PR 1912: Add Scheduler Documentation

Add new file for CBS documentation.

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+@c
+@c  COPYRIGHT (c) 2011.
+@c  On-Line Applications Research Corporation (OAR).
+@c  All rights reserved.
+@c
+@c  $Id:$
+@c
+
+@chapter Constant Bandwidth Server Scheduler API
+
+@cindex cbs
+
+@section Introduction
+
+Unlike simple schedulers, the Constant Bandwidth Server (CBS) requires
+a special API for tasks to indicate their scheduling parameters.
+The directives provided by the CBS API are:
+
+@itemize @bullet
+@item @code{@value{DIRPREFIX}cbs_initialize} - Initialize the CBS library
+@item @code{@value{DIRPREFIX}cbs_cleanup} - Cleanup the CBS library
+@item @code{@value{DIRPREFIX}cbs_create_server} - Create a new bandwidth server
+@item @code{@value{DIRPREFIX}cbs_attach_thread} - Attach a thread to server
+@item @code{@value{DIRPREFIX}cbs_detach_thread} - Detach a thread from server
+@item @code{@value{DIRPREFIX}cbs_destroy_server} - Destroy a bandwidth server
+@item @code{@value{DIRPREFIX}cbs_get_server_id} - Get an ID of a server
+@item @code{@value{DIRPREFIX}cbs_get_parameters} - Get scheduling parameters of a server
+@item @code{@value{DIRPREFIX}cbs_set_parameters} - Set scheduling parameters of a server
+@item @code{@value{DIRPREFIX}cbs_get_execution_time} - Get elapsed execution time
+@item @code{@value{DIRPREFIX}cbs_get_remaining_budget} - Get remainig execution time
+@item @code{@value{DIRPREFIX}cbs_get_approved_budget} - Get scheduler approved execution time
+@end itemize
+
+@section Background
+
+@subsection Constant Bandwidth Server Definitions
+
+@cindex CBS parameters
+
+@findex rtems_cbs_parameters
+
+The Constant Bandwidth Server API enables tasks to communicate with
+the scheduler and indicate its scheduling parameters. The scheduler
+has to be set up first (by defining @code{CONFIGURE_SCHEDULER_CBS} macro).
+
+The difference to a plain EDF is the presence of servers.
+It is a budget aware extention of the EDF scheduler, therefore, tasks
+attached to servers behave in a similar way as with EDF unless they
+exceed their budget.
+
+The intention of servers is reservation of a certain computation
+time (budget) of the processor for all subsequent periods. The structure
+@code{rtems_cbs_parameters} determines the behavior of
+a server. It contains @code{deadline} which is equal to period,
+and @code{budget} which is the time the server is allowed to
+spend on CPU per each period. The ratio between those two parameters
+yields the maximum percentage of the CPU the server can use
+(bandwidth). Moreover, thanks to this limitation the overall
+utilization of CPU is under control, and the sum of bandwidths
+of all servers in the system yields the overall reserved portion
+of processor. The rest is still available for ordinary tasks that
+are not attached to any server.
+
+In order to make the server effective to the executing tasks,
+tasks have to be attached to the servers. The
+@code{rtems_cbs_server_id} is a type denoting an id of a server
+and @code{rtems_id} a type for id of tasks.
+
+@subsection Handling Periodic Tasks
+
+@cindex CBS periodic tasks
+
+Each task's execution begins with a default background priority
+(see the chapter Scheduling Concepts to understand the concept of
+priorities in EDF). Once you decide the tasks should start periodic
+execution, you have two possibilities. Either you use only the Rate
+Monotonic manager which takes care of periodic behavior, or you declare
+deadline and budget using the CBS API in which case these properties
+are constant for all subsequent periods, unless you change them using
+the CBS API again. Task now only has to indicate and end of
+each period using @code{rtems_rate_monotonic_period}.
+
+@subsection Registering a Callback Function
+
+@cindex CBS overrun handler
+
+In case tasks attached to servers are not aware of their execution time
+and happen to exceed it, the scheduler does not guarantee execution any
+more and pulls the priority of the task to background, which would
+possibly lead to immediate preemption (if there is at least one ready
+task with a higher pirority). However, the task is not blocked but a
+callback function is invoked. The callback function
+(@code{rtems_cbs_budget_overrun}) might be optionally registered upon
+a server creation (@code{rtems_cbs_create_server}).
+
+This enables the user to define what should happen in case of budget
+overrun. There is obviously no space for huge operations because the
+priority is down and not real time any more, however, you still can at
+least in release resources for other tasks, restart the task or log an
+error information. Since the routine is called directly from kernel,
+use @code{printk()} instead of @code{printf()}.
+
+The calling convention of the callback function is:
+@ifset is-C
+@findex rtems_asr
+@example
+void overrun_handler(
+  rtems_cbs_server_id server_id
+);
+@end example
+@end ifset
+
+@subsection Limitations
+
+@cindex CBS limitations
+
+When using this scheduler you have to keep in mind several things:
+
+@itemize @bullet
+@c it_limitations
+@item In the current implementation it is possible to attach only
+a single task to each server.
+@item If you have a task attached to a server and you voluntatily
+block it in the beginning of its execution, its priority will be
+probably pulled to background upon unblock, thus not guaranteed
+deadline any more. This is because you are effectively raising
+computation time of the task. When unbocking, you should be always
+sure that the ratio between remaining computation time and remaining
+deadline is not higher that the utilization you have agreed with the
+scheduler.
+@end itemize
+
+@section Operations
+
+@subsection Setting up a server
+
+The directive @code{rtems_cbs_create_server} is used to create a new
+server that is characterized by @code{rtems_cbs_parameters}. You also
+might want to register the @code{rtems_cbs_budget_overrun} callback
+routine. After this step tasks can be attached to the server. The directive
+@code{rtems_cbs_set_parameters} can change the scheduling parameters
+to avoid destroying and creating a new server again.
+
+@subsection Attaching Task to a Server
+
+If a task is attached to a server using @code{rtems_cbs_attach_thread},
+the task's computation time per period is limited by the server and
+the deadline (period) of task is equal to deadline of the server which
+means if you conclude a period using @code{rate_monotonic_period},
+the length of next period is always determined by the server's property.
+
+The task has a guaranteed bandwidth given by the server but should not
+exceed it, otherwise the priority is pulled to background until the
+start of next period and the @code{rtems_cbs_budget_overrun} callback
+function is invoked.
+
+When attaching a task to server, the preemptability flag of the task
+is raised, otherwise it would not be possible to control the execution
+of the task.
+
+@subsection Detaching Task from a Server
+
+The directive @code{rtems_cbs_detach_thread} is just an inverse
+operation to the previous one, the task continues its execution with
+the initial priority.
+
+Preemptability of the task is restored to the initial value.
+
+@subsection Examples
+
+The following example presents a simple common use of the API.
+
+You can see the initialization and cleanup call here, if there are
+multiple tasks in the system, it is obvious that the initialization
+should be called before creating the task.
+
+Notice also that in this case we decided to register an overrun handler,
+instead of which there could be @code{NULL}. This handler just prints
+a message to terminal, what else may be done here depends on a specific
+application.
+
+During the periodic execution, remaining budget should be watched
+to avoid overrun.
+
+@example
+void overrun_handler (
+  rtems_cbs_server_id server_id
+)
+@{
+  printk( "Budget overrun, fixing the task\n" );
+  return;
+@}
+
+rtems_task Tasks_Periodic(
+  rtems_task_argument argument
+)
+@{
+  rtems_id          rmid;
+
+  rtems_cbs_server_id server_id;
+  rtems_cbs_parameters params;
+
+  params.deadline = 10;
+  params.budget = 4;
+
+  rtems_cbs_initialize();
+
+  rtems_cbs_create_server( &params, &overrun_handler, &server_id )
+
+  rtems_cbs_attach_thread( server_id, SELF );
+
+  rtems_rate_monotonic_create( argument, &rmid );
+
+  while ( 1 ) @{
+    if (rtems_rate_monotonic_period(rmid, params.deadline)==RTEMS_TIMEOUT)
+      break;
+
+    /* Perform some periodic action */
+  @}
+
+  rtems_rate_monotonic_delete( rmid );
+
+  rtems_cbs_cleanup();
+  exit( 1 );
+@}
+@end example
+
+@section Directives
+
+This section details the Constant Bandwidth Server's directives.
+A subsection is dedicated to each of this manager's directives
+and describes the calling sequence, related constants, usage,
+and status codes.
+
+@c
+@c
+@c
+@page
+@subsection CBS_INITIALIZE - Initialize the CBS library
+
+@cindex initialize the CBS library
+
+@subheading CALLING SEQUENCE:
+
+@ifset is-C
+@findex rtems_cbs_initialize
+@example
+int rtems_cbs_initialize( void );
+@end example
+@end ifset
+
+@subheading DIRECTIVE STATUS CODES:
+@code{@value{RPREFIX}CBS_OK} - successful initialization@*
+@code{@value{RPREFIX}CBS_ERROR_NO_MEMORY} - not enough memory for data@*
+
+@subheading DESCRIPTION:
+
+This routine initializes the library in terms of allocating necessary memory
+for the servers. In case not enough memory is available in the system,
+@code{@value{RPREFIX}CBS_ERROR_NO_MEMORY} is returned, otherwise
+@code{@value{RPREFIX}CBS_OK}.
+
+@subheading NOTES:
+
+Additional memory per each server is allocated upon invocation of
+@code{rtems_cbs_create_server}.
+
+Tasks in the system are not influenced, they still keep executing
+with their initial parameters.
+
+@c
+@c
+@c
+@page
+@subsection CBS_CLEANUP - Cleanup the CBS library
+
+@cindex cleanup the CBS library
+
+@subheading CALLING SEQUENCE:
+
+@ifset is-C
+@findex rtems_cbs_cleanup
+@example
+int rtems_cbs_cleanup( void );
+@end example
+@end ifset
+
+@subheading DIRECTIVE STATUS CODES:
+@code{@value{RPREFIX}CBS_OK} - always successful
+
+@subheading DESCRIPTION:
+
+This routine detaches all tasks from their servers, destroys all servers
+and returns memory back to the system.
+
+@subheading NOTES:
+
+All tasks continue executing with their initial priorities.
+
+@c
+@c
+@c
+@page
+@subsection CBS_CREATE_SERVER - Create a new bandwidth server
+
+@cindex create a new bandwidth server
+
+@subheading CALLING SEQUENCE:
+
+@ifset is-C
+@findex rtems_cbs_create_server
+@example
+int rtems_cbs_create_server (
+  rtems_cbs_parameters     *params,
+  rtems_cbs_budget_overrun  budget_overrun_callback,
+  rtems_cbs_server_id      *server_id
+);
+@end example
+@end ifset
+
+@subheading DIRECTIVE STATUS CODES:
+@code{@value{RPREFIX}CBS_OK} - successfully created@*
+@code{@value{RPREFIX}CBS_ERROR_NO_MEMORY} - not enough memory for data@*
+@code{@value{RPREFIX}CBS_ERROR_FULL} - maximum servers exceeded@*
+@code{@value{RPREFIX}CBS_ERROR_INVALID_PARAMETER} - invalid input argument@*
+
+@subheading DESCRIPTION:
+
+This routine prepares an instance of a constant bandwidth server.
+The input parameter @code{rtems_cbs_parameters} specifies scheduling
+parameters of the server (period and budget). If these are not valid,
+@code{@value{RPREFIX}CBS_ERROR_INVALID_PARAMETER} is returned.
+The @code{budget_overrun_callback} is an optional callback function, which is
+invoked in case the server's budget within one period is exceeded.
+Output parameter @code{server_id} becomes an id of the newly created server.
+If there is not enough memory, the @code{@value{RPREFIX}CBS_ERROR_NO_MEMORY}
+is returned. If the maximum server count in the system is exceeded,
+@code{@value{RPREFIX}CBS_ERROR_FULL} is returned.
+
+@subheading NOTES:
+
+No task execution is being influenced so far.
+
+@c
+@c
+@c
+@page
+@subsection CBS_ATTACH_THREAD - Attach a thread to server
+
+@cindex attach a thread to server
+
+@subheading CALLING SEQUENCE:
+
+@ifset is-C
+@findex rtems_cbs_attach_thread
+@example
+int rtems_cbs_attach_thread (
+  rtems_cbs_server_id server_id,
+  rtems_id            task_id
+);
+@end example
+@end ifset
+
+@subheading DIRECTIVE STATUS CODES:
+@code{@value{RPREFIX}CBS_OK} - successfully attached@*
+@code{@value{RPREFIX}CBS_ERROR_FULL} - server maximum tasks exceeded@*
+@code{@value{RPREFIX}CBS_ERROR_INVALID_PARAMETER} - invalid input argument@*
+@code{@value{RPREFIX}CBS_ERROR_NOSERVER} - server is not valid@*
+
+@subheading DESCRIPTION:
+
+Attaches a task (@code{task_id}) to a server (@code{server_id}).
+The server has to be previously created. Now, the task starts
+to be scheduled according to the server parameters and not
+using initial priority. This implementation allows only one task
+per server, if the user tries to bind another task to the same
+server, @code{@value{RPREFIX}CBS_ERROR_FULL} is returned.
+
+@subheading NOTES:
+
+Tasks attached to servers become preemptible.
+
+@c
+@c
+@c
+@page
+@subsection CBS_DETACH_THREAD - Detach a thread from server
+
+@cindex detach a thread from server
+
+@subheading CALLING SEQUENCE:
+
+@ifset is-C
+@findex rtems_cbs_detach_thread
+@example
+int rtems_cbs_detach_thread (
+  rtems_cbs_server_id server_id,
+  rtems_id            task_id
+);
+@end example
+@end ifset
+
+@subheading DIRECTIVE STATUS CODES:
+@code{@value{RPREFIX}CBS_OK} - successfully detached@*
+@code{@value{RPREFIX}CBS_ERROR_INVALID_PARAMETER} - invalid input argument@*
+@code{@value{RPREFIX}CBS_ERROR_NOSERVER} - server is not valid@*
+
+@subheading DESCRIPTION:
+
+This directive detaches a thread from server. The task continues its
+execution with initial priority.
+
+@subheading NOTES:
+
+The server can be reused for any other task.
+
+@c
+@c
+@c
+@page
+@subsection CBS_DESTROY_SERVER - Destroy a bandwidth server
+
+@cindex destroy a bandwidth server
+
+@subheading CALLING SEQUENCE:
+
+@ifset is-C
+@findex rtems_cbs_destroy_server
+@example
+int rtems_cbs_destroy_server (
+  rtems_cbs_server_id server_id
+);
+@end example
+@end ifset
+
+@subheading DIRECTIVE STATUS CODES:
+@code{@value{RPREFIX}CBS_OK} - successfully destroyed@*
+@code{@value{RPREFIX}CBS_ERROR_INVALID_PARAMETER} - invalid input argument@*
+@code{@value{RPREFIX}CBS_ERROR_NOSERVER} - server is not valid@*
+
+@subheading DESCRIPTION:
+
+This directive destroys a server. If any task was attached to the server,
+the task is detached and continues its execution according to EDF rules
+with initial properties.
+
+@subheading NOTES:
+
+This again enables one more task to be created.
+
+@c
+@c
+@c
+@page
+@subsection CBS_GET_SERVER_ID - Get an ID of a server
+
+@cindex get an ID of a server
+
+@subheading CALLING SEQUENCE:
+
+@ifset is-C
+@findex rtems_cbs_get_server_id
+@example
+int rtems_cbs_get_server_id (
+  rtems_id             task_id,
+  rtems_cbs_server_id *server_id
+);
+@end example
+@end ifset
+
+@subheading DIRECTIVE STATUS CODES:
+@code{@value{RPREFIX}CBS_OK} - successful@*
+@code{@value{RPREFIX}CBS_ERROR_NOSERVER} - server is not valid@*
+
+@subheading DESCRIPTION:
+
+This directive returns an id of server belonging to a given task.
+
+@c
+@c
+@c
+@page
+@subsection CBS_GET_PARAMETERS - Get scheduling parameters of a server
+
+@cindex get scheduling parameters of a server
+
+@subheading CALLING SEQUENCE:
+
+@ifset is-C
+@findex rtems_cbs_get_parameters
+@example
+rtems_cbs_get_parameters (
+  rtems_cbs_server_id   server_id,
+  rtems_cbs_parameters *params
+);
+@end example
+@end ifset
+
+@subheading DIRECTIVE STATUS CODES:
+@code{@value{RPREFIX}CBS_OK} - successful@*
+@code{@value{RPREFIX}CBS_ERROR_INVALID_PARAMETER} - invalid input argument@*
+@code{@value{RPREFIX}CBS_ERROR_NOSERVER} - server is not valid@*
+
+@subheading DESCRIPTION:
+
+This directive returns a structure with current scheduling parameters
+of a given server (period and execution time).
+
+@subheading NOTES:
+
+It makes no difference if any task is assigned or not.
+
+@c
+@c
+@c
+@page
+@subsection CBS_SET_PARAMETERS - Set scheduling parameters
+
+@cindex set scheduling parameters
+
+@subheading CALLING SEQUENCE:
+
+@ifset is-C
+@findex rtems_cbs_set_parameters
+@example
+int rtems_cbs_set_parameters (
+  rtems_cbs_server_id   server_id,
+  rtems_cbs_parameters *params
+);
+@end example
+@end ifset
+
+@subheading DIRECTIVE STATUS CODES:
+@code{@value{RPREFIX}CBS_OK} - successful@*
+@code{@value{RPREFIX}CBS_ERROR_INVALID_PARAMETER} - invalid input argument@*
+@code{@value{RPREFIX}CBS_ERROR_NOSERVER} - server is not valid@*
+
+@subheading DESCRIPTION:
+
+This directive sets new scheduling parameters to the server. This operation
+can be performed regardless of whether a task is assigned or not.
+If a task is assigned, the parameters become effective imediately, therefore it
+is recommended to apply the change between two subsequent periods.
+
+@subheading NOTES:
+
+There is an upper limit on both period and budget equal to (2^31)-1 ticks.
+
+@c
+@c
+@c
+@page
+@subsection CBS_GET_EXECUTION_TIME - Get elapsed execution time
+
+@cindex get elapsed execution time
+
+@subheading CALLING SEQUENCE:
+
+@ifset is-C
+@findex rtems_cbs_get_execution_time
+@example
+int rtems_cbs_get_execution_time (
+  rtems_cbs_server_id    server_id,
+  time_t                *exec_time,
+  time_t                *abs_time
+);
+@end example
+@end ifset
+
+@subheading DIRECTIVE STATUS CODES:
+@code{@value{RPREFIX}CBS_OK} - successful@*
+@code{@value{RPREFIX}CBS_ERROR_INVALID_PARAMETER} - invalid input argument@*
+@code{@value{RPREFIX}CBS_ERROR_NOSERVER} - server is not valid@*
+
+@subheading DESCRIPTION:
+
+This routine returns consumed execution time (@code{exec_time}) of a server
+during the current period.
+
+@subheading NOTES:
+
+Absolute time (@code{abs_time}) not supported now.
+
+@c
+@c
+@c
+@page
+@subsection CBS_GET_REMAINING_BUDGET - Get remaining execution time
+
+@cindex get remaining execution time
+
+@subheading CALLING SEQUENCE:
+
+@ifset is-C
+@findex rtems_cbs_get_remaining_budget
+@example
+int rtems_cbs_get_remaining_budget (
+  rtems_cbs_server_id  server_id,
+  time_t              *remaining_budget
+);
+@end example
+@end ifset
+
+@subheading DIRECTIVE STATUS CODES:
+@code{@value{RPREFIX}CBS_OK} - successful@*
+@code{@value{RPREFIX}CBS_ERROR_INVALID_PARAMETER} - invalid input argument@*
+@code{@value{RPREFIX}CBS_ERROR_NOSERVER} - server is not valid@*
+
+@subheading DESCRIPTION:
+
+This directive returns remaining execution time of a given server for
+current period.
+
+@subheading NOTES:
+
+If the execution time approaches zero, the assigned task should finish
+computations of the current period.
+
+@c
+@c
+@c
+@page
+@subsection CBS_GET_APPROVED_BUDGET - Get scheduler approved execution time
+
+@cindex get scheduler approved execution time
+
+@subheading CALLING SEQUENCE:
+
+@ifset is-C
+@findex rtems_cbs_get_approved_budget
+@example
+int rtems_cbs_get_approved_budget (
+  rtems_cbs_server_id  server_id,
+  time_t              *appr_budget
+);
+@end example
+@end ifset
+
+@subheading DIRECTIVE STATUS CODES:
+@code{@value{RPREFIX}CBS_OK} - successful@*
+@code{@value{RPREFIX}CBS_ERROR_INVALID_PARAMETER} - invalid input argument@*
+@code{@value{RPREFIX}CBS_ERROR_NOSERVER} - server is not valid@*
+
+@subheading DESCRIPTION:
+
+This directive returns server's approved budget for subsequent periods.