linux-kernel - Re: [RFC v2 2/7] Correctly track the active utilisation for migrating tasks

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Message-ID: <20160405122425.GV3430@twins.programming.kicks-ass.net>
Date:	Tue, 5 Apr 2016 14:24:25 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Luca Abeni <luca.abeni@...tn.it>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Juri Lelli <juri.lelli@....com>
Subject: Re: [RFC v2 2/7] Correctly track the active utilisation for
 migrating tasks

On Fri, Apr 01, 2016 at 05:12:28PM +0200, Luca Abeni wrote:
> Fix active utilisation accounting on migration: when a task is migrated
> from CPUi to CPUj, immediately subtract the task's utilisation from
> CPUi and add it to CPUj. This mechanism is implemented by modifying the
> pull and push functions.
> 
> Note: this is not fully correct from the theoretical point of view
> (the utilisation should be removed from CPUi only at the 0 lag time),
> but doing the right thing would be _MUCH_ more complex (leaving the
> timer armed when the task is on a different CPU... Inactive timers should
> be moved from per-task timers to per-runqueue lists of timers! Bah...)
> 
> Signed-off-by: Luca Abeni <luca.abeni@...tn.it>
> ---
>  kernel/sched/deadline.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index 3c64ebf..05cfccb 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -1530,7 +1530,9 @@ retry:
>  	}
>  
>  	deactivate_task(rq, next_task, 0);
> +	sub_running_bw(&next_task->dl, &rq->dl);
>  	set_task_cpu(next_task, later_rq->cpu);
> +	add_running_bw(&next_task->dl, &later_rq->dl);
>  	activate_task(later_rq, next_task, 0);
>  	ret = 1;
>  
> @@ -1618,7 +1620,9 @@ static void pull_dl_task(struct rq *this_rq)
>  			resched = true;
>  
>  			deactivate_task(src_rq, p, 0);
> +			sub_running_bw(&p->dl, &src_rq->dl);
>  			set_task_cpu(p, this_cpu);
> +			add_running_bw(&p->dl, &this_rq->dl);
>  			activate_task(this_rq, p, 0);
>  			dmin = p->dl.deadline;
>  

Are these the only places a DL task might be migrated from? In
particular I worry about the case where we assign an existing DL task to
a different cpuset.

Juri?