Re: [PATCH] sched/deadline: Add sched_dl documentation

From: Juri Lelli
Date: Tue Jan 21 2014 - 07:11:32 EST


I'd say that we need to re-write parts of the document, as we need more
consensus about terms and better explain to users what SCHED_DEADLINE can be
used for, and the type of guarantees it can provide. I'll try to do this with
Luca, taking into account Henrik's points.


- Juri

On 01/21/2014 12:35 PM, Luca Abeni wrote:
> On 01/21/2014 11:20 AM, Henrik Austad wrote:
>> On Mon, Jan 20, 2014 at 02:39:29PM +0100, Luca Abeni wrote:
>>> Hi all,
>>> On 01/20/2014 02:16 PM, Henrik Austad wrote:
>>> [...]
>>>>>>> + The typical -deadline task is composed of a computation phase (instance)
>>>>>>> + which is activated on a periodic or sporadic fashion. The expected (maximum)
>>>>>>> + duration of such computation is called the task's runtime; the time interval
>>>>>>> + by which each instance needs to be completed is called the task's relative
>>>>>>> + deadline. The task's absolute deadline is dynamically calculated as the
>>>>>>> + time instant a task (or, more properly) activates plus the relative
>>>>>>> + deadline.
>>>>>> activates - released?
>>>>> I'd keep (modifying a bit):
>>>>> "time instant a task activates plus the relative deadline."
>>>>> This is probably the nearest thing to what is implemented that we can say
>>>>> (without entering into the theory too much), a task that "activates" can mean
>>>>> that it is first released, enqueued, woken-up, etc.
>>>> So, if we look at release (yes, I'm avoiding activates for a little while)
>>>> as the time at the *beginning* of a new period, then, and only then should
>>>> the *absolute* deadline be computed.
>>>> If you den move on to use 'activate' as a term for when a task becomes
>>>> eligble to run, then 'release' becomes a subset of 'activate', and you
>>>> should only compute the absolute deadline at that time. Did that make
>>>> sense?
>>> I think things are a little bit complex here, because there are 2 different
>>> "deadlines" we can think about:
>>> - the "jobs deadlines" (the absolute job deadline can be computed at job
>>> arrival, as the arrival time + the relative deadline). These are generally
>>> used for performance metrics, to see if a job is respecting its timing
>>> constraints or not
>>> - the "scheduling deadlines", which are the ones used by the scheduler to
>>> schedule the tasks. These are computed at tasks' wake-up time - notice that
>>> for self-suspending jobs there can be wake-up times that are not job arrival
>>> times. And are assigned according to the rules described in the CBS paper.
>>> I think this can easily become very confusing, so I currently have no concrete
>>> proposals for improving the documentation... But I wanted to point out that
>>> things here are more complex than in the "traditional" real-time task model.
>> Traditional real-time as in the current real-time model used in Linux, or
>> traditional as in EDF terminology used by Liu & Layland?
> Sorry, by traditional real-time task model I meant the Liu & Layland one
> (which is similar to what is describe above "... a task is composed of
> a computation phase (instance) which is activated on a periodic or sporadic
> fashion"...). I just wanted to say that things in reality are more complex
> than this.
>>> Maybe a solution could be to simply describe scheduling deadlines (which are
>>> what sched_deadline uses) without going into the details of jobs' deadlines.
>> Huh?
>> We definately need a short dictionary. In fact, I'd like to have a
>> paragraph describing what deadline driven scheduling is.
>> For instance, I'm getting *Really* confused wrt to arrival time - you seem
>> to wrap several types of arrival into the same name, yet treat it
>> differently.
>> - arrival: when a job gets ready to run for the first time
>> - arrival: when a job unblocks on some resource
>> Or did I misunderstand?
> Well, my point was (but I might be wrong) that describing all of these details
> in this document might be confusing, so it might make sense to describe only
> the relevant things (leaving all of the theoretical details to other documents,
> like the papers and technical reports cited in this document).
> You are right: there are different concepts of "arrival" and "arrival time": the
> "job arrival" (when some activity - for example decoding a video frame - starts)
> and the "task wake up". But I think in this document it is not important to mention
> jobs (or task instances) and job arrivals: the only relevant things are task
> wake-ups.
>> So, the terminology I'm used to, an attempt to write up something to
>> clear up the terminology and establish common grounds. Please
>> edit/elaborate or shoot down as appropriate:
>> """
>> N. Crashcourse in deadline-terminology:
>> In a system, we typically look at a set of tasks. In Linux-kernel
>> terminology, a particular task is normally a thread. When a thread is
>> ready to run, we say that a *job* of that task is running.
> This would be true in the original Liu&Layland model (where a task blocks
> only when a job finishes), but I do not think it is correct in a real system...
> For example: (notice: this discussion might be slightly off-topic, and I do not
> think this should go in the document... I am writing just to clarify my point
> of view)
> - Let's consider a (over simplified) video decoder as an example of task
> - The task periodically read a video frame (from disk or network), decodes it,
> and displays it
> - So, each job starts when the frame is read, and finishes when the frame is
> displayed. And jobs are (in this case) activated periodically
> - During the execution of a job, the task might invoke a blocking system call,
> and block... When it wakes up, it is still in the same job (decoding the same
> video frame), and not in a different one.
> This is (IMHO) where all the confusion comes from.
>> It is
>> perhaps easiest to grasp this if one think only of periodic tasks, i.e. a
>> thread that need to run for 2ms every 10ms. Normally, we want one job to
>> finish before a new (of the same task) start, which implies that the
>> deadline for this task is also 10ms. Once this is clear, expanding one's
>> mind to aperiodic and/or sporadic tasks is easier.
>> * Periodic task: a task that needs to run for a while every N us.
>> * Sporadic task: a tasks that needs tor un for a while at most every N us
>> (jobs start no closer than N us apart)
>> * Aperiodic task: a task that have no particular period, but once
>> released, needs to complete before a given deadline.
> This is all correct, but I think it is not needed to understand how sched_deadline
> works
>> * Set of all deadline-tasks in the system: \tau
>> * One particluar task: \tau_i
>> * The j'th job of task i: \tau_{i,j}
>> * The (relative) deadline of task i: D_i
>> * The (periodic, relative) release time of task i: R_i
>> * Required execution time a tasks's job needs to complete. C_i
>> * Absolute release-time, the time when a new job is ready (when a thread is
>> woken up for a new period).
>> * The absolute deadline of a job, the actual point in time where a job
>> needs to be finished. This is what the scheduler looks at when it picks
>> the next thread to run.
> This is not completely correct... The scheduler does not use this deadline, but
> uses a "scheduling deadline". The difference is, for example, that the scheduling
> deadline is postponed when the task tries to execute for more than the runtime...
> And this is why I think (but I might be wrong) that the concepts above do not need
> to be introduced in this document.
> Notice that if runtime >= C_i and if the sched_deadline period is >= "R_i", then
> the scheduling deadlines and the absolute deadlines you mention above coincide, so
> the task is guaranteed to respect the jobs' absolute deadlines (this is what is
> called "hard schedulability property" in the CBS paper).
>> We can now construct a 3-tuple describing a perioic and sporadic tasks:
>> (C_i, R_i, D_i).
>> These 3 items is what you can use to describe your task to the scheduler.
>> """
>>> So, the document could avoid talking about instances (or jobs), and can say
>>> that a task is guaranteed to receive "runtime" time units every "period" time
>>> units (and these "runtime" time units are available within "deadline" time
>>> units from the beginning of the period). Every time the task wakes up, the
>>> scheduler computes a scheduling deadline consistent with this constraint,
>>> and tasks are scheduled using EDF on these scheduling deadlines.
>>> Every time "runtime" time units are consumed in a period, the scheduling
>>> deadline is postponed by a period.
>> What is wrong with using the CORRECT TERMINOLOGY?
> Well, what I mentioned above could be "correct terminology" :).
> I just avoided talking about the two different kinds of deadlines
> (jobs' deadlines and scheduling deadlines), because this could be confusing.
> And about jobs, in order to avoid talking about self-suspending jobs, and
> having to make a distinction about job arrivals and other task wake ups...
>> People looking at using
>> sched_deadline _need_ to understand what a _deadline_ scheduler is.
> I think the sentence I wrote above gives an idea about what sched_deadline does,
> and how to use the three parameters. I can be extended by clarifying that "using
> EDF on these scheduling deadlines" means "scheduling the task with the smallest
> scheduling deadline".
>> If the only goal of sched_deadline is to act as a bandwidth-gauge, then
>> fine, but *I* want to use sched_deadline for systems that HAVE DEADLINES.
> Ok, so this is a more advanced usage. A section about the "hard schedulability
> property" mentioned above can be added (later in the document, I think) to explain
> how sched_deadline can be used to provide guarantees to real-time tasks (so,
> the real-time task model you mentioned can be described later, and the condition
> I mentioned above can be explained).
> My point is: first explain what sched_deadline is and how it works (something about
> my paragraph above), then, later explain what a real-time task is and how sched_deadline
> can be used to properly schedule it.
>> I do NOT want to mess around with mapping deadlines to priorities in order to
>> meet my deadlines.
> I agree, this is not needed.
>> I suspect others would like to use sched_deadline for the same.
> Again, I might be wrong... But once the "runtime time units every period, served
> within a specified deadline" idea is clear, it is easier to understand how to use
> sched_deadline.
>>> This is of course an approximative description, but I think it can give an
>>> intuitive idea of how the scheduler works, and about the meaning of the three
>>> scheduling parameters.
>> Look, I'm not in favour of turning sched_deadline.txt into an academic
>> paper, but it is clear that we need to establish a baseline here, and then
>> we need to include tasks, instances/jobs, deadline, arrival/release and so
>> on.
> See above... IMHO, this is not needed to understand how sched_deadline works.
> But once the sched_deadline behaviour is defined, these concepts can be introduced
> and it can be explained how to use sched_deadline to respect the jobs' deadlines...
> Luca
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