From fd6dc8c8de4dbc7ecf8a82a597cd5b43476fc8e3 Mon Sep 17 00:00:00 2001 From: Amar Takhar Date: Sun, 17 Jan 2016 19:19:43 -0500 Subject: Split document into seperate files by section. --- c_user/constant_bandwidth_server.rst | 593 +++++++++++++++++++++++++++++++++++ 1 file changed, 593 insertions(+) create mode 100644 c_user/constant_bandwidth_server.rst (limited to 'c_user/constant_bandwidth_server.rst') diff --git a/c_user/constant_bandwidth_server.rst b/c_user/constant_bandwidth_server.rst new file mode 100644 index 0000000..6616623 --- /dev/null +++ b/c_user/constant_bandwidth_server.rst @@ -0,0 +1,593 @@ +Constant Bandwidth Server Scheduler API +####################################### + +.. index:: cbs + +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: + +- ``rtems_cbs_initialize`` - Initialize the CBS library + +- ``rtems_cbs_cleanup`` - Cleanup the CBS library + +- ``rtems_cbs_create_server`` - Create a new bandwidth server + +- ``rtems_cbs_attach_thread`` - Attach a thread to server + +- ``rtems_cbs_detach_thread`` - Detach a thread from server + +- ``rtems_cbs_destroy_server`` - Destroy a bandwidth server + +- ``rtems_cbs_get_server_id`` - Get an ID of a server + +- ``rtems_cbs_get_parameters`` - Get scheduling parameters of a server + +- ``rtems_cbs_set_parameters`` - Set scheduling parameters of a server + +- ``rtems_cbs_get_execution_time`` - Get elapsed execution time + +- ``rtems_cbs_get_remaining_budget`` - Get remainig execution time + +- ``rtems_cbs_get_approved_budget`` - Get scheduler approved execution time + +Background +========== + +Constant Bandwidth Server Definitions +------------------------------------- +.. index:: CBS parameters + +.. index:: 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 ``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``rtems_cbs_parameters`` determines the behavior of +a server. It contains ``deadline`` which is equal to period, +and ``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``rtems_cbs_server_id`` is a type denoting an id of a server +and ``rtems_id`` a type for id of tasks. + +Handling Periodic Tasks +----------------------- +.. index:: 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 ``rtems_rate_monotonic_period``. + +Registering a Callback Function +------------------------------- +.. index:: 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 +(``rtems_cbs_budget_overrun``) might be optionally registered upon +a server creation (``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 ``printk()`` instead of ``printf()``. + +The calling convention of the callback function is:.. index:: rtems_asr + +.. code:: c + + void overrun_handler( + rtems_cbs_server_id server_id + ); + +Limitations +----------- +.. index:: CBS limitations + +When using this scheduler you have to keep in mind several things: + +- it_limitations + +- In the current implementation it is possible to attach only + a single task to each server. + +- 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. + +Operations +========== + +Setting up a server +------------------- + +The directive ``rtems_cbs_create_server`` is used to create a new +server that is characterized by ``rtems_cbs_parameters``. You also +might want to register the ``rtems_cbs_budget_overrun`` callback +routine. After this step tasks can be attached to the server. The directive``rtems_cbs_set_parameters`` can change the scheduling parameters +to avoid destroying and creating a new server again. + +Attaching Task to a Server +-------------------------- + +If a task is attached to a server using ``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 ``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 ``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. + +Detaching Task from a Server +---------------------------- + +The directive ``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. + +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 ``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. +.. code:: c + + 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( ¶ms, &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 ); + } + +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. + +CBS_INITIALIZE - Initialize the CBS library +------------------------------------------- +.. index:: initialize the CBS library + +**CALLING SEQUENCE:** + +.. index:: rtems_cbs_initialize + +.. code:: c + + int rtems_cbs_initialize( void ); + +**DIRECTIVE STATUS CODES:** + +``RTEMS_CBS_OK`` - successful initialization +``RTEMS_CBS_ERROR_NO_MEMORY`` - not enough memory for data + +**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,``RTEMS_CBS_ERROR_NO_MEMORY`` is returned, otherwise``RTEMS_CBS_OK``. + +**NOTES:** + +Additional memory per each server is allocated upon invocation of``rtems_cbs_create_server``. + +Tasks in the system are not influenced, they still keep executing +with their initial parameters. + +CBS_CLEANUP - Cleanup the CBS library +------------------------------------- +.. index:: cleanup the CBS library + +**CALLING SEQUENCE:** + +.. index:: rtems_cbs_cleanup + +.. code:: c + + int rtems_cbs_cleanup( void ); + +**DIRECTIVE STATUS CODES:** + +``RTEMS_CBS_OK`` - always successful + +**DESCRIPTION:** + +This routine detaches all tasks from their servers, destroys all servers +and returns memory back to the system. + +**NOTES:** + +All tasks continue executing with their initial priorities. + +CBS_CREATE_SERVER - Create a new bandwidth server +------------------------------------------------- +.. index:: create a new bandwidth server + +**CALLING SEQUENCE:** + +.. index:: rtems_cbs_create_server + +.. code:: c + + int rtems_cbs_create_server ( + rtems_cbs_parameters \*params, + rtems_cbs_budget_overrun budget_overrun_callback, + rtems_cbs_server_id \*server_id + ); + +**DIRECTIVE STATUS CODES:** + +``RTEMS_CBS_OK`` - successfully created +``RTEMS_CBS_ERROR_NO_MEMORY`` - not enough memory for data +``RTEMS_CBS_ERROR_FULL`` - maximum servers exceeded +``RTEMS_CBS_ERROR_INVALID_PARAMETER`` - invalid input argument + +**DESCRIPTION:** + +This routine prepares an instance of a constant bandwidth server. +The input parameter ``rtems_cbs_parameters`` specifies scheduling +parameters of the server (period and budget). If these are not valid,``RTEMS_CBS_ERROR_INVALID_PARAMETER`` is returned. +The ``budget_overrun_callback`` is an optional callback function, which is +invoked in case the server’s budget within one period is exceeded. +Output parameter ``server_id`` becomes an id of the newly created server. +If there is not enough memory, the ``RTEMS_CBS_ERROR_NO_MEMORY`` +is returned. If the maximum server count in the system is exceeded,``RTEMS_CBS_ERROR_FULL`` is returned. + +**NOTES:** + +No task execution is being influenced so far. + +CBS_ATTACH_THREAD - Attach a thread to server +--------------------------------------------- +.. index:: attach a thread to server + +**CALLING SEQUENCE:** + +.. index:: rtems_cbs_attach_thread + +.. code:: c + + int rtems_cbs_attach_thread ( + rtems_cbs_server_id server_id, + rtems_id task_id + ); + +**DIRECTIVE STATUS CODES:** + +``RTEMS_CBS_OK`` - successfully attached +``RTEMS_CBS_ERROR_FULL`` - server maximum tasks exceeded +``RTEMS_CBS_ERROR_INVALID_PARAMETER`` - invalid input argument +``RTEMS_CBS_ERROR_NOSERVER`` - server is not valid + +**DESCRIPTION:** + +Attaches a task (``task_id``) to a server (``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, ``RTEMS_CBS_ERROR_FULL`` is returned. + +**NOTES:** + +Tasks attached to servers become preemptible. + +CBS_DETACH_THREAD - Detach a thread from server +----------------------------------------------- +.. index:: detach a thread from server + +**CALLING SEQUENCE:** + +.. index:: rtems_cbs_detach_thread + +.. code:: c + + int rtems_cbs_detach_thread ( + rtems_cbs_server_id server_id, + rtems_id task_id + ); + +**DIRECTIVE STATUS CODES:** + +``RTEMS_CBS_OK`` - successfully detached +``RTEMS_CBS_ERROR_INVALID_PARAMETER`` - invalid input argument +``RTEMS_CBS_ERROR_NOSERVER`` - server is not valid + +**DESCRIPTION:** + +This directive detaches a thread from server. The task continues its +execution with initial priority. + +**NOTES:** + +The server can be reused for any other task. + +CBS_DESTROY_SERVER - Destroy a bandwidth server +----------------------------------------------- +.. index:: destroy a bandwidth server + +**CALLING SEQUENCE:** + +.. index:: rtems_cbs_destroy_server + +.. code:: c + + int rtems_cbs_destroy_server ( + rtems_cbs_server_id server_id + ); + +**DIRECTIVE STATUS CODES:** + +``RTEMS_CBS_OK`` - successfully destroyed +``RTEMS_CBS_ERROR_INVALID_PARAMETER`` - invalid input argument +``RTEMS_CBS_ERROR_NOSERVER`` - server is not valid + +**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. + +**NOTES:** + +This again enables one more task to be created. + +CBS_GET_SERVER_ID - Get an ID of a server +----------------------------------------- +.. index:: get an ID of a server + +**CALLING SEQUENCE:** + +.. index:: rtems_cbs_get_server_id + +.. code:: c + + int rtems_cbs_get_server_id ( + rtems_id task_id, + rtems_cbs_server_id \*server_id + ); + +**DIRECTIVE STATUS CODES:** + +``RTEMS_CBS_OK`` - successful +``RTEMS_CBS_ERROR_NOSERVER`` - server is not valid + +**DESCRIPTION:** + +This directive returns an id of server belonging to a given task. + +CBS_GET_PARAMETERS - Get scheduling parameters of a server +---------------------------------------------------------- +.. index:: get scheduling parameters of a server + +**CALLING SEQUENCE:** + +.. index:: rtems_cbs_get_parameters + +.. code:: c + + rtems_cbs_get_parameters ( + rtems_cbs_server_id server_id, + rtems_cbs_parameters \*params + ); + +**DIRECTIVE STATUS CODES:** + +``RTEMS_CBS_OK`` - successful +``RTEMS_CBS_ERROR_INVALID_PARAMETER`` - invalid input argument +``RTEMS_CBS_ERROR_NOSERVER`` - server is not valid + +**DESCRIPTION:** + +This directive returns a structure with current scheduling parameters +of a given server (period and execution time). + +**NOTES:** + +It makes no difference if any task is assigned or not. + +CBS_SET_PARAMETERS - Set scheduling parameters +---------------------------------------------- +.. index:: set scheduling parameters + +**CALLING SEQUENCE:** + +.. index:: rtems_cbs_set_parameters + +.. code:: c + + int rtems_cbs_set_parameters ( + rtems_cbs_server_id server_id, + rtems_cbs_parameters \*params + ); + +**DIRECTIVE STATUS CODES:** + +``RTEMS_CBS_OK`` - successful +``RTEMS_CBS_ERROR_INVALID_PARAMETER`` - invalid input argument +``RTEMS_CBS_ERROR_NOSERVER`` - server is not valid + +**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. + +**NOTES:** + +There is an upper limit on both period and budget equal to (2^31)-1 ticks. + +CBS_GET_EXECUTION_TIME - Get elapsed execution time +--------------------------------------------------- +.. index:: get elapsed execution time + +**CALLING SEQUENCE:** + +.. index:: rtems_cbs_get_execution_time + +.. code:: c + + int rtems_cbs_get_execution_time ( + rtems_cbs_server_id server_id, + time_t \*exec_time, + time_t \*abs_time + ); + +**DIRECTIVE STATUS CODES:** + +``RTEMS_CBS_OK`` - successful +``RTEMS_CBS_ERROR_INVALID_PARAMETER`` - invalid input argument +``RTEMS_CBS_ERROR_NOSERVER`` - server is not valid + +**DESCRIPTION:** + +This routine returns consumed execution time (``exec_time``) of a server +during the current period. + +**NOTES:** + +Absolute time (``abs_time``) not supported now. + +CBS_GET_REMAINING_BUDGET - Get remaining execution time +------------------------------------------------------- +.. index:: get remaining execution time + +**CALLING SEQUENCE:** + +.. index:: rtems_cbs_get_remaining_budget + +.. code:: c + + int rtems_cbs_get_remaining_budget ( + rtems_cbs_server_id server_id, + time_t \*remaining_budget + ); + +**DIRECTIVE STATUS CODES:** + +``RTEMS_CBS_OK`` - successful +``RTEMS_CBS_ERROR_INVALID_PARAMETER`` - invalid input argument +``RTEMS_CBS_ERROR_NOSERVER`` - server is not valid + +**DESCRIPTION:** + +This directive returns remaining execution time of a given server for +current period. + +**NOTES:** + +If the execution time approaches zero, the assigned task should finish +computations of the current period. + +CBS_GET_APPROVED_BUDGET - Get scheduler approved execution time +--------------------------------------------------------------- +.. index:: get scheduler approved execution time + +**CALLING SEQUENCE:** + +.. index:: rtems_cbs_get_approved_budget + +.. code:: c + + int rtems_cbs_get_approved_budget ( + rtems_cbs_server_id server_id, + time_t \*appr_budget + ); + +**DIRECTIVE STATUS CODES:** + +``RTEMS_CBS_OK`` - successful +``RTEMS_CBS_ERROR_INVALID_PARAMETER`` - invalid input argument +``RTEMS_CBS_ERROR_NOSERVER`` - server is not valid + +**DESCRIPTION:** + +This directive returns server’s approved budget for subsequent periods. + +.. COMMENT: COPYRIGHT (c) 1989-2011. + +.. COMMENT: On-Line Applications Research Corporation (OAR). + +.. COMMENT: All rights reserved. + -- cgit v1.2.3