct cfs_rq *cfs_rq)
 {
 	return 0;
@@ -13151,10 +13161,13 @@ static void propagate_entity_cfs_rq(struct sched_entity *se)
 {
 	struct cfs_rq *cfs_rq = cfs_rq_of(se);
 
-	if (cfs_rq_throttled(cfs_rq))
-		return;
-
-	if (!throttled_hierarchy(cfs_rq))
+	/*
+	 * If a task gets attached to this cfs_rq and before being queued,
+	 * it gets migrated to another CPU due to reasons like cpuset change,
+	 * we need to make sure this cfs_rq stays on leaf cfs_rq list to
+	 * have that removed load decayed or it can cause faireness problem.
+	 */
+	if(!cfs_rq_pelt_clock_throttled(cfs_rq))
 		list_add_leaf_cfs_rq(cfs_rq);
 
 	/* Start to propagate at parent */
@@ -13165,10 +13178,7 @@ static void propagate_entity_cfs_rq(struct sched_entity *se)
 
 		update_load_avg(cfs_rq, se, UPDATE_TG);
 
-		if (cfs_rq_throttled(cfs_rq))
-			break;
-
-		if (!throttled_hierarchy(cfs_rq))
+		if (!cfs_rq_pelt_clock_throttled(cfs_rq))
 			list_add_leaf_cfs_rq(cfs_rq);
 	}
 }
-- 
2.39.5