sched: add sched_dl documentation.

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@linux.it>
Signed-off-by: Juri Lelli <juri.lelli@gmail.com>

Documentation/scheduler/sched-deadline.txt

@@ -0,0 +1,146 @@
+			Deadline Task and Group Scheduling
+			----------------------------------
+
+CONTENTS
+========
+
+0. WARNING
+1. Overview
+2. Task scheduling
+2. The Interface
+3. Bandwidth management
+  3.1 System-wide settings
+  2.2 Task interface
+  2.4 Default behavior
+3. 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 will be made up 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 (better, an instance) activates plus the relative
+ deadline.
+
+ The EDF algorithm selects the task with the smallest absolute deadline as
+ the one to be executed first, while the CBS 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.
+
+
+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 per-group controls that are added to the cgroupfs virtual file system are:
+  * /proc/sys/kernel/sched_dl_runtime_us,
+  * /proc/sys/kernel/sched_dl_period_us,
+
+ They accept (if written) and provides (if read) the new runtime and period,
+ respectively, for each CPU in each root_domain.
+
+ This means that, for a root_domain comprising M CPUs, -deadline tasks
+ can be created until the sum of their bandwidths stay below:
+
+   M * (sched_dl_runtime_us / sched_dl_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_dl_runtime_us.
+
+
+2.2 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.
+
+
+2.4 Default behavior
+---------------------
+
+The default values for SCHED_DEADLINE bandwidth is to have dl_runtime and
+dl_period equal to 500000 and 1000000, respectively. This means -deadline
+tasks can use at most 5%, multiplied by the number of CPUs that compose the
+root_domain, for each root_domain.
+
+When a -deadline task fork a child, its dl_runtime is set to 0, which means
+someone must call sched_setscheduler2() on it, or it won't even start.
+
+
+3. 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
+   hopefully we can have some demonstrative code soon.