Re: [PATCH] sched: Handle priority boosted tasks proper in setscheduler()

[Steven Rostedt]

On Tue, 5 May 2015 18:08:01 +0200 (CEST)
Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:

> Reported-by: Ronny Meeus <ronny.meeus@xxxxxxxxx>
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> ---
>  kernel/locking/rtmutex.c |   10 ++++++----
>  kernel/sched/core.c      |   11 +++++------
>  2 files changed, 11 insertions(+), 10 deletions(-)
> 
> Index: tip/kernel/locking/rtmutex.c
> ===================================================================
> --- tip.orig/kernel/locking/rtmutex.c
> +++ tip/kernel/locking/rtmutex.c
> @@ -265,15 +265,17 @@ struct task_struct *rt_mutex_get_top_tas
>  }
>  
>  /*
> - * Called by sched_setscheduler() to check whether the priority change
> - * is overruled by a possible priority boosting.
> + * Called by sched_setscheduler() to get the priority which will be
> + * effective after the change.
>   */
>  int rt_mutex_check_prio(struct task_struct *task, int newprio)
>  {
>  	if (!task_has_pi_waiters(task))
> -		return 0;
> +		return newprio;
>  
> -	return task_top_pi_waiter(task)->task->prio <= newprio;
> +	if (task_top_pi_waiter(task)->task->prio <= newprio)
> +		return task_top_pi_waiter(task)->task->prio;
> +	return newprio;
>  }
>  
>  /*
> Index: tip/kernel/sched/core.c
> ===================================================================
> --- tip.orig/kernel/sched/core.c
> +++ tip/kernel/sched/core.c
> @@ -3414,7 +3414,7 @@ static int __sched_setscheduler(struct t
>  	int newprio = dl_policy(attr->sched_policy) ? MAX_DL_PRIO - 1 :
>  		      MAX_RT_PRIO - 1 - attr->sched_priority;
>  	int retval, oldprio, oldpolicy = -1, queued, running;
> -	int policy = attr->sched_policy;
> +	int new_effective_prio, policy = attr->sched_policy;
>  	unsigned long flags;
>  	const struct sched_class *prev_class;
>  	struct rq *rq;
> @@ -3596,15 +3596,14 @@ change:
>  	oldprio = p->prio;
>  
>  	/*
> -	 * Special case for priority boosted tasks.
> -	 *
> -	 * If the new priority is lower or equal (user space view)
> -	 * than the current (boosted) priority, we just store the new
> +	 * Take priority boosted tasks into account. If the new
> +	 * effective priority is unchanged, we just store the new
>  	 * normal parameters and do not touch the scheduler class and
>  	 * the runqueue. This will be done when the task deboost
>  	 * itself.
>  	 */
> -	if (rt_mutex_check_prio(p, newprio)) {
> +	new_effective_prio = rt_mutex_check_prio(p, newprio);
> +	if (new_effective_prio == oldprio) {

When I first heard of this problem, I started writing code to fix this
and came up with pretty much the exact same answer.

I got pulled onto other things so I never finished it, but one thing
that worried me about this fix is this:

	T1 - FIFO policy (prio = 10)
	  lock(rtmutex);

	T2 (prio = 20)
	  lock(rtmutex)
	    boost T1 (prio = 20)

        TI (prio = 20)
	  sys_sched_setscheduler(prio = 30)
	  TI (prio = 30)

	T1 (prio = 30)
	  sys_sched_setscheduler(SCHED_OTHER)
	  new_effective_prio = 20, oldprio = 30

Before the code stopped at the rt_mutex_check_prio(), but now it
continues. Will having the policy change cause problems here?

-- Steve


>  		__setscheduler_params(p, attr);
>  		task_rq_unlock(rq, p, &flags);
>  		return 0;

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