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

[Xunlei Pang <xpang@redhat.com>]

On 05/12/2017 at 03:01 PM, luca abeni wrote:
> Hi,
>
> On Fri, 12 May 2017 14:53:33 +0800
> Xunlei Pang <xpang@redhat.com> wrote:
>
>> On 05/12/2017 at 01:57 PM, luca abeni wrote:
>>> Hi again,
>>>
>>> (sorry for the previous email; I replied from gmail and I did not
>>> realize I was sending it in html).
>>>
>>>
>>> On Fri, 12 May 2017 11:32:08 +0800
>>> Xunlei Pang <xlpang@redhat.com> wrote:
>>>  
>>>> dl_runtime_exceeded() only checks negative runtime, actually
>>>> when the current deadline past, we should start a new period
>>>> and zero out the remaining runtime as well.  
>>> In this case, I think global EDF wants to allow the task to run with
>>> its remaining runtime even also missing a deadline, so I think this
>>> change is not correct.
>>> (when using global EDF, tasks scheduled on multiple CPUs can miss
>>> their deadlines... Setting the runtime to 0 as soon as a deadline
>>> is missed would break global EDF scheduling)  
>> Hi Luca,
>>
>> Thanks for the comment, looks like I neglected the theoretical
>> analysis.
>>
>> Cited from Documentation/scheduler/sched-deadline.txt:
>>  "As a matter of fact, in this case it is possible to provide an
>> upper bound for tardiness (defined as the maximum between 0 and the
>> difference between the finishing time of a job and its absolute
>> deadline). More precisely, it can be proven that using a global EDF
>> scheduler the maximum tardiness of each task is smaller or equal than
>>     ((M − 1) · WCET_max − WCET_min)/(M − (M − 2) · U_max) + WCET_m
>>  where WCET_max = max{WCET_i} is the maximum WCET,
>> WCET_min=min{WCET_i} is the minimum WCET, and U_max = max{WCET_i/P_i}
>> is the maximum utilization[12]."
>>
>> And
>>  "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?

Regards,
Xunlei