linux-kernel - Re: [PATCH]sched/rt.c: leaf_rt_rq_list contains empty queues. Fix it

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Date:	Fri, 21 Sep 2012 10:31:30 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Kirill Tkhai <tkhai@...dex.ru>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH]sched/rt.c: leaf_rt_rq_list contains empty queues. Fix it

On Fri, 2012-09-21 at 17:35 +0400, Kirill Tkhai wrote:
> Function __enqueue_rt_entity() adds an empty queue to leaf_rt_rq_list.
> 
> So, pick_next_highest_task_rt() picks empty queues. Fix it.

Do you have an example where this breaks?

> 
> Signed-off-by: Kirill Tkhai <tkhai@...dex.ru>
> ---
>  kernel/sched/rt.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index 944cb68..b07edb9 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -1103,7 +1103,7 @@ static void __enqueue_rt_entity(struct sched_rt_entity *rt_se, bool head)
>  	if (group_rq && (rt_rq_throttled(group_rq) || !group_rq->rt_nr_running))
>  		return;
>  
> -	if (!rt_rq->rt_nr_running)
> +	if (rt_rq->rt_nr_running)
>  		list_add_leaf_rt_rq(rt_rq);

Note, the list_add_leaf_rt_rq() is called when the first task is queued
up. The function here is __enqueue_rt_entity(), which will increment
rt_rq->rt_nr_running. If this is the first entry we add the queue now,
as the __dequeue_rt_entity() does the list_del_leaf_rt_rq() when it goes
to zero.

-- Steve

>  
>  	if (head)


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