Re: [PATCH v4 3/5] sched/fair: Switch to task based throttle model

[Aaron Lu <ziqianlu@bytedance.com>]

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:

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 3e927b9b7eeb6..97ae561c60f5b 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5234,6 +5234,7 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
 
 static void check_enqueue_throttle(struct cfs_rq *cfs_rq);
 static inline int cfs_rq_throttled(struct cfs_rq *cfs_rq);
+static inline int cfs_rq_pelt_clock_throttled(struct cfs_rq *cfs_rq);
 
 static void
 requeue_delayed_entity(struct sched_entity *se);
@@ -5729,6 +5730,11 @@ static inline int cfs_rq_throttled(struct cfs_rq *cfs_rq)
 	return cfs_bandwidth_used() && cfs_rq->throttled;
 }
 
+static inline int 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 +6727,11 @@ static inline int cfs_rq_throttled(struct cfs_rq *cfs_rq)
 	return 0;
 }
 
+static inline int cfs_rq_pelt_clock_throttled(struct cfs_rq *cfs_rq)
+{
+	return 0;
+}
+
 static inline int throttled_hierarchy(struct cfs_rq *cfs_rq)
 {
 	return 0;
@@ -13154,10 +13165,7 @@ 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 (!cfs_rq_pelt_clock_throttled(cfs_rq))
 		list_add_leaf_cfs_rq(cfs_rq);
 
 	/* Start to propagate at parent */
@@ -13168,10 +13176,7 @@ 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);
 	}
 }

So this means when a task left/joined a cfs_rq, we will do propagate
immediately, no matter if the cfs_rq is throttled or has its pelt clock
stopped or not; if cfs_rq still has pelt clock running, it will be added
to leaf cfs_rq list to make sure its load can be decayed. If cfs_rq's
pelt clock is stopped, it will be added to leaf cfs_rq list if necessary
by enqueue_task_fair() or when it's unthrottled.