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 09:01:58 +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

Hi,

On Fri, 12 May 2017 14:53:33 +0800
Xunlei Pang <xpang@...hat.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@...hat.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).



				Luca