linux-kernel - Re: [PATCH 1/2] sched/deadline: add per rq tracking of admitted bandwidth

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Date:	Wed, 10 Feb 2016 13:48:48 +0100
From:	luca abeni <luca.abeni@...tn.it>
To:	Juri Lelli <juri.lelli@....com>
Cc:	rostedt@...dmis.org, linux-kernel@...r.kernel.org,
	peterz@...radead.org, mingo@...hat.com, vincent.guittot@...aro.org,
	wanpeng.li@...mail.com
Subject: Re: [PATCH 1/2] sched/deadline: add per rq tracking of admitted
 bandwidth

Hi,

On Wed, 10 Feb 2016 11:32:58 +0000
Juri Lelli <juri.lelli@....com> wrote:
[...]
> From 62f70ca3051672dce209e8355cf5eddc9d825c2a Mon Sep 17 00:00:00 2001
> From: Juri Lelli <juri.lelli@....com>
> Date: Sat, 6 Feb 2016 12:41:09 +0000
> Subject: [PATCH 1/2] sched/deadline: add per rq tracking of admitted
> bandwidth
> 
> Currently SCHED_DEADLINE scheduling policy tracks bandwidth of tasks
> that passed admission control at root_domain level only. This creates
> problems when such data structure(s) are destroyed, when we
> reconfigure scheduling domains for example.
> 
> This is part one of two changes required to fix the problem. In this
> patch we add per-rq tracking of admitted bandwidth. Tasks bring with
> them their bandwidth contribution when they enter the system and are
> enqueued for the first time. Contributions are then moved around when
> migrations happen and removed when tasks die.

I think this patch actually does two different things (addressing two
separate problems):
1) it introduces the tracking of per-rq utilization (used in your next
   patch to address the root domain issues)
2) it fixes a bug in the current utilization tracking mechanism.
   Currently, a task doing
	while(1) {
		switch to SCHED_DEADLINE
		switch to SCHED_OTHER
	}
   brings dl_b->total_bw below 0. Thanks to Juri for showing me this
   problem (and how to reproduce it) in a private email.
   This happens because when the task switches back from SCHED_DEADLINE
   to SCHED_OTHER, switched_from_dl() does not clear its deadline
   parameters (they will be cleared by the deadline timer when it
   fires). But dl_overflow() removes its utilization from
   dl_b->total_bw. When the task switches back to SCHED_DEADLINE, the
   if (new_bw == p->dl.dl_bw) check in dl_overflow() prevents
   __dl_add() from being called, and so when the task switches back to
   SCHED_OTHER dl_b->total_bw becomes negative.

> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 9503d59..0ee0ec2 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2432,7 +2432,7 @@ static int dl_overflow(struct task_struct *p,
> int policy, u64 new_bw = dl_policy(policy) ? to_ratio(period,
> runtime) : 0; int cpus, err = -1;
>  
> -	if (new_bw == p->dl.dl_bw)
> +	if (task_has_dl_policy(p) && new_bw == p->dl.dl_bw)
>  		return 0;

This hunk actually fixes issue 2) mentioned above, so I think it should
be committed in a short time (independently from the rest of the
patch). And maybe is a good candidate for backporting to stable kernels?



			Thanks,
				Luca