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<chapter id="scheduling">

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  <title>Scheduling</title>

  <para>One of the key aims of the operating system is to create and support

  the impression that several activities are executing contemporarily. This is

  true for both uniprocessor as well as multiprocessor systems. In the case of

  multiprocessor systems, the activities are truly happening in parallel. The

  scheduler helps to materialize this impression by planning threads on as

  many processors as possible and, when this strategy reaches its limits, by

  quickly switching among threads executing on a single processor.</para>

  <section>

    <title>Contexts</title>

    <para>The term <emphasis>context</emphasis> refers to the set of processor

    resources that define the current state of the computation or the

    environment and the kernel understands it in several more or less narrow

    sences:</para>

    <itemizedlist>

      <listitem>

        <para>synchronous register context,</para>

      </listitem>

      <listitem>

        <para>asynchronous register context,</para>

      </listitem>

      <listitem>

        <para>FPU context and</para>

      </listitem>

      <listitem>

        <para>memory management context.</para>

      </listitem>

    </itemizedlist>

    <para>The most narrow sense refers to the the synchronous register

    context. It includes all the preserved registers as defined by the

    architecture. To highlight some, the program counter and stack pointer

    take part in the synchronous register context. These registers must be

    preserved across a procedure call and during synchronous context

    switches.</para>

    <para>The next type of the context understood by the kernel is the

    asynchronous register context. On an interrupt, the interrupted execution

    flow's state must be guaranteed to be eventually completely restored.

    Therefore the interrupt context includes, among other things, the scratch

    registers as defined by the architecture. As a special optimization and if

    certain conditions are met, it need not include the architecture's

    preserved registers. The condition mentioned in the previous sentence is

    that the low-level assembly language interrupt routines don't modify the

    preserved registers. The handlers usually call a higher-level C routine.

    The preserved registers are then saved on the stack by the compiler

    generated code of the higher-level function. In HelenOS, several

    architectures can be compiled with this optimization.</para>

    <para>Although the kernel does not do any floating point

    arithmetics<footnote>

        <para>Some architectures (e.g. ia64) inevitably use a fixed set of

        floating point registers to carry out their normal operations.</para>

      </footnote>, it must protect FPU context of userspace threads against

    destruction by other threads. Moreover, only a fraction of userspace

    programs use the floating point unit. HelenOS contains a generic framework

    for switching FPU context only when the switch is forced (i.e. a thread

    uses a floating point instruction and its FPU context is not loaded in the

    processor).</para>

    <para>The last member of the context family is the memory management

    context. It includes memory management registers that identify address

    spaces on hardware level (i.e. ASIDs and page tables pointers).</para>

    <section>

      <title>Synchronous Context Switches</title>

      <para>The scheduler, but also other pieces of the kernel, make heavy use

      of synchronous context switches, because it is a natural vehicle not

      only for changes in control flow, but also for switching between two

      kernel stacks. Two functions figure in a synchronous context switch

      implementation: <code>context_save()</code> and

      <code>context_restore()</code>. Note that these two functions break the

      natural perception of the linear C code execution flow starting at

      function's entry point and ending on one of the function's exit

      points.</para>

      <para>When the <code>context_save()</code> function is called, the

      synchronous context is saved in a memory structure passed to it. After

      executing <code>context_save()</code>, the caller is returned 1 as a

      return value. The execution of instructions continues as normally until

      <code>context_restore()</code> is called. For the caller, it seems like

      the call never returns<footnote>

          <para>Which might be a source of problems with variable liveliness

          after <code>context_restore()</code>.</para>

        </footnote>. Nevertheless, a synchronous register context, which is

      saved in a memory structure passed to <code>context_restore()</code>, is

      restored, thus transfering the control flow to the place of occurrence

      of the corresponding call to <code>context_save()</code>. From the

      perspective of the caller of the corresponding

      <code>context_save()</code>, it looks like a return from

      <code>context_save()</code>. However, this time a return value of 0 is

      returned.</para>

    </section>

  </section>

  <section>

    <title>Threads</title>

    <para>A thread is the basic executable entity with some code and a stack.

    While the code, implemented by a C language function, can be shared by

    several threads, the stack is always private to each instance of the

    thread. Each thread belongs to exactly one task through which it shares

    address space with its sibling threads. Threads that execute purely in the

    kernel don't have any userspace memory allocated. However, when a thread

    has ambitions to run in userspace, it must be allocated a userspace stack.

    The distinction between the purely kernel threads and threads running also

    in userspace is made by refering to the former group as to kernel threads

    and to the latter group as to userspace threads. Both kernel and userspace

    threads are visible to the scheduler and can become a subject of kernel

    preemption and thread migration anytime when preemption is not

    disabled.</para>

    <formalpara>

      <title>Thread States</title>

      <para>In each moment, a thread exists in one of six possible thread

      states. When the thread is created and first inserted into the

      scheduler's run queues or when a thread is migrated to a new processor,

      it is put into the <constant>Entering</constant> state. After some time

      elapses, the scheduler picks up the thread and starts executing it. A

      thread being currently executed on a processor is in the

      <constant>Running</constant> state. From there, the thread has three

      possibilities. It either runs until it is preemtped, in which case the

      state changes to <constant>Ready</constant>, goes to the

      <constant>Sleeping</constant> state by going to sleep or enters the

      <constant>Exiting</constant> state when it reaches termination. When the

      thread exits, its kernel structure usually stays in memory, until the

      thread is detached by another thread using <code>thread_detach()</code>

      function. Terminated but undetached threads are in the

      <constant>Lingering</constant> state. When the thread is detached or

      detaches itself during its life, it is destroyed in the

      <constant>Exiting</constant> state and the

      <constant>Lingering</constant> state is not reached.<figure float="1">

          <title>Transitions among thread states.</title>

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    </formalpara>

    <formalpara>

      <title>Fibrils</title>

      <para>HelenOS userspace layer knows even smaller units of execution.

      Each userspace thread can make use of an arbitrary number of fibrils.

      These fibrils have their own synchronous register context, userspace

      code and stack. They live their own life within the userspace thread and

      the scheduler does not have any idea about them because they are

      completely implemented by the userspace library. This implies several

      things:<itemizedlist>

          <listitem>

            <para>fibrils schedule themselves cooperatively within the time

            slice given to their userspace thread,</para>

          </listitem>

          <listitem>

            <para>fibrils share FPU context of their containing thread

            and</para>

          </listitem>

          <listitem>

            <para>all fibrils of one userspace thread block when one of them

            goes to sleep.</para>

          </listitem>

        </itemizedlist></para>

    </formalpara>

  </section>

  <section>

    <title>Scheduler</title>

    <section>

      <title>Run Queues</title>

      <para>There is an array of several run queues on each processor. The

      current version of HelenOS uses 16 run queues implemented by 16 doubly

      linked lists. Each of the run queues is associated with thread priority.

      The lower the run queue index in the array is, the higher is the

      priority of threads linked in that run queue and the shorter is the time

      in which those threads will execute. When kernel code wants to access

      the run queue, it must first acquire its lock.</para>

    </section>

    <section>

      <title>Scheduler Operation</title>

      <para>The scheduler is invoked either explicitly when a thread calls the

      <code>scheduler()</code> function (e.g. goes to sleep or merely wants to

      relinquish the processor for a while) or implicitly on a periodic basis

      when the generic clock interrupt preempts the current thread. After its

      invocation, the scheduler saves the synchronous register context of the

      current thread and switches to its private stack. Afterwards, a new

      thread is selected according to the scheduling policy. If there is no

      suitable thread, the processor is idle and no thread executes on it.

      Note that the act of switching to the private scheduler stack is

      essential. If the processor kept running using the stack of the

      preempted thread it could damage it because the old thread can be

      migrated to another processor and scheduled there. In the worst case

      scenario, two execution flows would be using the same stack.</para>

      <para>The scheduling policy is implemented in the function

      <code>find_best_thread()</code>. This function walks the processor run

      queues from lower towards higher indices and looks for a thread. If the

      visited run queue is empty, it simply searches the next run queue. If it

      is known in advance that there are no ready threads waiting for

      execution, <code>find_best_thread()</code> interruptibly halts the

      processor or busy waits until some threads arrive. This process repeats

      until <code>find_best_thread()</code> succeeds.</para>

      <para>After the best thread is chosen, the scheduler switches to the

      thread's task and memory management context. Finally, the saved

      synchronous register context is restored and the thread runs. Each

      scheduled thread is given a time slice depending on its priority (i.e.

      run queue). The higher priority, the shorter timeslice. To summarize,

      this policy schedules threads with high priorities more frequently but

      gives them smaller time slices. On the other hand, lower priority

      threads are scheduled less frequently, but run for longer periods of

      time.</para>

      <para>When a thread uses its entire time slice, it is preempted and put

      back into the run queue that immediately follows the previous run queue

      from which the thread ran. Threads that are woken up from a sleep are

      put into the biggest priority run queue. Low priority threads are

      therefore those that don't go to sleep so often and just occupy the

      processor.</para>

      <para>In order to avoid complete starvation of the low priority threads,

      from time to time, the scheduler will provide them with a bonus of one

      point priority increase. In other words, the scheduler will now and then

      move the entire run queues one level up.</para>

    </section>

    <section>

      <title>Processor Load Balancing</title>

      <para>Normally, for the sake of cache locality, threads are scheduled on

      one of the processors and don't leave it. Nevertheless, a situation in

      which one processor is heavily overloaded while others sit idle can

      occur. HelenOS deploys special kernel threads to help mitigate this

      problem. Each processor is associated with one load balancing thread

      called <code>kcpulb</code> that wakes up regularly to see whether its

      processor is underbalanced or not. If it is, the thread attempts to

      migrate threads from other overloaded processors to its own processor's

      run queues. When the job is done or there is no need for load balancing,

      the thread goes to sleep.</para>

      <para>The balancing threads operate very gently and try to migrate low

      priority threads first; one <code>kcpulb</code> never takes from one

      processor twice in a row. The load balancing threads as well as threads

      that were just stolen cannot be migrated. The <code>kcpulb</code>

      threads are wired to their processors and cannot be migrated whatsoever.

      The ordinary threads are protected only until they are

      rescheduled.</para>

    </section>

  </section>

</chapter>