Re: [PATCH] sched/deadline: Fix missing ENQUEUE_REPLENISH during PI de-boosting

[Juri Lelli <juri.lelli@redhat.com>]

Hi Peter,

On 09/02/26 10:46, Juri Lelli wrote:
> On 07/02/26 09:45, Peter Zijlstra wrote:
> > On Fri, Feb 06, 2026 at 02:25:52PM +0100, Juri Lelli wrote:
> > 
> > > @@ -284,6 +285,33 @@ static bool check_same_owner(struct task_struct *p)
> > >  		uid_eq(cred->euid, pcred->uid));
> > >  }
> > >  
> > > +#ifdef CONFIG_RT_MUTEXES
> > > +static void __setscheduler_dl(struct task_struct *p,
> > > +			      struct sched_change_ctx *scope)
> > > +{
> > > +	struct task_struct *pi_task = rt_mutex_get_top_task(p);
> > > +
> > > +	/*
> > > +	 * In case a former DEADLINE task (either proper or boosted) gets
> > > +	 * setscheduled to a lower priority class, check if it neeeds to
> > > +	 * inherit parameters from a potential pi_task. In that case make
> > > +	 * sure replenishment happens with the next enqueue.
> > > +	 */
> > > +	if (!dl_prio(p->normal_prio) &&
> > > +	    (pi_task && dl_prio(pi_task->prio))) {
> > > +		p->dl.pi_se = pi_task->dl.pi_se;
> > > +
> > > +		if (scope && scope->queued)
> > > +			scope->flags |= ENQUEUE_REPLENISH;
> > > +	}
> > > +}
> > > +#else /* !CONFIG_RT_MUTEXES */
> > > +static void __setscheduler_dl(struct task_struct *p,
> > > +			      struct sched_change_ctx *scope)
> > > +{
> > > +}
> > > +#endif /* !CONFIG_RT_MUTEXES */
> > > +
> > >  #ifdef CONFIG_UCLAMP_TASK
> > >  
> > >  static int uclamp_validate(struct task_struct *p,
> > > @@ -657,6 +685,7 @@ int __sched_setscheduler(struct task_struct *p,
> > >  			p->prio = newprio;
> > >  		}
> > >  		__setscheduler_uclamp(p, attr);
> > > +		__setscheduler_dl(p, scope);
> > >  
> > >  		if (scope->queued) {
> > >  			/*
> > > 
> > 
> > Urgh... :-)
> 
> Yeah.
> 
> > So normally it would be __setscheduler_params(), but that funks out
> > because !dl_policy() -- after all, we're demoting the boosted task to be
> > !DL.
> 
> In this particular case we have a DEADLINE task (holder) that didn't
> take the chance of being boosted by another DEADLINE task (donor),
> because donor had longer dynamic deadline when rt_mutex_setprio() was
> called. So now p->dl.pi_se still points to &p->dl and so enqueue_task_dl
> doesn't recognize the holder as boosted and takes the wrong path at the
> start.
> 
> > So then we need to fix up things to the effective priority.
> 
> We can use effective priority, right.
> 
> > Should this not be inside the !KEEP_PARAMS thing? Something like so?
> 
> And do it inside !KEEP_PARAMS, indeed.
> 
> > (afaict nothing clears dl_se::pi_se except rt_mutex_setprio() so that
> > should still be valid here -- so we don't need to go find it again)
> 
> But, maybe with something like this? I believe we need to make things
> right at this point "promoting" the now becoming lower prio class task
> to DEADLINE (inheriting from the task it didn't inherit from in the
> past). Maybe we can avoid checking pi_task since dl_prio(newprio). And
> also move everything in an helper to remove ifdeffery.
> 
> ---
> diff --git a/kernel/sched/syscalls.c b/kernel/sched/syscalls.c
> index 6f10db3646e7f..856df1a22e3ca 100644
> --- a/kernel/sched/syscalls.c
> +++ b/kernel/sched/syscalls.c
> @@ -655,6 +655,16 @@ int __sched_setscheduler(struct task_struct *p,
>  			__setscheduler_params(p, attr);
>  			p->sched_class = next_class;
>  			p->prio = newprio;
> +#ifdef CONFIG_RT_MUTEXES
> +			if (dl_prio(newprio) && !dl_policy(policy)) {
> +				struct task_struct *pi_task = rt_mutex_get_top_task(p);
> +
> +				if (pi_task) {
> +					p->dl.pi_se = pi_task->dl.pi_se;
> +					scope->flags |= ENQUEUE_REPLENISH;
> +				}
> +			}
> +#endif
>  		}
>  		__setscheduler_uclamp(p, attr);

When you have a minute, can you take a look at the above?

Thanks!
Juri