[RTEMS Project] #2809: Reduce interrupt latency on SMP configurations during thread dispatch

RTEMS trac trac at rtems.org
Mon Nov 14 11:16:00 UTC 2016

#2809: Reduce interrupt latency on SMP configurations during thread dispatch
 Reporter:  sebastian.huber  |      Owner:  sebastian.huber
     Type:  enhancement      |     Status:  new
 Priority:  normal           |  Milestone:  4.12
Component:  SMP              |    Version:  4.12
 Severity:  normal           |   Keywords:
 Currently we have this situation:


 "On SMP systems, scheduling decisions on one processor must be propagated
 to other processors through inter-processor interrupts. So, a thread
 dispatch which must be carried out on another processor happens not
 instantaneous. 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 and it is possible that some of them may be out of date before the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
 * an boolean flag indicating if a thread dispatch is necessary or not.

 Updates of the heir thread and the thread dispatch necessary indicator are
 synchronized via explicit memory barriers without the use of locks. A
 thread can be an heir thread on at most one processor in the system. 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. The thread
 post-switch actions use a per-processor lock. This implementation turned
 out to be quite efficient and no lock contention was observed in the test

 The current implementation of thread dispatching has some implications
 with respect to the interrupt latency. It is crucial to preserve the
 system invariant that a thread can execute on at most one processor in the
 system at a time. This is accomplished with a boolean indicator in the
 thread context. The processor architecture specific context switch code
 will mark that a thread 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 thread (the boolean indicator is basically a
 TTAS lock). So, there is one point in time in which a processor is without
 a thread. This is essential to avoid cyclic dependencies in case multiple
 threads migrate at once. Otherwise some supervising entity is necessary to
 prevent deadlocks. Such a global supervisor would lead to scalability
 problems so this approach is not used. Currently the context switch is
 performed with interrupts disabled. Thus in case the heir thread is
 currently executing on another processor, the time of disabled interrupts
 is prolonged since one processor has to wait for another processor to make

 It is difficult to avoid this issue with the interrupt latency since
 interrupts normally store the context of the interrupted thread on its
 stack. In case a thread is marked as not executing, we must not use its
 thread stack to store such an interrupt context. We cannot use the heir
 stack before it stopped execution on another processor. If we enable
 interrupts during this transition, then we have to provide an alternative
 thread independent stack for interrupts in this time frame. This issue
 needs further investigation.

 The problematic situation occurs in case we have a thread which executes
 with thread dispatching disabled and should execute on another processor
 (e.g. it is an heir thread on another processor). In this case the
 interrupts on this other processor are disabled until the thread enables
 thread dispatching and starts the thread dispatch sequence. The scheduler
 (an exception is the scheduler with thread processor affinity support)
 tries to avoid such a situation and checks if a new scheduled thread
 already executes on a processor. In case the assigned processor differs
 from the processor on which the thread already executes and this processor
 is a member of the processor set managed by this scheduler instance, it
 will reassign the processors to keep the already executing thread in
 place. Therefore normal scheduler requests will not lead to such a
 situation. Explicit thread migration requests, however, can lead to this
 situation. Explicit thread migrations may occur due to the scheduler
 helping protocol or explicit scheduler instance changes. The situation can
 also be provoked by interrupts which suspend and resume threads multiple
 times and produce stale asynchronous thread dispatch requests in the

 Add an interrupt frame to the per-CPU control which can be used during
 context switches on SMP configurations.

Ticket URL: <http://devel.rtems.org/ticket/2809>
RTEMS Project <http://www.rtems.org/>
RTEMS Project

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