This describes how processes are scheduled by the Tock kernel.

Tock Scheduling

The kernel defines a Scheduler trait that the main kernel loop uses to determine which process to execute next. Here is a simplified view of that trait:

fn main() {
pub trait Scheduler {
    /// Decide which process to run next.
    fn next(&self) -> SchedulingDecision;

    /// Inform the scheduler of why the last process stopped executing, and how
    /// long it executed for.
    fn result(&self, result: StoppedExecutingReason, execution_time_us: Option<u32>);

    /// Tell the scheduler to execute kernel work such as interrupt bottom
    /// halves and dynamic deferred calls. Most schedulers will use the default
    /// implementation.
    unsafe fn execute_kernel_work(&self, chip: &C) {...}

    /// Ask the scheduler whether to take a break from executing userspace
    /// processes to handle kernel tasks.
    unsafe fn do_kernel_work_now(&self, chip: &C) -> bool {...}

    /// Ask the scheduler whether to continue trying to execute a process.
    /// Most schedulers will use this default implementation.
    unsafe fn continue_process(&self, _id: ProcessId, chip: &C) -> bool {...}

Individual boards can choose which scheduler to use, and implementing new schedulers just requires implementing this trait.

Process State

In Tock, a process can be in one of these states:

  • Running: Normal operation. A Running process is eligible to be scheduled for execution, although is subject to being paused by Tock to allow interrupt handlers or other processes to run. During normal operation, a process remains in the Running state until it explicitly yields. Upcalls from other kernel operations are not delivered to Running processes (i.e. upcalls do not interrupt processes), rather they are enqueued until the process yields.
  • Yielded: Suspended operation. A Yielded process will not be scheduled by Tock. Processes often yield while they are waiting for I/O or other operations to complete and have no immediately useful work to do. Whenever the kernel issues an upcall to a Yielded process, the process is transitioned to the Running state.
  • YieldedFor: Suspended operation. Like a Yielded process, a YieldedFor process will not be scheduled by Tock. A YieldedFor process is waiting for a specific UpcallId (i.e., a specific upcall for a specific driver). The process will only be transitioned to the Running state when the kernel issues that specific upcall.
  • Fault: Erroneous operation. A Fault-ed process will not be scheduled by Tock. Processes enter the Fault state by performing an illegal operation, such as accessing memory outside of their address space.
  • Terminated: The process ended itself by calling the Exit system call and the kernel has not restarted it.
  • Stopped: The process was running or yielded but was then explicitly stopped by the kernel (e.g., by the process console). A process in these states will not be made runnable until it is started, at which point it will continue execution where it was stopped.