Re: [PATCH 5/5] sched/fair: Unify cfs_rq throttling via account_cfs_rq_runtime()

[K Prateek Nayak]

Hello Peter,

On 6/1/2026 7:18 PM, Peter Zijlstra wrote:
> On Thu, May 28, 2026 at 09:48:30AM +0000, K Prateek Nayak wrote:
> 
>> @@ -9893,8 +9882,15 @@ static struct task_struct *pick_task_fair(struct rq *rq, struct rq_flags *rf)
>>  		/* Might not have done put_prev_entity() */
>>  		if (cfs_rq->curr && cfs_rq->curr->on_rq)
>>  			update_curr(cfs_rq);
>> -
>> -		throttled |= check_cfs_rq_runtime(cfs_rq);
>> +		/*
>> +		 * For the current hierarchy, update_curr() above would
>> +		 * have set the throttle indicators if the cfs_rq has
>> +		 * run out of bandwidth. For others, enqueue / last
>> +		 * update_curr() for the cfs_rq would have ensured the
>> +		 * throttle indicators are set if bandwidth was not
>> +		 * available.
>> +		 */
>> +		throttled |= cfs_rq_throttled(cfs_rq);
>>  
>>  		se = pick_next_entity(rq, cfs_rq, true);
>>  		if (!se)
> 
>> @@ -15074,15 +15070,19 @@ static void __set_next_task_fair(struct rq *rq, struct task_struct *p, bool firs
>>  static void set_next_task_fair(struct rq *rq, struct task_struct *p, bool first)
>>  {
>>  	struct sched_entity *se = &p->se;
>> +	bool throttled = false;
>>  
>>  	for_each_sched_entity(se) {
>>  		struct cfs_rq *cfs_rq = cfs_rq_of(se);
>>  
>>  		set_next_entity(cfs_rq, se, first);
>>  		/* ensure bandwidth has been allocated on our new cfs_rq */
>> -		account_cfs_rq_runtime(cfs_rq, 0);
>> +		throttled |= account_cfs_rq_runtime(cfs_rq, 0);
>>  	}
>>  
>> +	if (throttled)
>> +		task_throttle_setup_work(p);
>> +
>>  	__set_next_task_fair(rq, p, first);
>>  }
> 
> (noticed while trying to rebase flat on top)
> 
> Why do we have both? Isn't just set_next_task_fair(.first=true)
> sufficient?

I misread this bit and only called account_cfs_rq_runtime() for !first in my
v2 [1] but even the following changes on top of my v2 [1] yields similar
results in my testing (slightly better on performance) so feel free to squash
this bit into the last patch:

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index ce5cf494b934..fa8c0b1a1cf1 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9892,15 +9892,6 @@ struct task_struct *pick_task_fair(struct rq *rq, struct rq_flags *rf)
 		/* Might not have done put_prev_entity() */
 		if (cfs_rq->curr && cfs_rq->curr->on_rq)
 			update_curr(cfs_rq);
-		/*
-		 * For the current hierarchy, update_curr() above would
-		 * have set the throttle indicators if the cfs_rq has
-		 * run out of bandwidth. For others, enqueue / last
-		 * update_curr() for the cfs_rq would have ensured the
-		 * throttle indicators are set if bandwidth was not
-		 * available.
-		 */
-		throttled |= cfs_rq_throttled(cfs_rq);
 
 		se = pick_next_entity(rq, cfs_rq, true);
 		if (!se)
@@ -15012,12 +15003,8 @@ static void set_next_task_fair(struct rq *rq, struct task_struct *p, bool first)
 			break;
 
 		set_next_entity(cfs_rq, se, first);
-		/*
-		 * Ensure bandwidth has been allocated on our new cfs_rq
-		 * if we've reached here for reasons other than pick.
-		 */
-		if (!first)
-			throttled |= account_cfs_rq_runtime(cfs_rq, 0);
+		/* ensure bandwidth has been allocated on our new cfs_rq */
+		throttled |= account_cfs_rq_runtime(cfs_rq, 0);
 	}
 
 	if (throttled)
---

My mind is taking a while to grasp the ->pick_next_task() removal.

[1] https://lore.kernel.org/lkml/20260602050005.11160-1-kprateek.nayak@xxxxxxx/

-- 
Thanks and Regards,
Prateek