On Mon, Jan 27, 2014 at 12:20:15PM +0100, Juri Lelli wrote: > From: Dario Faggioli <[email protected]> > > Add in Documentation/scheduler/ some hints about the design > choices, the usage and the future possible developments of the > sched_dl scheduling class and of the SCHED_DEADLINE policy. > > Cc: [email protected] > Cc: [email protected] > Cc: [email protected] > Cc: [email protected] > Cc: [email protected] > Cc: [email protected] > Cc: [email protected] > Cc: [email protected] > Cc: [email protected] > Cc: [email protected] > Cc: [email protected] > Cc: [email protected] > Cc: [email protected] > Cc: [email protected] > Cc: [email protected] > Cc: [email protected] > Cc: [email protected] > Cc: [email protected] > Cc: [email protected] > Cc: [email protected] > Cc: [email protected] > Cc: [email protected] > Cc: [email protected] > Signed-off-by: Dario Faggioli <[email protected]> > Signed-off-by: Juri Lelli <[email protected]> > Signed-off-by: Peter Zijlstra <[email protected]> > [ Re-wrote sections 2 and 3. ] > Signed-off-by: Luca Abeni <[email protected]> > --- > Documentation/scheduler/00-INDEX | 2 + > Documentation/scheduler/sched-deadline.txt | 281 > ++++++++++++++++++++++++++++ > kernel/sched/deadline.c | 3 +- > 3 files changed, 285 insertions(+), 1 deletion(-) > create mode 100644 Documentation/scheduler/sched-deadline.txt > > diff --git a/Documentation/scheduler/00-INDEX > b/Documentation/scheduler/00-INDEX > index d2651c4..46702e4 100644 > --- a/Documentation/scheduler/00-INDEX > +++ b/Documentation/scheduler/00-INDEX > @@ -10,5 +10,7 @@ sched-nice-design.txt > - How and why the scheduler's nice levels are implemented. > sched-rt-group.txt > - real-time group scheduling. > +sched-deadline.txt > + - deadline scheduling. > sched-stats.txt > - information on schedstats (Linux Scheduler Statistics). > diff --git a/Documentation/scheduler/sched-deadline.txt > b/Documentation/scheduler/sched-deadline.txt > new file mode 100644 > index 0000000..18adc92 > --- /dev/null > +++ b/Documentation/scheduler/sched-deadline.txt > @@ -0,0 +1,281 @@ > + Deadline Task Scheduling > + ------------------------ > + > +CONTENTS > +======== > + > + 0. WARNING > + 1. Overview > + 2. Scheduling algorithm > + 3. Scheduling Real-Time Tasks > + 4. Bandwidth management > + 4.1 System-wide settings > + 4.2 Task interface > + 4.3 Default behavior > + 5. Tasks CPU affinity > + 5.1 SCHED_DEADLINE and cpusets HOWTO > + 6. Future plans > + > + > +0. WARNING > +========== > + > + Fiddling with these settings can result in an unpredictable or even unstable > + system behavior. As for -rt (group) scheduling, it is assumed that root > users > + know what they're doing. > + > + > +1. Overview > +=========== > + > + The SCHED_DEADLINE policy contained inside the sched_dl scheduling class is > + basically an implementation of the Earliest Deadline First (EDF) scheduling > + algorithm, augmented with a mechanism (called Constant Bandwidth Server, > CBS) > + that makes it possible to isolate the behavior of tasks between each other. > + > + > +2. Scheduling algorithm > +================== > + > + SCHED_DEADLINE uses three parameters, named "runtime", "period", and > + "deadline" to schedule tasks. A SCHED_DEADLINE task is guaranteed to receive > + "runtime" microseconds of execution time every "period" microseconds, and > + these "runtime" microseconds are available within "deadline" microseconds > + from the beginning of the period. In order to implement this behaviour, > + every time the task wakes up, the scheduler computes a "scheduling deadline" > + consistent with the guarantee (using the CBS[2,3] algorithm). Tasks are then > + scheduled using EDF[1] on these scheduling deadlines (the task with the > + smallest scheduling deadline is selected for execution). Notice that this > + guaranteed is respected if a proper "admission control" strategy (see > Section > + "4. Bandwidth management") is used. > + > + Summing up, the CBS[2,3] algorithms assigns scheduling deadlines to tasks so > + that each task runs for at most its runtime every period, avoiding any > + interference between different tasks (bandwidth isolation), while the EDF[1] > + algorithm selects the task with the smallest scheduling deadline as the one > + to be executed first. Thanks to this feature, also tasks that do not > + strictly comply with the "traditional" real-time task model (see Section 3) > + can effectively use the new policy. > + > + In more details, the CBS algorithm assigns scheduling deadlines to > + tasks in the following way: > + > + - Each SCHED_DEADLINE task is characterised by the "runtime", > + "deadline", and "period" parameters; > + > + - The state of the task is described by a "scheduling deadline", and > + a "current runtime". These two parameters are initially set to 0; > + > + - When a SCHED_DEADLINE task wakes up (becomes ready for execution), > + the scheduler checks if > + > + current runtime runtime > + ---------------------------------- > ---------------- > + scheduling deadline - current time period > + > + then, if the scheduling deadline is smaller than the current time, or > + this condition is verified, the scheduling deadline and the > + current budget are re-initialised as
Current runtime: time spent running _this_ period? or is _remaining_
runtime this period? I get the feeling it's the latter.
So, roughly, it is the ration
remaining_runtime / relative_time_to_deadline
which needs to be greater than the assigned CPU bandwidth, and if so, the
budget should be replensihed?
Shouldn't there be something about not refilling the budget before a new
period has started?
> + scheduling deadline = current time + deadline
> + current runtime = runtime
> +
> + otherwise, the scheduling deadline and the current runtime are
> + left unchanged;
> +
> + - When a SCHED_DEADLINE task executes for an amount of time t, its
> + current runtime is decreased as
> +
> + current runtime = current runtime - t
> +
> + (technically, the runtime is decreased at every tick, or when the
> + task is descheduled / preempted);
Aha, there it is. Having it here makes sense, but it does wrapping ones
head around this a bit harder than strictly necessary perhaps.
> + - When the current runtime becomes less or equal than 0, the task is
> + said to be "throttled" (also known as "depleted" in real-time literature)
> + and cannot be scheduled until its scheduling deadline. The "replenishment
> + time" for this task (see next item) is set to be equal to the current
> + value of the scheduling deadline;
> +
> + - When the current time is equal to the replenishment time of a
> + throttled task, the scheduling deadline and the current runtime are
> + updated as
> +
> + scheduling deadline = scheduling deadline + period
> + current runtime = current runtime + runtime
ok, this section makes sense now
> +3. Scheduling Real-Time Tasks
> +=============================
> +
> + * BIG FAT WARNING ******************************************************
> + *
> + * This section contains a (not-thorough) summary on classical deadline
> + * scheduling theory, and how it applies to SCHED_DEADLINE.
"This section should not be considered a complete summary of classical
deadline scheduling theroy in any way AT ALL."
(not-thorough sounds a bit strange)
> + * The reader can "safely" skip to Section 4 if only interested in seeing
> + * how the scheduling policy can be used. Anyway, we strongly recommend
> + * to come back here and continue reading (once the urge for testing is
> + * satisfied :P) to be sure of fully understanding all technical details.
> + ************************************************************************
> +
> + There are no limitations on what kind of task can exploit this new
> + scheduling discipline, even if it must be said that it is particularly
> + suited for periodic or sporadic real-time tasks that need guarantees on
> their
> + timing behavior, e.g., multimedia, streaming, control applications, etc.
> +
> + A typical real-time task is composed of a repetition of computation phases
> + (task instances, or jobs) which are activated on a periodic or sporadic
> + fashion.
> + Each job J_j (where J_j is the j^th job of the task) is characterised by an
> + arrival time r_j (the time when the job starts), an amount of computation
> + time c_j needed to finish the job, and a job absolute deadline d_j, which
> + is the time within which the job should be finished. The maximum execution
> + time max_j{c_j} is called "Worst Case Execution Time" (WCET) for the task.
> + A real-time task can be periodic with period P if r_{j+1} = r_j + P, or
> + sporadic with minimum inter-arrival time P is r_{j+1} >= r_j + P. Finally,
> + d_j = r_j + D, where D is the task's relative deadline.
\o/
Great, thanks!
> + SCHED_DEADLINE can be used to schedule real-time tasks guaranteeing that
> + the jobs' deadlines of a task are respected. In order to do this, a task
> + must be scheduled by setting:
> +
> + - runtime >= WCET
> + - deadline = D
> + - period <= P
> +
> + IOW, if runtime >= WCET and if period is >= P, then the scheduling deadlines
> + and the absolute deadlines (d_j) coincide, so a proper admission control
> + allows to respect the jobs' absolute deadlines for this task (this is what
> is
> + called "hard schedulability property" and is an extension of Lemma 1 of
> [2]).
> +
> + References:
> + 1 - C. L. Liu and J. W. Layland. Scheduling algorithms for multiprogram-
> + ming in a hard-real-time environment. Journal of the Association for
> + Computing Machinery, 20(1), 1973.
> + 2 - L. Abeni , G. Buttazzo. Integrating Multimedia Applications in Hard
> + Real-Time Systems. Proceedings of the 19th IEEE Real-time Systems
> + Symposium, 1998.
> http://retis.sssup.it/~giorgio/paps/1998/rtss98-cbs.pdf
> + 3 - L. Abeni. Server Mechanisms for Multimedia Applications. ReTiS Lab
> + Technical Report. http://xoomer.virgilio.it/lucabe72/pubs/tr-98-01.ps
> +
> +4. Bandwidth management
> +=======================
> +
> + In order for the -deadline scheduling to be effective and useful, it is
> + important to have some method to keep the allocation of the available CPU
> + bandwidth to the tasks under control.
> + This is usually called "admission control" and if it is not performed at
> all,
> + no guarantee can be given on the actual scheduling of the -deadline tasks.
> +
> + Since when RT-throttling has been introduced each task group has a bandwidth
> + associated, calculated as a certain amount of runtime over a period.
> + Moreover, to make it possible to manipulate such bandwidth,
> readable/writable
> + controls have been added to both procfs (for system wide settings) and
> cgroupfs
> + (for per-group settings).
> + Therefore, the same interface is being used for controlling the bandwidth
> + distrubution to -deadline tasks.
> +
> + However, more discussion is needed in order to figure out how we want to
> manage
> + SCHED_DEADLINE bandwidth at the task group level. Therefore, SCHED_DEADLINE
> + uses (for now) a less sophisticated, but actually very sensible, mechanism
> to
> + ensure that a certain utilization cap is not overcome per each root_domain.
> +
> + Another main difference between deadline bandwidth management and
> RT-throttling
> + is that -deadline tasks have bandwidth on their own (while -rt ones don't!),
> + and thus we don't need an higher level throttling mechanism to enforce the
> + desired bandwidth.
> +
> +4.1 System wide settings
> +------------------------
> +
> + The system wide settings are configured under the /proc virtual file system.
> +
> + For now the -rt knobs are used for dl admission control and the -deadline
> + runtime is accounted against the -rt runtime. We realise that this isn't
> + entirely desirable; however, it is better to have a small interface for now,
> + and be able to change it easily later. The ideal situation (see 5.) is to
> run
> + -rt tasks from a -deadline server; in which case the -rt bandwidth is a
> direct
> + subset of dl_bw.
> +
> + This means that, for a root_domain comprising M CPUs, -deadline tasks
> + can be created while the sum of their bandwidths stays below:
> +
> + M * (sched_rt_runtime_us / sched_rt_period_us)
> +
> + It is also possible to disable this bandwidth management logic, and
> + be thus free of oversubscribing the system up to any arbitrary level.
> + This is done by writing -1 in /proc/sys/kernel/sched_rt_runtime_us.
> +
> +
> +4.2 Task interface
> +------------------
> +
> + Specifying a periodic/sporadic task that executes for a given amount of
> + runtime at each instance, and that is scheduled according to the urgency of
> + its own timing constraints needs, in general, a way of declaring:
> + - a (maximum/typical) instance execution time,
> + - a minimum interval between consecutive instances,
> + - a time constraint by which each instance must be completed.
> +
> + Therefore:
> + * a new struct sched_attr, containing all the necessary fields is
> + provided;
> + * the new scheduling related syscalls that manipulate it, i.e.,
> + sched_setattr() and sched_getattr() are implemented.
> +
> +
> +4.3 Default behavior
> +---------------------
> +
> + The default value for SCHED_DEADLINE bandwidth is to have rt_runtime equal
> to
> + 950000. With rt_period equal to 1000000, by default, it means that -deadline
> + tasks can use at most 95%, multiplied by the number of CPUs that compose the
> + root_domain, for each root_domain.
> +
> + A -deadline task cannot fork.
> +
> +5. Tasks CPU affinity
> +=====================
> +
> + -deadline tasks cannot have an affinity mask smaller that the entire
> + root_domain they are created on. However, affinities can be specified
> + through the cpuset facility (Documentation/cgroups/cpusets.txt).
> +
> +5.1 SCHED_DEADLINE and cpusets HOWTO
> +------------------------------------
> +
> + An example of a simple configuration (pin a -deadline task to CPU0)
> + follows (rt-app is used to create a -deadline task).
> +
> + mkdir /dev/cpuset
> + mount -t cgroup -o cpuset cpuset /dev/cpuset
> + cd /dev/cpuset
> + mkdir cpu0
> + echo 0 > cpu0/cpuset.cpus
> + echo 0 > cpu0/cpuset.mems
> + echo 1 > cpuset.cpu_exclusive
> + echo 0 > cpuset.sched_load_balance
> + echo 1 > cpu0/cpuset.cpu_exclusive
> + echo 1 > cpu0/cpuset.mem_exclusive
> + echo $$ > cpu0/tasks
> + rt-app -t 100000:10000:d:0 -D5 (it is now actually superfluous to specify
> + task affinity)
> +
> +6. Future plans
> +===============
> +
> + Still missing:
> +
> + - refinements to deadline inheritance, especially regarding the possibility
> + of retaining bandwidth isolation among non-interacting tasks. This is
> + being studied from both theoretical and practical points of view, and
> + hopefully we should be able to produce some demonstrative code soon;
> + - (c)group based bandwidth management, and maybe scheduling;
> + - access control for non-root users (and related security concerns to
> + address), which is the best way to allow unprivileged use of the
> mechanisms
> + and how to prevent non-root users "cheat" the system?
> +
> + As already discussed, we are planning also to merge this work with the EDF
> + throttling patches [https://lkml.org/lkml/2010/2/23/239] but we still are in
> + the preliminary phases of the merge and we really seek feedback that would
> + help us decide on the direction it should take.
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index 0de2482..0dd5e09 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -351,7 +351,8 @@ static void replenish_dl_entity(struct sched_dl_entity
> *dl_se,
> * disrupting the schedulability of the system. Otherwise, we should
> * refill the runtime and set the deadline a period in the future,
> * because keeping the current (absolute) deadline of the task would
> - * result in breaking guarantees promised to other tasks.
> + * result in breaking guarantees promised to other tasks (refer to
> + * Documentation/scheduler/sched-deadline.txt for more informations).
> *
> * This function returns true if:
> *
> --
> 1.7.9.5
>
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Nice! /me is very happy
Reviewed-by: Henrik Austad <[email protected]>
--
Henrik Austad
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