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Drop the <rtems/score/percpu.h> include since this file exposes a lot of
implementation details.
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Do not extract the idle threads from the ready set so that there is
always a thread available for comparison.
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The following scheduler operations return a thread in need for help
- unblock,
- change priority, and
- yield.
A thread in need for help is a thread that encounters a scheduler state
change from scheduled to ready or a thread that cannot be scheduled in
an unblock operation. Such a thread can ask threads which depend on
resources owned by this thread for help.
Add a new ask for help scheduler operation. This operation is used by
_Scheduler_Ask_for_help() to help threads in need for help returned by
the operations mentioned above. This operation is also used by
_Scheduler_Thread_change_resource_root() in case the root of a resource
sub-tree changes. A use case is the ownership change of a resource.
In case it is not possible to schedule a thread in need for help, then
the corresponding scheduler node will be placed into the set of ready
scheduler nodes of the scheduler instance. Once a state change from
ready to scheduled happens for this scheduler node it may be used to
schedule the thread in need for help.
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Add a chain node to the scheduler node to decouple the thread and
scheduler nodes. It is now possible to enqueue a thread in a thread
wait queue and use its scheduler node at the same for other threads,
e.g. a resouce owner.
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The function to change a thread priority was too complex. Simplify it
with a new scheduler operation. This increases the average case
performance due to the simplified logic. The interrupt disabled
critical section is a bit prolonged since now the extract, update and
enqueue steps are executed atomically. This should however not impact
the worst-case interrupt latency since at least for the Deterministic
Priority Scheduler this sequence can be carried out with a wee bit of
instructions and no loops.
Add _Scheduler_Change_priority() to replace the sequence of
- _Thread_Set_transient(),
- _Scheduler_Extract(),
- _Scheduler_Enqueue(), and
- _Scheduler_Enqueue_first().
Delete STATES_TRANSIENT, _States_Is_transient() and
_Thread_Set_transient() since this state is now superfluous.
With this change it is possible to get rid of the
SCHEDULER_SMP_NODE_IN_THE_AIR state. This considerably simplifies the
implementation of the new SMP locking protocols.
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Rename scheduler per-thread information into scheduler nodes using
Scheduler_Node as the base type. Use inheritance for specialized
schedulers.
Move the scheduler specific states from the thread control block into
the scheduler node structure.
Validate the SMP scheduler node state transitions in case RTEMS_DEBUG is
defined.
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The current implementation of task migration in RTEMS has some
implications with respect to the interrupt latency. It is crucial to
preserve the system invariant that a task can execute on at most one
processor in the system at a time. This is accomplished with a boolean
indicator in the task context. The processor architecture specific
low-level task context switch code will mark that a task context is no
longer executing and waits that the heir context stopped execution
before it restores the heir context and resumes execution of the heir
task. So there is one point in time in which a processor is without a
task. This is essential to avoid cyclic dependencies in case multiple
tasks migrate at once. Otherwise some supervising entity is necessary to
prevent life-locks. Such a global supervisor would lead to scalability
problems so this approach is not used. Currently the thread dispatch is
performed with interrupts disabled. So in case the heir task is
currently executing on another processor then this prolongs the time of
disabled interrupts since one processor has to wait for another
processor to make progress.
It is difficult to avoid this issue with the interrupt latency since
interrupts normally store the context of the interrupted task on its
stack. In case a task is marked as not executing we must not use its
task stack to store such an interrupt context. We cannot use the heir
stack before it stopped execution on another processor. So if we enable
interrupts during this transition we have to provide an alternative task
independent stack for this time frame. This issue needs further
investigation.
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Do not allocate the scheduler control structures from the workspace.
This is a preparation step for configuration of clustered/partitioned
schedulers on SMP.
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Delete global variables _Priority_Major_bit_map and _Priority_Bit_map.
This makes it possible to use multiple priority scheduler instances for
example with clustered/partitioned scheduling on SMP.
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Rename _Scheduler_simple_smp_Start_idle() to
_Scheduler_SMP_Start_idle().
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Replace Scheduler_simple_smp_Control with Scheduler_SMP_Control. Rename
_Scheduler_simple_smp_Instance() to _Scheduler_SMP_Instance().
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