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

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Date:   Fri, 12 May 2017 22:58:09 +0200
From:   luca abeni <luca.abeni@...tannapisa.it>
To:     Xunlei Pang <xpang@...hat.com>
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 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.



				Luca