On 05/02/2014 06:23 PM, Gedare Bloom wrote:
On Fri, May 2, 2014 at 7:39 AM, Sebastian Huber
<[email protected]> wrote:
>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.
Thanks for the good explanation. Is the problem of using the
non-executing task's stack that the task might get resumed on some
other processor whlie the interrupt is running?
Yes, as soon as you set is_executing to false it may start execution on
another processor.
When you say "cannot
use the heir stack" do you mean cannot use it on the "idle" processor
because the heir is executing on some oher processor where the heir
should be getting blocked soon?
Yes, exactly, the heir may currently execute on another processor. On
this other processor it must have an heir thread and a thread dispatch
must be pending.
For example lets have a look at _Scheduler_Set() and assume that
the_thread is the currently executing thread:
RTEMS_INLINE_ROUTINE void _Scheduler_Set(
const Scheduler_Control *scheduler,
Thread_Control *the_thread
)
{
const Scheduler_Control *current_scheduler = _Scheduler_Get(
the_thread );
if ( current_scheduler != scheduler ) {
_Thread_Set_state( the_thread, STATES_MIGRATING );
<-- here we are still executing on this processor (thread
dispatching is disabled), but we have an heir for this processor
_Scheduler_Free( _Scheduler_Get( the_thread ), the_thread );
the_thread->scheduler = scheduler;
_Scheduler_Allocate( scheduler, the_thread );
_Scheduler_Update( scheduler, the_thread );
_Thread_Clear_state( the_thread, STATES_MIGRATING );
<-- here we are still executing on this processor (thread
dispatching is still disabled) and we may be an heir on another
processor which just received an inter-processor interrupt
}
}
<-- high level code will eventually call _Thread_Dispatch() and we
finish the migration
>---
> c/src/lib/libbsp/sparc/shared/irq_asm.S | 18 ++
> c/src/lib/libcpu/powerpc/new-exceptions/cpu.c | 4 +
> c/src/lib/libcpu/powerpc/new-exceptions/cpu_asm.S | 18 ++
> cpukit/libmisc/cpuuse/cpuusagereport.c | 2 +-
> cpukit/rtems/include/rtems/rtems/tasks.h | 2 -
> cpukit/rtems/src/tasksetscheduler.c | 5 +-
> cpukit/score/cpu/arm/cpu.c | 12 +
> cpukit/score/cpu/arm/cpu_asm.S | 18 ++
> cpukit/score/cpu/arm/rtems/score/cpu.h | 16 ++
> cpukit/score/cpu/i386/cpu.c | 18 ++
> cpukit/score/cpu/i386/cpu_asm.S | 29 ++-
> cpukit/score/cpu/i386/rtems/score/cpu.h | 27 ++
> cpukit/score/cpu/no_cpu/rtems/score/cpu.h | 18 ++
> cpukit/score/cpu/powerpc/cpu.c | 4 +
> cpukit/score/cpu/powerpc/rtems/score/cpu.h | 14 +-
> cpukit/score/cpu/sparc/cpu.c | 8 +
> cpukit/score/cpu/sparc/rtems/score/cpu.h | 13 +
> cpukit/score/include/rtems/score/percpu.h | 11 +-
> cpukit/score/include/rtems/score/scheduler.h | 2 +-
> cpukit/score/include/rtems/score/schedulerimpl.h | 24 +--
> cpukit/score/include/rtems/score/schedulersmp.h | 4 +-
> .../score/include/rtems/score/schedulersmpimpl.h | 33 ++-
> cpukit/score/include/rtems/score/statesimpl.h | 2 +
> cpukit/score/include/rtems/score/thread.h | 11 +
> cpukit/score/include/rtems/score/threadimpl.h | 35 +++-
> cpukit/score/src/smp.c | 5 +-
> cpukit/score/src/threaddispatch.c | 31 +--
> cpukit/score/src/threadhandler.c | 4 +-
> cpukit/score/src/threadinitialize.c | 6 +-
> cpukit/score/src/threadrestart.c | 2 +-
> cpukit/score/src/threadstartmultitasking.c | 18 +-
> doc/user/smp.t | 46 ++++
> testsuites/smptests/Makefile.am | 1 +
> testsuites/smptests/configure.ac | 1 +
> testsuites/smptests/smpmigration02/Makefile.am | 19 ++
> testsuites/smptests/smpmigration02/init.c | 255
++++++++++++++++++++
> .../smptests/smpmigration02/smpmigration02.doc | 12 +
> .../smptests/smpmigration02/smpmigration02.scn | 7 +
> testsuites/smptests/smpscheduler02/init.c | 4 +-
> 39 files changed, 679 insertions(+), 80 deletions(-)
> create mode 100644 testsuites/smptests/smpmigration02/Makefile.am
> create mode 100644 testsuites/smptests/smpmigration02/init.c
> create mode 100644 testsuites/smptests/smpmigration02/smpmigration02.doc
> create mode 100644 testsuites/smptests/smpmigration02/smpmigration02.scn
>
>diff --git a/c/src/lib/libbsp/sparc/shared/irq_asm.S
b/c/src/lib/libbsp/sparc/shared/irq_asm.S
>index 3a86ad5..cdaaf6d 100644
>--- a/c/src/lib/libbsp/sparc/shared/irq_asm.S
>+++ b/c/src/lib/libbsp/sparc/shared/irq_asm.S
>@@ -163,6 +163,21 @@ done_flushing:
> nop
> nop
>
>+#if defined(RTEMS_SMP)
>+ ! Indicate that this context is no longer executing
>+ stb %g0, [%o0 + SPARC_CONTEXT_CONTROL_IS_EXECUTING_OFFSET]
>+
The register usage should be documented at the declaration of
CPU_Context_restore_heir above. Probably in the other architectures it
should be documented as well.
%g0 is the zero register on SPARC and %o0 is the executing context, so I
don't think there is something to document here.
--
Sebastian Huber, embedded brains GmbH
Address : Dornierstr. 4, D-82178 Puchheim, Germany
Phone : +49 89 189 47 41-16
Fax : +49 89 189 47 41-09
E-Mail : [email protected]
PGP : Public key available on request.
Diese Nachricht ist keine geschäftliche Mitteilung im Sinne des EHUG.
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