linux-kernel - Re: [PATCH 2/6] sched/dl: fix yield task artificial overrun

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Date:	Thu, 30 Oct 2014 06:49:51 +0800
From:	Wanpeng Li <kernellwp@...il.com>
To:	Juri Lelli <juri.lelli@....com>,
	Wanpeng Li <wanpeng.li@...ux.intel.com>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/6] sched/dl: fix yield task artificial overrun

Hi Juri,
2014/10/30 0:33, Juri Lelli:
> Hi,
>
> On 27/10/14 01:41, Wanpeng Li wrote:
>> The yield semantic of deadline class is to reduce remaining runtime to
>> zero, and then update_curr_dl() will stop it. However, comsumed bandwidth
>> is reduced from the budget of yield task again even if it has already been
>> set to zero which leads to artificial overrun. This patch fix it by reduce
>> remaining runtime to zero if there is still remaining runtime after comsumed
>> bandwidth is accumulated.
>>
> Oh, right. But, how about what below instead (with a proper comment
> and changelog)?
>
> Thanks,
>
> - Juri
>
>  From 108ecdff52b154ea2c79d4aac552ddf1ead871c7 Mon Sep 17 00:00:00 2001
> From: Juri Lelli <juri.lelli@....com>
> Date: Wed, 29 Oct 2014 16:09:06 +0000
> Subject: [PATCH] sched/deadline: fix artificial overrun introduced by
>   yield_task_dl
>
> Signed-off-by: Juri Lelli <juri.lelli@....com>
> ---
>   kernel/sched/deadline.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index 2e31a30..db6ad38 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -633,7 +633,7 @@ static void update_curr_dl(struct rq *rq)
>   
>   	sched_rt_avg_update(rq, delta_exec);
>   
> -	dl_se->runtime -= delta_exec;
> +	dl_se->runtime -= dl_se->dl_yielded ? 0 : delta_exec;

+	if (dl_se->dl_yielded && dl_se->runtime > 0)
+		dl_se->runtime = 0;

Maybe this can be moved to update_curr_dl().

I think the consumed bandwidth still should be reduced from remaining 
runtime even if yield, then the remaining runtime will be reset to 0 if 
there is still remaining runtime as what my patch do. What's your option?

Regards,
Wanpeng Li

>   	if (dl_runtime_exceeded(rq, dl_se)) {
>   		__dequeue_task_dl(rq, curr, 0);
>   		if (likely(start_dl_timer(dl_se, curr->dl.dl_boosted)))

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