linux-kernel - Re: Bug in scheduler when using rt

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Date:	Tue, 18 Jan 2011 16:25:27 +0200
From:	Onkalo Samu <samu.p.onkalo@...ia.com>
To:	ext Peter Zijlstra <peterz@...radead.org>
Cc:	Yong Zhang <yong.zhang0@...il.com>, mingo@...e.hu,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	tglx <tglx@...utronix.de>
Subject: Re: Bug in scheduler when using rt_mutex

On Tue, 2011-01-18 at 14:35 +0100, ext Peter Zijlstra wrote:
> On Tue, 2011-01-18 at 16:59 +0800, Yong Zhang wrote:
> > On Tue, Jan 18, 2011 at 4:23 PM, Onkalo Samu <samu.p.onkalo@...ia.com> wrote:
> > > On Mon, 2011-01-17 at 17:00 +0100, ext Peter Zijlstra wrote:
> > >> On Mon, 2011-01-17 at 16:42 +0200, Onkalo Samu wrote:
> > >> >
> > >> > Failure case:
> > >> > - user process locks rt_mutex
> > >> > - and goes to sleep (wait_for_completion etc.)
> > >> > - user process is dequeued to sleep state
> > >> > -> vruntime is not updated in dequeue_entity
> > >> >
> > >>
> > >> Does the below (completely untested) patch help?
> > >
> > > Unfortunately no.
> > 
> > It couldn't work because place_entity() will not place it
> > backwards.
> 
> Ah indeed, I was somehow assuming it was way left, but that is not at
> all true, something like the below then..
> 

First trials show that my test case is not stucked. I'll continue
testing. Thanks.

Howabout this i2c-core case. Do you see that rt_mutex must be taken
away? It reduces latencies from the I2C accesses in the irq-threads.

-Samu

> ---
> Subject: sched: Fix switch_to_fair()
> From: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> Date: Mon Jan 17 17:03:27 CET 2011
> 
> When a task is placed back into fair_sched_class, we must update its
> placement, since we don't know how long its been gone, hence its
> vruntime is stale and cannot be trusted.
> 
> There is also a case where it was moved from fair_sched_class when it
> was in a blocked state and moved back while it is running, this causes
> an imbalance between DEQUEUE_SLEEP/DEQUEUE_WAKEUP for the fair class
> and leaves vruntime way out there (due to the min_vruntime
> adjustment).
> 
> Also update sysrq-n to call the ->switch_{to,from} methods.
> 
> Reported-by: Onkalo Samu <samu.p.onkalo@...ia.com>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> ---
>  kernel/sched.c      |    4 ++++
>  kernel/sched_fair.c |   16 ++++++++++++++++
>  2 files changed, 20 insertions(+)
> 
> Index: linux-2.6/kernel/sched.c
> ===================================================================
> --- linux-2.6.orig/kernel/sched.c
> +++ linux-2.6/kernel/sched.c
> @@ -8106,6 +8106,8 @@ EXPORT_SYMBOL(__might_sleep);
>  #ifdef CONFIG_MAGIC_SYSRQ
>  static void normalize_task(struct rq *rq, struct task_struct *p)
>  {
> +	struct sched_class *prev_class = p->sched_class;
> +	int old_prio = p->prio;
>  	int on_rq;
>  
>  	on_rq = p->se.on_rq;
> @@ -8116,6 +8118,8 @@ static void normalize_task(struct rq *rq
>  		activate_task(rq, p, 0);
>  		resched_task(rq->curr);
>  	}
> +
> +	check_class_changed(rq, p, prev_class, old_prio, task_current(rq, p));
>  }
>  
>  void normalize_rt_tasks(void)
> Index: linux-2.6/kernel/sched_fair.c
> ===================================================================
> --- linux-2.6.orig/kernel/sched_fair.c
> +++ linux-2.6/kernel/sched_fair.c
> @@ -4075,6 +4075,22 @@ static void prio_changed_fair(struct rq
>  static void switched_to_fair(struct rq *rq, struct task_struct *p,
>  			     int running)
>  {
> +	struct sched_entity *se = &p->se;
> +	struct cfs_rq *cfs_rq = cfs_rq_of(se);
> +
> +	if (se->on_rq && cfs_rq->curr != se)
> +		__dequeue_entity(cfs_rq, se);
> +
> +	/*
> +	 * se->vruntime can be completely out there, there is no telling
> +	 * how long this task was !fair and on what CPU if any it became
> +	 * !fair. Therefore, reset it to a known, reasonable value.
> +	 */
> +	se->vruntime = cfs_rq->min_vruntime;
> +
> +	if (se->on_rq && cfs_rq->curr != se)
> +		__enqueue_entity(cfs_rq, se);
> +
>  	/*
>  	 * We were most likely switched from sched_rt, so
>  	 * kick off the schedule if running, otherwise just see
> 


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