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c-user: Add SMP implementation details section

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Sebastian Huber 2017-02-02 10:49:56 +01:00
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c-user/symmetric_multiprocessing_services.rst
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@ -333,44 +333,6 @@ of the OpenMP master thread that created it. In the scheduler instance
``WRK1`` there are three thread pools available and their worker threads run at
priority four.
Thread Dispatch Details
-----------------------
This section gives background information to developers interested in the
interrupt latencies introduced by thread dispatching. A thread dispatch
consists of all work which must be done to stop the currently executing thread
on a processor and hand over this processor to an heir thread.
In SMP systems, scheduling decisions on one processor must be propagated
to other processors through inter-processor interrupts. A thread dispatch
which must be carried out on another processor does not happen instantaneously.
Thus, several thread dispatch requests might be in the air and it is possible
that some of them may be out of date before the corresponding processor has
time to deal with them. The thread dispatch mechanism uses three per-processor
variables,
- the executing thread,
- the heir thread, and
- a boolean flag indicating if a thread dispatch is necessary or not.
Updates of the heir thread are done via a normal store operation. The thread
dispatch necessary indicator of another processor is set as a side-effect of an
inter-processor interrupt. So, this change notification works without the use
of locks. The thread context is protected by a TTAS lock embedded in the
context to ensure that it is used on at most one processor at a time.
Normally, only thread-specific or per-processor locks are used during a thread
dispatch. This implementation turned out to be quite efficient and no lock
contention was observed in the testsuite. The heavy-weight thread dispatch
sequence is only entered in case the thread dispatch indicator is set.
The context-switch is performed with interrupts enabled. During the transition
from the executing to the heir thread neither the stack of the executing nor
the heir thread must be used during interrupt processing. For this purpose a
temporary per-processor stack is set up which may be used by the interrupt
prologue before the stack is switched to the interrupt stack.
Application Issues
==================
@ -562,6 +524,47 @@ on a suitable platform, e.g. QorIQ T4240. High-performance SMP applications
need full control of the object storage :cite:`Drepper:2007:Memory`.
Therefore, self-contained synchronization objects are now available for RTEMS.
Implementation Details
======================
Thread Dispatch Details
-----------------------
This section gives background information to developers interested in the
interrupt latencies introduced by thread dispatching. A thread dispatch
consists of all work which must be done to stop the currently executing thread
on a processor and hand over this processor to an heir thread.
In SMP systems, scheduling decisions on one processor must be propagated
to other processors through inter-processor interrupts. A thread dispatch
which must be carried out on another processor does not happen instantaneously.
Thus, several thread dispatch requests might be in the air and it is possible
that some of them may be out of date before the corresponding processor has
time to deal with them. The thread dispatch mechanism uses three per-processor
variables,
- the executing thread,
- the heir thread, and
- a boolean flag indicating if a thread dispatch is necessary or not.
Updates of the heir thread are done via a normal store operation. The thread
dispatch necessary indicator of another processor is set as a side-effect of an
inter-processor interrupt. So, this change notification works without the use
of locks. The thread context is protected by a TTAS lock embedded in the
context to ensure that it is used on at most one processor at a time.
Normally, only thread-specific or per-processor locks are used during a thread
dispatch. This implementation turned out to be quite efficient and no lock
contention was observed in the testsuite. The heavy-weight thread dispatch
sequence is only entered in case the thread dispatch indicator is set.
The context-switch is performed with interrupts enabled. During the transition
from the executing to the heir thread neither the stack of the executing nor
the heir thread must be used during interrupt processing. For this purpose a
temporary per-processor stack is set up which may be used by the interrupt
prologue before the stack is switched to the interrupt stack.
Directives
==========