From 72a62ad88f82fe1ffee50024db4dd0f3fa5806f7 Mon Sep 17 00:00:00 2001 From: Chris Johns Date: Thu, 3 Nov 2016 16:58:08 +1100 Subject: Rename all manuals with an _ to have a -. It helps released naming of files. --- c-user/constant_bandwidth_server.rst | 686 +++++++++++++++++++++++++++++++++++ 1 file changed, 686 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..135dfdb --- /dev/null +++ b/c-user/constant_bandwidth_server.rst @@ -0,0 +1,686 @@ +.. comment SPDX-License-Identifier: CC-BY-SA-4.0 + +.. COMMENT: COPYRIGHT (c) 1989-2011. +.. COMMENT: On-Line Applications Research Corporation (OAR). +.. COMMENT: All rights reserved. + +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-block:: 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-block:: 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. + +.. _rtems_cbs_initialize: + +CBS_INITIALIZE - Initialize the CBS library +------------------------------------------- +.. index:: initialize the CBS library + +**CALLING SEQUENCE:** + +.. index:: rtems_cbs_initialize + +.. code-block:: c + + int rtems_cbs_initialize( void ); + +**DIRECTIVE STATUS CODES:** + +.. list-table:: + :class: rtems-table + + * - ``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. + +.. _rtems_cbs_cleanup: + +CBS_CLEANUP - Cleanup the CBS library +------------------------------------- +.. index:: cleanup the CBS library + +**CALLING SEQUENCE:** + +.. index:: rtems_cbs_cleanup + +.. code-block:: c + + int rtems_cbs_cleanup( void ); + +**DIRECTIVE STATUS CODES:** + +.. list-table:: + :class: rtems-table + + * - ``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. + +.. _rtems_cbs_create_server: + +CBS_CREATE_SERVER - Create a new bandwidth server +------------------------------------------------- +.. index:: create a new bandwidth server + +**CALLING SEQUENCE:** + +.. index:: rtems_cbs_create_server + +.. code-block:: 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:** + +.. list-table:: + :class: rtems-table + + * - ``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. + +.. _rtems_cbs_attach_thread: + +CBS_ATTACH_THREAD - Attach a thread to server +--------------------------------------------- +.. index:: attach a thread to server + +**CALLING SEQUENCE:** + +.. index:: rtems_cbs_attach_thread + +.. code-block:: c + + int rtems_cbs_attach_thread ( + rtems_cbs_server_id server_id, + rtems_id task_id + ); + +**DIRECTIVE STATUS CODES:** + +.. list-table:: + :class: rtems-table + + * - ``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. + +.. _rtems_cbs_detach_thread: + +CBS_DETACH_THREAD - Detach a thread from server +----------------------------------------------- +.. index:: detach a thread from server + +**CALLING SEQUENCE:** + +.. index:: rtems_cbs_detach_thread + +.. code-block:: c + + int rtems_cbs_detach_thread ( + rtems_cbs_server_id server_id, + rtems_id task_id + ); + +**DIRECTIVE STATUS CODES:** + +.. list-table:: + :class: rtems-table + + * - ``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. + +.. _rtems_cbs_destroy_server: + +CBS_DESTROY_SERVER - Destroy a bandwidth server +----------------------------------------------- +.. index:: destroy a bandwidth server + +**CALLING SEQUENCE:** + +.. index:: rtems_cbs_destroy_server + +.. code-block:: c + + int rtems_cbs_destroy_server ( + rtems_cbs_server_id server_id + ); + +**DIRECTIVE STATUS CODES:** + +.. list-table:: + :class: rtems-table + + * - ``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. + +.. _rtems_cbs_get_server_id: + +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-block:: c + + int rtems_cbs_get_server_id ( + rtems_id task_id, + rtems_cbs_server_id *server_id + ); + +**DIRECTIVE STATUS CODES:** + +.. list-table:: + :class: rtems-table + + * - ``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. + +.. _rtems_cbs_get_parameters: + +CBS_GET_PARAMETERS - Get scheduling parameters of a server +---------------------------------------------------------- +.. index:: get scheduling parameters of a server + +**CALLING SEQUENCE:** + +.. index:: rtems_cbs_get_parameters + +.. code-block:: c + + rtems_cbs_get_parameters ( + rtems_cbs_server_id server_id, + rtems_cbs_parameters *params + ); + +**DIRECTIVE STATUS CODES:** + +.. list-table:: + :class: rtems-table + + * - ``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. + +.. _rtems_cbs_set_parameters: + +CBS_SET_PARAMETERS - Set scheduling parameters +---------------------------------------------- +.. index:: set scheduling parameters + +**CALLING SEQUENCE:** + +.. index:: rtems_cbs_set_parameters + +.. code-block:: c + + int rtems_cbs_set_parameters ( + rtems_cbs_server_id server_id, + rtems_cbs_parameters *params + ); + +**DIRECTIVE STATUS CODES:** + +.. list-table:: + :class: rtems-table + + * - ``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. + +.. _rtems_cbs_get_execution_time: + +CBS_GET_EXECUTION_TIME - Get elapsed execution time +--------------------------------------------------- +.. index:: get elapsed execution time + +**CALLING SEQUENCE:** + +.. index:: rtems_cbs_get_execution_time + +.. code-block:: c + + int rtems_cbs_get_execution_time ( + rtems_cbs_server_id server_id, + time_t *exec_time, + time_t *abs_time + ); + +**DIRECTIVE STATUS CODES:** + +.. list-table:: + :class: rtems-table + + * - ``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. + +.. _rtems_cbs_get_remaining_budget: + +CBS_GET_REMAINING_BUDGET - Get remaining execution time +------------------------------------------------------- +.. index:: get remaining execution time + +**CALLING SEQUENCE:** + +.. index:: rtems_cbs_get_remaining_budget + +.. code-block:: c + + int rtems_cbs_get_remaining_budget ( + rtems_cbs_server_id server_id, + time_t *remaining_budget + ); + +**DIRECTIVE STATUS CODES:** + +.. list-table:: + :class: rtems-table + + * - ``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. + +.. _rtems_cbs_get_approved_budget: + +CBS_GET_APPROVED_BUDGET - Get scheduler approved execution time +--------------------------------------------------------------- +.. index:: get scheduler approved execution time + +**CALLING SEQUENCE:** + +.. index:: rtems_cbs_get_approved_budget + +.. code-block:: c + + int rtems_cbs_get_approved_budget ( + rtems_cbs_server_id server_id, + time_t *appr_budget + ); + +**DIRECTIVE STATUS CODES:** + +.. list-table:: + :class: rtems-table + + * - ``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. -- cgit v1.2.3