Date:	Wed, 2 Mar 2016 21:16:57 +0100
From:	luca abeni <luca.abeni@...tn.it>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Juri Lelli <juri.lelli@....com>, linux-kernel@...r.kernel.org
Subject: Re: Question about prio_changed_dl()

On Wed, 2 Mar 2016 20:02:58 +0100
Peter Zijlstra <peterz@...radead.org> wrote:

> On Sat, Feb 27, 2016 at 12:37:57PM +0100, luca abeni wrote:
> > Subject: sched/core: fix __sched_setscheduler() to properly invoke prio_changed_dl()
> > 
> > Currently, prio_changed_dl() is not called when __sched_setscheduler()
> > changes the parameters of a SCHED_DEADLINE task. This happens because
> > when changing parameters deadline tasks do not change their priority,
> > so new_effective_prio == oldprio.
> > The issue is fixed by explicitly checking if the task is a deadline task.
> > 
> > Signed-off-by: Luca Abeni <luca.abeni@...tn.it>
> > ---
> >  kernel/sched/core.c | 2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index 9503d59..5646bde 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -4079,6 +4079,8 @@ change:
> >  		new_effective_prio = rt_mutex_get_effective_prio(p, newprio);
> >  		if (new_effective_prio == oldprio) {
> >  			__setscheduler_params(p, attr);
> > +			if (p->sched_class == &dl_sched_class)
> > +				p->sched_class->prio_changed(rq, p, oldprio);
> >  			task_rq_unlock(rq, p, &flags);
> >  			return 0;
> >  		}
> 
> That code no longer exists like that.
Uh... I must have done something wrong, then (I was convinced I pulled
from master and rebased my patch before testing and sending it, but
probably something went wrong).
Sorry about that.


> Can you see if the below patch (which caused that) also fixes your
> problem?
Ok; in the next days I'll make sure to have this patch applied and
I'll test it.


			Thanks,
				Luca
> 
> ---
> commit ff77e468535987b3d21b7bd4da15608ea3ce7d0b
> Author: Peter Zijlstra <peterz@...radead.org>
> Date:   Mon Jan 18 15:27:07 2016 +0100
> 
>     sched/rt: Fix PI handling vs. sched_setscheduler()
>     
>     Andrea Parri reported:
>     
>     > I found that the following scenario (with CONFIG_RT_GROUP_SCHED=y) is not
>     > handled correctly:
>     >
>     >     T1 (prio = 20)
>     >        lock(rtmutex);
>     >
>     >     T2 (prio = 20)
>     >        blocks on rtmutex  (rt_nr_boosted = 0 on T1's rq)
>     >
>     >     T1 (prio = 20)
>     >        sys_set_scheduler(prio = 0)
>     >           [new_effective_prio == oldprio]
>     >           T1 prio = 20    (rt_nr_boosted = 0 on T1's rq)
>     >
>     > The last step is incorrect as T1 is now boosted (c.f., rt_se_boosted());
>     > in particular, if we continue with
>     >
>     >    T1 (prio = 20)
>     >       unlock(rtmutex)
>     >          wakeup(T2)
>     >          adjust_prio(T1)
>     >             [prio != rt_mutex_getprio(T1)]
>     >	    dequeue(T1)
>     >	       rt_nr_boosted = (unsigned long)(-1)
>     >	       ...
>     >             T1 prio = 0
>     >
>     > then we end up leaving rt_nr_boosted in an "inconsistent" state.
>     >
>     > The simple program attached could reproduce the previous scenario; note
>     > that, as a consequence of the presence of this state, the "assertion"
>     >
>     >     WARN_ON(!rt_nr_running && rt_nr_boosted)
>     >
>     > from dec_rt_group() may trigger.
>     
>     So normally we dequeue/enqueue tasks in sched_setscheduler(), which
>     would ensure the accounting stays correct. However in the early PI path
>     we fail to do so.
>     
>     So this was introduced at around v3.14, by:
>     
>       c365c292d059 ("sched: Consider pi boosting in setscheduler()")
>     
>     which fixed another problem exactly because that dequeue/enqueue, joy.
>     
>     Fix this by teaching rt about DEQUEUE_SAVE/ENQUEUE_RESTORE and have it
>     preserve runqueue location with that option. This requires decoupling
>     the on_rt_rq() state from being on the list.
>     
>     In order to allow for explicit movement during the SAVE/RESTORE,
>     introduce {DE,EN}QUEUE_MOVE. We still must use SAVE/RESTORE in these
>     cases to preserve other invariants.
>     
>     Respecting the SAVE/RESTORE flags also has the (nice) side-effect that
>     things like sys_nice()/sys_sched_setaffinity() also do not reorder
>     FIFO tasks (whereas they used to before this patch).
>     
>     Reported-by: Andrea Parri <parri.andrea@...il.com>
>     Tested-by: Andrea Parri <parri.andrea@...il.com>
>     Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
>     Cc: Juri Lelli <juri.lelli@....com>
>     Cc: Linus Torvalds <torvalds@...ux-foundation.org>
>     Cc: Mike Galbraith <efault@....de>
>     Cc: Peter Zijlstra <peterz@...radead.org>
>     Cc: Steven Rostedt <rostedt@...dmis.org>
>     Cc: Thomas Gleixner <tglx@...utronix.de>
>     Signed-off-by: Ingo Molnar <mingo@...nel.org>
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index a292c4b7e94c..87e5f9886ac8 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1293,6 +1293,8 @@ struct sched_rt_entity {
>  	unsigned long timeout;
>  	unsigned long watchdog_stamp;
>  	unsigned int time_slice;
> +	unsigned short on_rq;
> +	unsigned short on_list;
>  
>  	struct sched_rt_entity *back;
>  #ifdef CONFIG_RT_GROUP_SCHED
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 291552b4d8ee..bb565b4663c8 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2221,6 +2221,10 @@ static void __sched_fork(unsigned long clone_flags, struct task_struct *p)
>  	__dl_clear_params(p);
>  
>  	INIT_LIST_HEAD(&p->rt.run_list);
> +	p->rt.timeout		= 0;
> +	p->rt.time_slice	= sched_rr_timeslice;
> +	p->rt.on_rq		= 0;
> +	p->rt.on_list		= 0;
>  
>  #ifdef CONFIG_PREEMPT_NOTIFIERS
>  	INIT_HLIST_HEAD(&p->preempt_notifiers);
> @@ -3531,7 +3535,7 @@ EXPORT_SYMBOL(default_wake_function);
>   */
>  void rt_mutex_setprio(struct task_struct *p, int prio)
>  {
> -	int oldprio, queued, running, enqueue_flag = ENQUEUE_RESTORE;
> +	int oldprio, queued, running, queue_flag = DEQUEUE_SAVE | DEQUEUE_MOVE;
>  	struct rq *rq;
>  	const struct sched_class *prev_class;
>  
> @@ -3559,11 +3563,15 @@ void rt_mutex_setprio(struct task_struct *p, int prio)
>  
>  	trace_sched_pi_setprio(p, prio);
>  	oldprio = p->prio;
> +
> +	if (oldprio == prio)
> +		queue_flag &= ~DEQUEUE_MOVE;
> +
>  	prev_class = p->sched_class;
>  	queued = task_on_rq_queued(p);
>  	running = task_current(rq, p);
>  	if (queued)
> -		dequeue_task(rq, p, DEQUEUE_SAVE);
> +		dequeue_task(rq, p, queue_flag);
>  	if (running)
>  		put_prev_task(rq, p);
>  
> @@ -3581,7 +3589,7 @@ void rt_mutex_setprio(struct task_struct *p, int prio)
>  		if (!dl_prio(p->normal_prio) ||
>  		    (pi_task && dl_entity_preempt(&pi_task->dl, &p->dl))) {
>  			p->dl.dl_boosted = 1;
> -			enqueue_flag |= ENQUEUE_REPLENISH;
> +			queue_flag |= ENQUEUE_REPLENISH;
>  		} else
>  			p->dl.dl_boosted = 0;
>  		p->sched_class = &dl_sched_class;
> @@ -3589,7 +3597,7 @@ void rt_mutex_setprio(struct task_struct *p, int prio)
>  		if (dl_prio(oldprio))
>  			p->dl.dl_boosted = 0;
>  		if (oldprio < prio)
> -			enqueue_flag |= ENQUEUE_HEAD;
> +			queue_flag |= ENQUEUE_HEAD;
>  		p->sched_class = &rt_sched_class;
>  	} else {
>  		if (dl_prio(oldprio))
> @@ -3604,7 +3612,7 @@ void rt_mutex_setprio(struct task_struct *p, int prio)
>  	if (running)
>  		p->sched_class->set_curr_task(rq);
>  	if (queued)
> -		enqueue_task(rq, p, enqueue_flag);
> +		enqueue_task(rq, p, queue_flag);
>  
>  	check_class_changed(rq, p, prev_class, oldprio);
>  out_unlock:
> @@ -3960,6 +3968,7 @@ static int __sched_setscheduler(struct task_struct *p,
>  	const struct sched_class *prev_class;
>  	struct rq *rq;
>  	int reset_on_fork;
> +	int queue_flags = DEQUEUE_SAVE | DEQUEUE_MOVE;
>  
>  	/* may grab non-irq protected spin_locks */
>  	BUG_ON(in_interrupt());
> @@ -4142,17 +4151,14 @@ static int __sched_setscheduler(struct task_struct *p,
>  		 * itself.
>  		 */
>  		new_effective_prio = rt_mutex_get_effective_prio(p, newprio);
> -		if (new_effective_prio == oldprio) {
> -			__setscheduler_params(p, attr);
> -			task_rq_unlock(rq, p, &flags);
> -			return 0;
> -		}
> +		if (new_effective_prio == oldprio)
> +			queue_flags &= ~DEQUEUE_MOVE;
>  	}
>  
>  	queued = task_on_rq_queued(p);
>  	running = task_current(rq, p);
>  	if (queued)
> -		dequeue_task(rq, p, DEQUEUE_SAVE);
> +		dequeue_task(rq, p, queue_flags);
>  	if (running)
>  		put_prev_task(rq, p);
>  
> @@ -4162,15 +4168,14 @@ static int __sched_setscheduler(struct task_struct *p,
>  	if (running)
>  		p->sched_class->set_curr_task(rq);
>  	if (queued) {
> -		int enqueue_flags = ENQUEUE_RESTORE;
>  		/*
>  		 * We enqueue to tail when the priority of a task is
>  		 * increased (user space view).
>  		 */
> -		if (oldprio <= p->prio)
> -			enqueue_flags |= ENQUEUE_HEAD;
> +		if (oldprio < p->prio)
> +			queue_flags |= ENQUEUE_HEAD;
>  
> -		enqueue_task(rq, p, enqueue_flags);
> +		enqueue_task(rq, p, queue_flags);
>  	}
>  
>  	check_class_changed(rq, p, prev_class, oldprio);
> @@ -7958,7 +7963,7 @@ void sched_move_task(struct task_struct *tsk)
>  	queued = task_on_rq_queued(tsk);
>  
>  	if (queued)
> -		dequeue_task(rq, tsk, DEQUEUE_SAVE);
> +		dequeue_task(rq, tsk, DEQUEUE_SAVE | DEQUEUE_MOVE);
>  	if (unlikely(running))
>  		put_prev_task(rq, tsk);
>  
> @@ -7982,7 +7987,7 @@ void sched_move_task(struct task_struct *tsk)
>  	if (unlikely(running))
>  		tsk->sched_class->set_curr_task(rq);
>  	if (queued)
> -		enqueue_task(rq, tsk, ENQUEUE_RESTORE);
> +		enqueue_task(rq, tsk, ENQUEUE_RESTORE | ENQUEUE_MOVE);
>  
>  	task_rq_unlock(rq, tsk, &flags);
>  }
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index 8ec86abe0ea1..406a9c20b210 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -436,7 +436,7 @@ static void dequeue_top_rt_rq(struct rt_rq *rt_rq);
>  
>  static inline int on_rt_rq(struct sched_rt_entity *rt_se)
>  {
> -	return !list_empty(&rt_se->run_list);
> +	return rt_se->on_rq;
>  }
>  
>  #ifdef CONFIG_RT_GROUP_SCHED
> @@ -482,8 +482,8 @@ static inline struct rt_rq *group_rt_rq(struct sched_rt_entity *rt_se)
>  	return rt_se->my_q;
>  }
>  
> -static void enqueue_rt_entity(struct sched_rt_entity *rt_se, bool head);
> -static void dequeue_rt_entity(struct sched_rt_entity *rt_se);
> +static void enqueue_rt_entity(struct sched_rt_entity *rt_se, unsigned int flags);
> +static void dequeue_rt_entity(struct sched_rt_entity *rt_se, unsigned int flags);
>  
>  static void sched_rt_rq_enqueue(struct rt_rq *rt_rq)
>  {
> @@ -499,7 +499,7 @@ static void sched_rt_rq_enqueue(struct rt_rq *rt_rq)
>  		if (!rt_se)
>  			enqueue_top_rt_rq(rt_rq);
>  		else if (!on_rt_rq(rt_se))
> -			enqueue_rt_entity(rt_se, false);
> +			enqueue_rt_entity(rt_se, 0);
>  
>  		if (rt_rq->highest_prio.curr < curr->prio)
>  			resched_curr(rq);
> @@ -516,7 +516,7 @@ static void sched_rt_rq_dequeue(struct rt_rq *rt_rq)
>  	if (!rt_se)
>  		dequeue_top_rt_rq(rt_rq);
>  	else if (on_rt_rq(rt_se))
> -		dequeue_rt_entity(rt_se);
> +		dequeue_rt_entity(rt_se, 0);
>  }
>  
>  static inline int rt_rq_throttled(struct rt_rq *rt_rq)
> @@ -1166,7 +1166,30 @@ void dec_rt_tasks(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq)
>  	dec_rt_group(rt_se, rt_rq);
>  }
>  
> -static void __enqueue_rt_entity(struct sched_rt_entity *rt_se, bool head)
> +/*
> + * Change rt_se->run_list location unless SAVE && !MOVE
> + *
> + * assumes ENQUEUE/DEQUEUE flags match
> + */
> +static inline bool move_entity(unsigned int flags)
> +{
> +	if ((flags & (DEQUEUE_SAVE | DEQUEUE_MOVE)) == DEQUEUE_SAVE)
> +		return false;
> +
> +	return true;
> +}
> +
> +static void __delist_rt_entity(struct sched_rt_entity *rt_se, struct rt_prio_array *array)
> +{
> +	list_del_init(&rt_se->run_list);
> +
> +	if (list_empty(array->queue + rt_se_prio(rt_se)))
> +		__clear_bit(rt_se_prio(rt_se), array->bitmap);
> +
> +	rt_se->on_list = 0;
> +}
> +
> +static void __enqueue_rt_entity(struct sched_rt_entity *rt_se, unsigned int flags)
>  {
>  	struct rt_rq *rt_rq = rt_rq_of_se(rt_se);
>  	struct rt_prio_array *array = &rt_rq->active;
> @@ -1179,26 +1202,37 @@ static void __enqueue_rt_entity(struct sched_rt_entity *rt_se, bool head)
>  	 * get throttled and the current group doesn't have any other
>  	 * active members.
>  	 */
> -	if (group_rq && (rt_rq_throttled(group_rq) || !group_rq->rt_nr_running))
> +	if (group_rq && (rt_rq_throttled(group_rq) || !group_rq->rt_nr_running)) {
> +		if (rt_se->on_list)
> +			__delist_rt_entity(rt_se, array);
>  		return;
> +	}
>  
> -	if (head)
> -		list_add(&rt_se->run_list, queue);
> -	else
> -		list_add_tail(&rt_se->run_list, queue);
> -	__set_bit(rt_se_prio(rt_se), array->bitmap);
> +	if (move_entity(flags)) {
> +		WARN_ON_ONCE(rt_se->on_list);
> +		if (flags & ENQUEUE_HEAD)
> +			list_add(&rt_se->run_list, queue);
> +		else
> +			list_add_tail(&rt_se->run_list, queue);
> +
> +		__set_bit(rt_se_prio(rt_se), array->bitmap);
> +		rt_se->on_list = 1;
> +	}
> +	rt_se->on_rq = 1;
>  
>  	inc_rt_tasks(rt_se, rt_rq);
>  }
>  
> -static void __dequeue_rt_entity(struct sched_rt_entity *rt_se)
> +static void __dequeue_rt_entity(struct sched_rt_entity *rt_se, unsigned int flags)
>  {
>  	struct rt_rq *rt_rq = rt_rq_of_se(rt_se);
>  	struct rt_prio_array *array = &rt_rq->active;
>  
> -	list_del_init(&rt_se->run_list);
> -	if (list_empty(array->queue + rt_se_prio(rt_se)))
> -		__clear_bit(rt_se_prio(rt_se), array->bitmap);
> +	if (move_entity(flags)) {
> +		WARN_ON_ONCE(!rt_se->on_list);
> +		__delist_rt_entity(rt_se, array);
> +	}
> +	rt_se->on_rq = 0;
>  
>  	dec_rt_tasks(rt_se, rt_rq);
>  }
> @@ -1207,7 +1241,7 @@ static void __dequeue_rt_entity(struct sched_rt_entity *rt_se)
>   * Because the prio of an upper entry depends on the lower
>   * entries, we must remove entries top - down.
>   */
> -static void dequeue_rt_stack(struct sched_rt_entity *rt_se)
> +static void dequeue_rt_stack(struct sched_rt_entity *rt_se, unsigned int flags)
>  {
>  	struct sched_rt_entity *back = NULL;
>  
> @@ -1220,31 +1254,31 @@ static void dequeue_rt_stack(struct sched_rt_entity *rt_se)
>  
>  	for (rt_se = back; rt_se; rt_se = rt_se->back) {
>  		if (on_rt_rq(rt_se))
> -			__dequeue_rt_entity(rt_se);
> +			__dequeue_rt_entity(rt_se, flags);
>  	}
>  }
>  
> -static void enqueue_rt_entity(struct sched_rt_entity *rt_se, bool head)
> +static void enqueue_rt_entity(struct sched_rt_entity *rt_se, unsigned int flags)
>  {
>  	struct rq *rq = rq_of_rt_se(rt_se);
>  
> -	dequeue_rt_stack(rt_se);
> +	dequeue_rt_stack(rt_se, flags);
>  	for_each_sched_rt_entity(rt_se)
> -		__enqueue_rt_entity(rt_se, head);
> +		__enqueue_rt_entity(rt_se, flags);
>  	enqueue_top_rt_rq(&rq->rt);
>  }
>  
> -static void dequeue_rt_entity(struct sched_rt_entity *rt_se)
> +static void dequeue_rt_entity(struct sched_rt_entity *rt_se, unsigned int flags)
>  {
>  	struct rq *rq = rq_of_rt_se(rt_se);
>  
> -	dequeue_rt_stack(rt_se);
> +	dequeue_rt_stack(rt_se, flags);
>  
>  	for_each_sched_rt_entity(rt_se) {
>  		struct rt_rq *rt_rq = group_rt_rq(rt_se);
>  
>  		if (rt_rq && rt_rq->rt_nr_running)
> -			__enqueue_rt_entity(rt_se, false);
> +			__enqueue_rt_entity(rt_se, flags);
>  	}
>  	enqueue_top_rt_rq(&rq->rt);
>  }
> @@ -1260,7 +1294,7 @@ enqueue_task_rt(struct rq *rq, struct task_struct *p, int flags)
>  	if (flags & ENQUEUE_WAKEUP)
>  		rt_se->timeout = 0;
>  
> -	enqueue_rt_entity(rt_se, flags & ENQUEUE_HEAD);
> +	enqueue_rt_entity(rt_se, flags);
>  
>  	if (!task_current(rq, p) && p->nr_cpus_allowed > 1)
>  		enqueue_pushable_task(rq, p);
> @@ -1271,7 +1305,7 @@ static void dequeue_task_rt(struct rq *rq, struct task_struct *p, int flags)
>  	struct sched_rt_entity *rt_se = &p->rt;
>  
>  	update_curr_rt(rq);
> -	dequeue_rt_entity(rt_se);
> +	dequeue_rt_entity(rt_se, flags);
>  
>  	dequeue_pushable_task(rq, p);
>  }
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 3dc7b8b3f94c..d3606e40ea0d 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1130,18 +1130,40 @@ static inline void finish_lock_switch(struct rq *rq, struct task_struct *prev)
>  extern const int sched_prio_to_weight[40];
>  extern const u32 sched_prio_to_wmult[40];
>  
> +/*
> + * {de,en}queue flags:
> + *
> + * DEQUEUE_SLEEP  - task is no longer runnable
> + * ENQUEUE_WAKEUP - task just became runnable
> + *
> + * SAVE/RESTORE - an otherwise spurious dequeue/enqueue, done to ensure tasks
> + *                are in a known state which allows modification. Such pairs
> + *                should preserve as much state as possible.
> + *
> + * MOVE - paired with SAVE/RESTORE, explicitly does not preserve the location
> + *        in the runqueue.
> + *
> + * ENQUEUE_HEAD      - place at front of runqueue (tail if not specified)
> + * ENQUEUE_REPLENISH - CBS (replenish runtime and postpone deadline)
> + * ENQUEUE_WAKING    - sched_class::task_waking was called
> + *
> + */
> +
> +#define DEQUEUE_SLEEP		0x01
> +#define DEQUEUE_SAVE		0x02 /* matches ENQUEUE_RESTORE */
> +#define DEQUEUE_MOVE		0x04 /* matches ENQUEUE_MOVE */
> +
>  #define ENQUEUE_WAKEUP		0x01
> -#define ENQUEUE_HEAD		0x02
> +#define ENQUEUE_RESTORE		0x02
> +#define ENQUEUE_MOVE		0x04
> +
> +#define ENQUEUE_HEAD		0x08
> +#define ENQUEUE_REPLENISH	0x10
>  #ifdef CONFIG_SMP
> -#define ENQUEUE_WAKING		0x04	/* sched_class::task_waking was called */
> +#define ENQUEUE_WAKING		0x20
>  #else
>  #define ENQUEUE_WAKING		0x00
>  #endif
> -#define ENQUEUE_REPLENISH	0x08
> -#define ENQUEUE_RESTORE	0x10
> -
> -#define DEQUEUE_SLEEP		0x01
> -#define DEQUEUE_SAVE		0x02
>  
>  #define RETRY_TASK		((void *)-1UL)
>  

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