Re: [PATCH 14/14] sched: add sched_dl documentation.

[Randy Dunlap]

Hi,

Just a few minor edits...


On 11/07/13 05:47, Juri Lelli wrote:
> From: Dario Faggioli <raistlin@xxxxxxxx>
> 
> 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.
> 
> Signed-off-by: Dario Faggioli <raistlin@xxxxxxxx>
> Signed-off-by: Juri Lelli <juri.lelli@xxxxxxxxx>
> ---
>  Documentation/scheduler/sched-deadline.txt |  196 ++++++++++++++++++++++++++++
>  kernel/sched/deadline.c                    |    3 +-
>  2 files changed, 198 insertions(+), 1 deletion(-)
>  create mode 100644 Documentation/scheduler/sched-deadline.txt
> 
> diff --git a/Documentation/scheduler/sched-deadline.txt b/Documentation/scheduler/sched-deadline.txt
> new file mode 100644
> index 0000000..4d1ed52
> --- /dev/null
> +++ b/Documentation/scheduler/sched-deadline.txt
> @@ -0,0 +1,196 @@
> +			  Deadline Task Scheduling
> +			  ------------------------
> +
> +CONTENTS
> +========
> +
> +0. WARNING
> +1. Overview
> +2. Task scheduling
> +2. The Interface
> +3. Bandwidth management
> +  3.1 System-wide settings
> +  3.2 Task interface
> +  3.4 Default behavior
> +4. Tasks CPU affinity
> +  4.1 SCHED_DEADLINE and cpusets HOWTO
> +5. 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. Task scheduling
> +==================
> +
> + 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.
> +
> + The EDF[1] algorithm selects the task with the smallest absolute deadline as
> + the one to be executed first, while the CBS[2,3] ensures that each task runs
> + for at most its runtime every period, avoiding any interference between
> + different tasks (bandwidth isolation).
> + Thanks to this feature, also tasks that do not strictly comply with the
> + computational model described above can effectively use the new policy.
> + IOW, 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 tasks that need guarantees on their
> + timing behavior, e.g., multimedia, streaming, control applications, etc.
> +
> + 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
> +
> +3. 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 and task groups, i.e., new controls but with
> + similar names, equivalent meaning and with the same usage paradigm are added.
> +
> + 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.
> +
> +3.1 System wide settings
> +------------------------
> +
> + The system wide settings are configured under the /proc virtual file system.
> +
> + The control knob that is added to the /proc virtual file system is
> + /proc/sys/kernel/sched_dl_runtime_us. It accepts (if written) and provides (if
> + read) the new runtime for each CPU in each root_domain. The period control knob
> + is instead shared with -rt settings (/proc/sys/kernel/sched_rt_period_us). 
> +
> + The CPU bandwidth available to -deadline tasks is actually a sub-quota of
> + the -rt bandwidth. By default 95% of system bandwidth is allocate to -rt tasks;
> + among this, a 40% quota is reserved for -dl tasks. To have the actual quota a

s/among/within/

> + simple multiplication is needed: .95 * .40 = .38 (38% of system bandwidth for
> + deadline tasks).
> +
> + This means that, for a root_domain comprising M CPUs, -deadline tasks
> + can be created until the sum of their bandwidths stay below:

                   while                             stays

> +
> +   M * (sched_dl_runtime_us * rt_bw)
> +
> + 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_dl_runtime_us or
> + in /proc/sys/kernel/sched_rt_runtime_us.
> +
> +
> +3.2 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_param2, containing all the necessary fields is
> +    provided;
> +  * the new scheduling related syscalls that manipulate it, i.e.,
> +    sched_setscheduler2(), sched_setparam2() and sched_getparam2()
> +    are implemented.
> +
> +
> +3.3 Default behavior
> +---------------------
> +
> +The default value for SCHED_DEADLINE bandwidth is to have dl_runtime equal to
> +40000. Being rt_period equal to 1000000, by default, it means that -deadline

          With rt_period equal to 1000000,

> +tasks can use at most 40%, multiplied by the number of CPUs that compose the
> +root_domain, for each root_domain.
> +
> +A -deadline task cannot fork.
> +
> +4. 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).
> +
> +4.1 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)
> +
> +5. 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 point of views, and

                                                        points of view,

> +   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.


-- 
~Randy
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