linux-kernel - Re: [RFC] sched/deadline: Prevent rt

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Date:	Fri, 21 Feb 2014 16:09:25 +0400
From:	Kirill Tkhai <tkhai@...dex.ru>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Juri Lelli <juri.lelli@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [RFC] sched/deadline: Prevent rt_time growth to infinity



21.02.2014, 15:39, "Kirill Tkhai" <tkhai@...dex.ru>:
> 21.02.2014, 14:37, "Peter Zijlstra" <peterz@...radead.org>:
>
>>  On Thu, Feb 20, 2014 at 02:16:00AM +0400, Kirill Tkhai wrote:
>>>   Since deadline tasks share rt bandwidth, we must care about
>>>   bandwidth timer set. Otherwise rt_time may grow up to infinity
>>>   in update_curr_dl(), if there are no other available RT tasks
>>>   on top level bandwidth.
>>>
>>>   I'm going to decide the problem the way below. Almost untested
>>>   because of I skipped almost all of recent patches which haveto be applied from lkml.
>>>
>>>   Please say, if I skipped anything in idea. Maybe better put
>>>   start_top_rt_bandwidth() into set_curr_task_dl()?
>>  How about we only increment rt_time when there's an RT bandwidth timer
>>  active?
>
> This case RT and DL may eat all the time:
>
> --------------  time ------------------>
> |RT's working |DL's working|
> ----------------------------
> |rt_runtime   |            |
> ----------------------------
> |        rt_period         |
>
> Or at least more, than it's allowed.
>
> It looks like, if we want to limit time of high priority classes
> execution, we have to set the timer anyway.

Oh, above is confusing. Sorry.

I mean one RT task and

-----------------  time -------------------------->
|DL's working    |RT's working| |DL's working    |
--------------------------------------------------
|                |rt_runtime  | |                |
--------------------------------------------------
|                |   rt_period  |                |

In this case FAIR receives less ratio, than (rt_period-rt_runtime)/rt_period.

>>  ---
>>  --- a/kernel/sched/rt.c
>>  +++ b/kernel/sched/rt.c
>>  @@ -568,6 +568,12 @@ static inline struct rt_bandwidth *sched
>
> [snipped by dumb mail client]
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