linux-kernel - Re: [PATCH v2 2/3] sched/deadline: Throttle the task when missing its deadline

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Date:   Mon, 15 May 2017 11:50:17 +0800
From:   Xunlei Pang <xpang@...hat.com>
To:     luca abeni <luca.abeni@...tannapisa.it>
Cc:     Xunlei Pang <xlpang@...hat.com>, linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Luca Abeni <luca.abeni@...tn.it>
Subject: Re: [PATCH v2 2/3] sched/deadline: Throttle the task when missing its
 deadline

On 05/13/2017 at 04:58 AM, luca abeni wrote:
> On Fri, 12 May 2017 15:19:55 +0800
> Xunlei Pang <xpang@...hat.com> wrote:
> [...]
>>>>  "As seen, enforcing that the total utilization is smaller than M
>>>> does not guarantee that global EDF schedules the tasks without
>>>> missing any deadline (in other words, global EDF is not an optimal
>>>> scheduling algorithm). However, a total utilization smaller than M
>>>> is enough to guarantee that non real-time tasks are not starved
>>>> and that the tardiness of real-time tasks has an upper bound[12]
>>>> (as previously noted). Different bounds on the maximum tardiness
>>>> experienced by real-time tasks have been developed in various
>>>> papers[13,14], but the theoretical result that is important for
>>>> SCHED_DEADLINE is that if the total utilization is smaller or equal
>>>> than M then the response times of the tasks are limited."
>>>>
>>>> Do you mean there is some tardiness allowed in theory(global EDF is
>>>> not an optimal scheduling algorithm), thus missed deadline is
>>>> allowed for global EDF?  
>>> Right.
>>>
>>> With the admission test currently used by the kernel (sum of
>>> utilizations <= 1), tasks are guaranteed to have a tardiness smaller
>>> than a theoretical maximum... But this theoretical maximum can be
>>> larger than 0.
>>>
>>> If you want to strictly respect all of the deadlines, you need a
>>> stricter admission test (for example, the one based on WCET_max
>>> that is mentioned above).  
>> Understood.
>>
>> I think in Patch 3, it is still worthy to add the accounting in
>> dl_runtime_exceeded(), to track the dl scheduling tardiness(after all
>> tardiness is not a good thing) like: if
>> (dl_time_before(dl_se->deadline, rq_clock(rq)) && dl_se->runtime > 0)
>> ++dl_se->nr_underrun_sched;
>>
>> Maybe changing the name to use "nr_underrun_tardy" is better, large
>> value does need our attention. What do you think?
> I do not know, I never used statistics like these...
>
> If there are enough people having a good usecase for these statistics,
> it might be worth adding them, but I do not know other people's
> opinions about this.
>

Hi Luca,

Thanks for the feedback.

I think I can defer the statistics patch after Daniel's "sched/deadline: Use the
revised wakeup rule for suspending constrained dl tasks", since there will
be another underrun case in the fix, let's wait for other's opinions then.

Regards,
Xunlei