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The EventQueue API

One of the optional Arm Mbed OS features is an event loop mechanism that you can use to defer the execution of code to a different context. In particular, a common use of an event loop is to postpone the execution of a code sequence from an interrupt handler to a user context. This is useful because of the specific constraints of code that runs in an interrupt handler:

The execution of certain functions (notably some functions in the C library) is not safe.
You cannot use various RTOS objects and functions from an interrupt context.
As a general rule, the code needs to finish as fast as possible, to allow other interrupts to be handled.

The event loop offers a solution to these issues in the form of an API that can defer execution of code from the interrupt context to the user context. More generally, the event loop can be used anywhere in a program (not necessarily in an interrupt handler) to defer code execution to a different context.

In Mbed OS, events are pointers to functions (and optionally function arguments). An event loop extracts events from a queue and executes them.

Creating an event loop

You must create and start the event loop manually. The simplest way to achieve that is to create a Thread and run the event queue's dispatch method in the thread:

#include "mbed.h"

// Create a queue that can hold a maximum of 32 events
EventQueue queue(32 * EVENTS_EVENT_SIZE);
// Create a thread that'll run the event queue's dispatch function
Thread t;

int main () {
    // Start the event queue's dispatch thread
    t.start(callback(&queue, &EventQueue::dispatch_forever));
}

Note that though this document assumes the presence of a single event loop in the system, there's nothing preventing the programmer from running more than one event loop, simply by following the create/start pattern above for each of them.

Using the event loop

Once you start the event loop, it can post events. Let's consider an example of a program that attaches two interrupt handlers for an InterruptIn object, using the InterruptIn rise and fall functions. The rise handler will run in interrupt context, and the fall handler will run in user context (more specifically, in the context of the event loop's thread). The full code for the example can be found below:

main.cppImport into Mbed IDE

/*
 * Copyright (c) 2020 Arm Limited and affiliates.
 * SPDX-License-Identifier: Apache-2.0
 */
#include "mbed.h"

DigitalOut led1(LED1);
InterruptIn sw(SW2);
EventQueue queue(32 * EVENTS_EVENT_SIZE);
Thread t;


void rise_handler(void)
{
    queue.call(printf, "rise_handler in context %p\n", ThisThread::get_id());
    // Toggle LED
    led1 = !led1;
}

void fall_handler(void)
{
    printf("rise_handler in context %p\n", ThisThread::get_id());
    // Toggle LED
    led1 = !led1;
}

int main()
{
    // Start the event queue
    t.start(callback(&queue, &EventQueue::dispatch_forever));
    printf("Starting in context %p\r\n", ThisThread::get_id());
    // The 'rise' handler will execute in IRQ context
    sw.rise(rise_handler);
    // The 'fall' handler will execute in the context of thread 't'
    sw.fall(queue.event(fall_handler));
}

The above code executes two handler functions (rise_handler and fall_handler) in two different contexts:

In interrupt context when a rising edge is detected on SW2 (rise_handler).
In the context of the event loop's thread function when a falling edge is detected on SW2 (fall_handler). queue.event() is called with fall_handler as an argument to specify that fall_handler will run in user context instead of interrupt context.

This is the output of the above program on an FRDM-K64F board. We reset the board and pressed the SW2 button twice:

Starting in context 20001fe0
fall_handler in context 20000b1c
rise_handler in context 00000000
fall_handler in context 20000b1c
rise_handler in context 00000000

The program starts in the context of the thread that runs the main function (20001fe0). When the user presses SW2, fall_handler is automatically queued in the event queue, and it runs later in the context of thread t (20000b1c). When the user releases the button, rise_handler is executed immediately, and it displays 00000000, indicating that the code ran in interrupt context.

The code for rise_handler is problematic because it calls printf in interrupt context, which is a potentially unsafe operation. Fortunately, this is exactly the kind of problem that event queues can solve. We can make the code safe by running rise_handler in user context (like we already do with fall_handler) by replacing this line:

sw.rise(rise_handler);

with this line:

sw.rise(queue.event(rise_handler));

The code is safe now, but we may have introduced another problem: latency. After the change above, the call to rise_handler will be queued, which means that it no longer runs immediately after the interrupt is raised. For this example code, this isn't a problem, but some applications might need the code to respond as fast as possible to an interrupt.

Let's assume that rise_handler must toggle the LED as quickly as possible in response to the user's action on SW2. To do that, it must run in interrupt context. However, rise_handler still needs to print a message indicating that the handler was called; that's problematic because it's not safe to call printf from an interrupt context.

The solution is to split rise_handler into two parts: the time critical part will run in interrupt context, while the non-critical part (displaying the message) will run in user context. This is easily doable using queue.call:

void rise_handler_user_context(void) {
    printf("rise_handler_user_context in context %p\r\n", Thread::gettid());
}

void rise_handler(void) {
    // Execute the time critical part first
    led1 = !led1;
    // The rest can execute later in user context (and can contain code that's not interrupt safe).
    // We use the 'queue.call' function to add an event (the call to 'rise_handler_user_context') to the queue.
    queue.call(rise_handler_user_context);
}

After replacing the code for rise_handler as above, the output of our example becomes:

Starting in context 0x20002c50
fall_handler in context 0x20002c90
rise_handler_user_context in context 0x20002c90
fall_handler in context 0x20002c90
rise_handler_user_context in context 0x20002c90

The scenario above (splitting an interrupt handler's code into time critical code and non-time critical code) is another common pattern that you can easily implement with event queues; queuing code that's not interrupt safe is not the only thing you can use event queues for. Any kind of code can be queued and deferred for later execution.

We used InterruptIn for the example above, but the same kind of code can be used with any attach()-like functions in the SDK. Examples include Serial::attach(), Ticker::attach(), Ticker::attach_us(), Timeout::attach().

Prioritization

The EventQueue has no concept of event priority. If you schedule events to run at the same time, the order in which the events run relative to one another is undefined. The EventQueue only schedules events based on time. If you want to separate your events into different priorities, you must instantiate an EventQueue for each priority. You must appropriately set the priority of the thread dispatching each EventQueue instance.

EventQueue memory pool

When you create an instance of the EventQueue, you specify a fixed size for its memory. Because allocating from the general purpose heap is not IRQ safe, the EventQueue allocates this fixed size block of memory during its creation. Although the EventQueue memory size is fixed, the Eventqueue supports various sized events.

Various sized events introduce fragmentation to the memory region. This fragmentation makes it difficult to determine how many more events the EventQueue can dispatch. The EventQueue may be able to dispatch many small events, but fragmentation may prevent it from allocating one large event.

Calculating the number of events

If your project only uses fix-sized events, you can use a counter that tracks the number of events the EventQueue has dispatched.

If your projects uses variable-sized events, you can calculate the number of available events of a specific size because successfully allocated memory is never fragmented further. However, untouched space can service any event that fits, which complicates such a calculation.

// event size in words: 9 + callback_size (4) + arguments_size (where 9 is internal space for event data)
void func1(int);
void func3(int, int, int);

EventQueue queue(2*(9+4+3)*sizeof(int)); // 32 words of storage (store two callbacks with three arguments at max)
queue.call(func1, 1);       // requires 14 words of storage (9+4+1)
queue.call(func3, 1, 2, 3); // requires 16 words of storage (9+4+3)
// after this we have 2 words of storage left

queue.dispatch(); // free all pending events

queue.call(func, 1, 2, 3); // requires 16 words of storage (9+4+3)
queue.call(func, 1, 2, 3); // fails
// storage has been fragmented into two events with 14 and 16 words
// of storage, no space is left for an another 16 word event even though two words
// exist in the memory region

Failure due to fragmentation

The following example would fail because of fragmentation:

// event size in words: 9 + callback_size (4) + arguments_size (where 9 is internal space for event data)
void func0();
void func3(int, int, int);

EventQueue queue(4*(9+4)*sizeof(int)); // 52 words of storage
queue.call(func0);       // requires 13 words of storage (9+4)
queue.call(func0);       // requires 13 words of storage (9+4)
queue.call(func0);       // requires 13 words of storage (9+4)
queue.call(func0);       // requires 13 words of storage (9+4)
// 0 words of storage remain

queue.dispatch();  // free all pending events
// all memory is free again (52 words) and in 13-word chunks

queue.call(func3, 1, 2, 3); // requires 16 words of storage (9+4+3), so allocation fails

52 words of storage are free but only for allocations of 13 words or fewer. The solution to this failure is to increase the size of your EventQueue. Having the proper sized EventQueue prevents you from running out of space for events in the future.

More about events

This is only a small part of how event queues work in Mbed OS. The EventQueue, Event and UserAllocatedEvent classes in the mbed-events library offer a lot of features that this document does not cover, including calling functions with arguments, queueing functions to be called after a delay or queueing functions to be called periodically. The README of the mbed-events library shows more ways to use events and event queues. To see the implementation of the events library, review the equeue library.

Static EventQueue

The EventQueue API provides a mechanism for creating a static queue, a queue that doesn't use any dynamic memory allocation and accepts only user-allocated events. After you create a static queue (by passing zero as size to its constructor), you can post UserAllocatedEvent to it. Using static EventQueue combined with UserAllocatedEvent ensures no dynamic memory allocation will take place during queue creation and events posting and dispatching. You can also declare queues and events as static objects (static in the C++ sense), and then memory for them will be reserved at compile time: