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Diffstat (limited to 'doc/developer/process-architecture.rst')
| -rw-r--r-- | doc/developer/process-architecture.rst | 54 |
1 files changed, 28 insertions, 26 deletions
diff --git a/doc/developer/process-architecture.rst b/doc/developer/process-architecture.rst index 33ef278c4d..85126cab30 100644 --- a/doc/developer/process-architecture.rst +++ b/doc/developer/process-architecture.rst @@ -46,7 +46,8 @@ implemented in FRR. This doc should be expanded and broken off into its own section. For now it provides basic information necessary to understand the interplay between the event system and kernel threads. -The core event system is implemented in :file:`lib/thread.[ch]`. The primary +The core event system is implemented in :file:`lib/event.c` and +:file:`lib/frrevent.h`. The primary structure is ``struct event_loop``, hereafter referred to as a ``threadmaster``. A ``threadmaster`` is a global state object, or context, that holds all the tasks currently pending execution as well as statistics on tasks @@ -57,41 +58,41 @@ execute. At initialization, a daemon will typically create one fetch each task and execute it. These tasks have various types corresponding to their general action. The types -are given by integer macros in :file:`event.h` and are: +are given by integer macros in :file:`frrevent.h` and are: -``THREAD_READ`` +``EVENT_READ`` Task which waits for a file descriptor to become ready for reading and then executes. -``THREAD_WRITE`` +``EVENT_WRITE`` Task which waits for a file descriptor to become ready for writing and then executes. -``THREAD_TIMER`` +``EVENT_TIMER`` Task which executes after a certain amount of time has passed since it was scheduled. -``THREAD_EVENT`` +``EVENT_EVENT`` Generic task that executes with high priority and carries an arbitrary integer indicating the event type to its handler. These are commonly used to implement the finite state machines typically found in routing protocols. -``THREAD_READY`` +``EVENT_READY`` Type used internally for tasks on the ready queue. -``THREAD_UNUSED`` +``EVENT_UNUSED`` Type used internally for ``struct event`` objects that aren't being used. The event system pools ``struct event`` to avoid heap allocations; this is the type they have when they're in the pool. -``THREAD_EXECUTE`` +``EVENT_EXECUTE`` Just before a task is run its type is changed to this. This is used to show - ``X`` as the type in the output of :clicmd:`show thread cpu`. + ``X`` as the type in the output of :clicmd:`show event cpu`. The programmer never has to work with these types explicitly. Each type of task is created and queued via special-purpose functions (actually macros, but irrelevant for the time being) for the specific type. For example, to add a -``THREAD_READ`` task, you would call +``EVENT_READ`` task, you would call :: @@ -113,9 +114,9 @@ sockets needed for peerings or IPC. To retrieve the next task to run the program calls ``event_fetch()``. ``event_fetch()`` internally computes which task to execute next based on -rudimentary priority logic. Events (type ``THREAD_EVENT``) execute with the +rudimentary priority logic. Events (type ``EVENT_EVENT``) execute with the highest priority, followed by expired timers and finally I/O tasks (type -``THREAD_READ`` and ``THREAD_WRITE``). When scheduling a task a function and an +``EVENT_READ`` and ``EVENT_WRITE``). When scheduling a task a function and an arbitrary argument are provided. The task returned from ``event_fetch()`` is then executed with ``event_call()``. @@ -135,23 +136,23 @@ Mapping the general names used in the figure to specific FRR functions: - ``task`` is ``struct event *`` - ``fetch`` is ``event_fetch()`` -- ``exec()`` is ``event_call`` +- ``exec()`` is ``event_call()`` - ``cancel()`` is ``event_cancel()`` - ``schedule()`` is any of the various task-specific ``event_add_*`` functions Adding tasks is done with various task-specific function-like macros. These -macros wrap underlying functions in :file:`thread.c` to provide additional +macros wrap underlying functions in :file:`event.c` to provide additional information added at compile time, such as the line number the task was scheduled from, that can be accessed at runtime for debugging, logging and informational purposes. Each task type has its own specific scheduling function -that follow the naming convention ``event_add_<type>``; see :file:`event.h` +that follow the naming convention ``event_add_<type>``; see :file:`frrevent.h` for details. There are some gotchas to keep in mind: - I/O tasks are keyed off the file descriptor associated with the I/O operation. This means that for any given file descriptor, only one of each - type of I/O task (``THREAD_READ`` and ``THREAD_WRITE``) can be scheduled. For + type of I/O task (``EVENT_READ`` and ``EVENT_WRITE``) can be scheduled. For example, scheduling two write tasks one after the other will overwrite the first task with the second, resulting in total loss of the first task and difficult bugs. @@ -209,7 +210,8 @@ Kernel Thread Wrapper The basis for the integration of pthreads and the event system is a lightweight wrapper for both systems implemented in :file:`lib/frr_pthread.[ch]`. The header provides a core datastructure, ``struct frr_pthread``, that encapsulates -structures from both POSIX threads and :file:`thread.[ch]`. In particular, this +structures from both POSIX threads and :file:`event.c`, :file:`frrevent.h`. +In particular, this datastructure has a pointer to a ``threadmaster`` that runs within the pthread. It also has fields for a name as well as start and stop functions that have signatures similar to the POSIX arguments for ``pthread_create()``. @@ -217,18 +219,18 @@ signatures similar to the POSIX arguments for ``pthread_create()``. Calling ``frr_pthread_new()`` creates and registers a new ``frr_pthread``. The returned structure has a pre-initialized ``threadmaster``, and its ``start`` and ``stop`` functions are initialized to defaults that will run a basic event -loop with the given threadmaster. Calling ``frr_pthread_run`` starts the thread +loop with the given threadmaster. Calling ``frr_pthread_run()`` starts the thread with the ``start`` function. From there, the model is the same as the regular event model. To schedule tasks on a particular pthread, simply use the regular -:file:`thread.c` functions as usual and provide the ``threadmaster`` pointed to +:file:`event.c` functions as usual and provide the ``threadmaster`` pointed to from the ``frr_pthread``. As part of implementing the wrapper, the -:file:`thread.c` functions were made thread-safe. Consequently, it is safe to +:file:`event.c` functions were made thread-safe. Consequently, it is safe to schedule events on a ``threadmaster`` belonging both to the calling thread as well as *any other pthread*. This serves as the basis for inter-thread communication and boils down to a slightly more complicated method of message passing, where the messages are the regular task events as used in the event-driven model. The only difference is thread cancellation, which requires -calling ``event_cancel_async()`` instead of ``event_cancel`` to cancel a task +calling ``event_cancel_async()`` instead of ``event_cancel()`` to cancel a task currently scheduled on a ``threadmaster`` belonging to a different pthread. This is necessary to avoid race conditions in the specific case where one pthread wants to guarantee that a task on another pthread is cancelled before @@ -236,16 +238,16 @@ proceeding. In addition, the existing commands to show statistics and other information for tasks within the event driven model have been expanded to handle multiple -pthreads; running :clicmd:`show thread cpu` will display the usual event +pthreads; running :clicmd:`show event cpu` will display the usual event breakdown, but it will do so for each pthread running in the program. For example, :ref:`bgpd` runs a dedicated I/O pthread and shows the following -output for :clicmd:`show thread cpu`: +output for :clicmd:`show event cpu`: :: - frr# show thread cpu + frr# show event cpu - Thread statistics for bgpd: + Event statistics for bgpd: Showing statistics for pthread main ------------------------------------ |