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<h1 class="menu-title">Real-Time Interrupt-driven Concurrency</h1>
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<main>
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<h1 id="task-priorities"><a class="header" href="#task-priorities">Task priorities</a></h1>
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<h2 id="priorities"><a class="header" href="#priorities">Priorities</a></h2>
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<p>The <code>priority</code> argument declares the static priority of each <code>task</code>.</p>
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<p>For Cortex-M, tasks can have priorities in the range <code>1..=(1 << NVIC_PRIO_BITS)</code>
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where <code>NVIC_PRIO_BITS</code> is a constant defined in the <code>device</code> crate.</p>
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<p>Omitting the <code>priority</code> argument the task priority defaults to <code>1</code>.
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The <code>idle</code> task has a non-configurable static priority of <code>0</code>, the lowest priority.</p>
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<blockquote>
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<p>A higher number means a higher priority in RTIC, which is the opposite from what
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Cortex-M does in the NVIC peripheral.
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Explicitly, this means that number <code>10</code> has a <strong>higher</strong> priority than number <code>9</code>.</p>
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</blockquote>
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<p>The highest static priority task takes precedence when more than one
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task are ready to execute.</p>
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<p>The following scenario demonstrates task prioritization:
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Spawning a higher priority task A during execution of a lower priority task B suspends
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task B. Task A has higher priority thus preempting task B which gets suspended
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until task A completes execution. Thus, when task A completes task B resumes execution.</p>
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<pre><code class="language-text">Task Priority
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┌────────────────────────────────────────────────────────┐
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│ │
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│ │
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3 │ Preempts │
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2 │ A─────────► │
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1 │ B─────────► - - - - B────────► │
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0 │Idle┌─────► Resumes ┌──────────► │
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├────┴──────────────────────────────────┴────────────────┤
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│ │
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└────────────────────────────────────────────────────────┘Time
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</code></pre>
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<p>The following example showcases the priority based scheduling of tasks:</p>
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<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>//! examples/preempt.rs
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#![no_main]
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#![no_std]
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use panic_semihosting as _;
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use rtic::app;
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#[app(device = lm3s6965, dispatchers = [SSI0, QEI0])]
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mod app {
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use cortex_m_semihosting::{debug, hprintln};
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#[shared]
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struct Shared {}
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#[local]
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struct Local {}
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#[init]
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fn init(_: init::Context) -> (Shared, Local, init::Monotonics) {
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foo::spawn().unwrap();
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(Shared {}, Local {}, init::Monotonics())
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}
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#[task(priority = 1)]
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fn foo(_: foo::Context) {
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hprintln!("foo - start");
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baz::spawn().unwrap();
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hprintln!("foo - end");
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debug::exit(debug::EXIT_SUCCESS); // Exit QEMU simulator
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}
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#[task(priority = 2)]
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fn bar(_: bar::Context) {
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hprintln!(" bar");
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}
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#[task(priority = 2)]
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fn baz(_: baz::Context) {
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hprintln!(" baz - start");
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bar::spawn().unwrap();
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hprintln!(" baz - end");
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}
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}
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<span class="boring">}</span></code></pre></pre>
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<pre><code class="language-console">$ cargo run --target thumbv7m-none-eabi --example preempt
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foo - start
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baz - start
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baz - end
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bar
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foo - end
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</code></pre>
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<p>Note that the task <code>bar</code> does <em>not</em> preempt task <code>baz</code> because its priority
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is the <em>same</em> as <code>baz</code>'s. The higher priority task <code>bar</code> runs before <code>foo</code>
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when <code>baz</code>returns. When <code>bar</code> returns <code>foo</code> can resume.</p>
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<p>One more note about priorities: choosing a priority higher than what the device
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supports will result in a compilation error.</p>
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<p>The error is cryptic due to limitations in the Rust language
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if <code>priority = 9</code> for task <code>uart0_interrupt</code> in <code>example/common.rs</code> this looks like:</p>
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<pre><code class="language-text"> error[E0080]: evaluation of constant value failed
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--> examples/common.rs:10:1
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10 | #[rtic::app(device = lm3s6965, dispatchers = [SSI0, QEI0])]
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| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ attempt to compute `8_usize - 9_usize`, which would overflow
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= note: this error originates in the attribute macro `rtic::app` (in Nightly builds, run with -Z macro-backtrace for more info)
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</code></pre>
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<p>The error message incorrectly points to the starting point of the macro, but at least the
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value subtracted (in this case 9) will suggest which task causes the error.</p>
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