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

[Peter Zijlstra]

On Fri, Oct 30, 2026 at 01:21:30PM +0000, Christian Loehle wrote:
> 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.

Right, every class that has uclamp_enabled needs to be audited -- this
seems to include sched_ext.