linux-kernel - Re: [PATCH] sched/deadline: Update total bandwidth when adding new task

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Date:	Wed, 19 Feb 2014 11:07:30 +0100
From:	Juri Lelli <juri.lelli@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] sched/deadline: Update total bandwidth when adding new
 task

On 02/19/2014 03:56 AM, Steven Rostedt wrote:
> While debugging the crash with the bad nr_running accounting, I hit
> another bug where, after running my sched deadline test, I was getting
> failures to take a CPU offline. It was giving me a -EBUSY error.
> 
> Adding a bunch of trace_printk()s around, I found that the cpu
> notifier that called sched_cpu_inactive() was returning a failure. The
> overflow value was coming up negative?
> 
> Adding more trace_printk()s, I found that task_dead_dl() function was
> subtracting the exact amount that was keeping the CPU from going
> offline. I then realized that the task_dead_dl() was updating the
> total_bw for the task that was going away, but there was nothing that
> added to the total_bw when the task came alive. 

We call __dl_add() from dl_overflow(), right before calling __setscheduler().

> If total_bw is not
> zero for a CPU, it will keep that CPU from going offline.
> 
> Signed-off-by: Steven Rostedt <rostedt@...dmis.org>
> ---
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index b46131e..17f4830 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3178,6 +3178,7 @@ static void
>  __setparam_dl(struct task_struct *p, const struct sched_attr *attr)
>  {
>  	struct sched_dl_entity *dl_se = &p->dl;
> +	struct dl_bw *dl_b = dl_bw_of(task_cpu(p));
>  
>  	init_dl_task_timer(dl_se);
>  	dl_se->dl_runtime = attr->sched_runtime;
> @@ -3187,6 +3188,7 @@ __setparam_dl(struct task_struct *p, const struct sched_attr *attr)
>  	dl_se->dl_bw = to_ratio(dl_se->dl_period, dl_se->dl_runtime);
>  	dl_se->dl_throttled = 0;
>  	dl_se->dl_new = 1;
> +	__dl_add(dl_b, dl_se->dl_bw);
>  }

This is intended to set params for a task that is going to become -deadline. I
don't think it is the right place to update dl_b. And it seems you add it twice
(inside dl_overflow and inside __setscheduler).

I performed some tests, and it seems that dl_b is updated correctly, once when
the task is started and once when task_dead_dl() is called.

That said, I'm still testing with your stress-cpu-hotplug.

Thanks,

- Juri

>  
>  /* Actually do priority change: must hold pi & rq lock. */
> 
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