Re: [PATCH v4 1/2] sched/uclamp: Fix race condition leading to imbalanced uclamp_rq

[Christian Loehle <christian.loehle@arm.com>]

On 10/30/26 12:41, Qais Yousef wrote:
> On 10/30/26 13:15, Peter Zijlstra wrote:
>> On Fri, Oct 30, 2026 at 11:28:25AM +0000, Qais Yousef wrote:
>>
>>> Peter/Vincent, what do you think? It seems we have a schrodenger task that is
>>> both enqueued and dequeued simultaneously in these throttling paths. Shouldn't
>>> this be fixed? If yes, any thoughts on the right approach? Should we throttle
>>> if there are no other tasks on the rq too? I didn't realize the semantics are
>>> that strong for enforcing the quota.
>>
>> Urgh, nothing is ever straight forward is it ...
> 
> Exactly my thoughts..
> 
>>
>> Bah, so this is because the core does uclamp (for classes that have it
>> on) but then the class does the throttle without going through the core
>> {en,de}queue.
>>
>> Does it make sense to have throttle_Cfs_rq_work() and tg_unthrottle_up()
>> do uclamp_rq_{dec,inc}() respectively, along with the
>> {de,en}queue_task_fair() they're already doing?
> 
> Yes it does, just wanted to make sure we agree the bug is there rather than in
> the current patchset approach.
> 
> I remember seeing a patch to remove the uclamp_enabled bit and defer to
> enqueue/dequeue_task_$class to handle this instead. Maybe I can revive it if it
> makes more sense to go that way.
> 
> Let me find the path of least resistance and add it to the stack. Not sure we
> want to tag as Fixes for LTS to pick up this independent fix. Note we have rt
> group throttling path flagged up - which I also thought was false positive but
> not sure anymore - but it seems to do partial dequeue and must be fixed in that
> regard anyway even if it doesn't force idle.

I can reproduce the idle path without uclamp_rq_dec through sched_rt_rq_dequeue(),
too, so IMO that needs fixing up as well.