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author | Sebastian Huber <sebastian.huber@embedded-brains.de> | 2015-09-25 14:34:24 +0200 |
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committer | Sebastian Huber <sebastian.huber@embedded-brains.de> | 2015-09-28 13:56:57 +0200 |
commit | 258ad71e9626c16f30b40e06c321326636c976ff (patch) | |
tree | da6e210947d590159796434bf04cf364247ac20a /cpukit/score/include/rtems | |
parent | SMP: Simplify thread lock operations (diff) | |
download | rtems-258ad71e9626c16f30b40e06c321326636c976ff.tar.bz2 |
SMP: Fix and optimize thread dispatching
According to the C11 and C++11 memory models only a read-modify-write
operation guarantees that we read the last value written in modification
order. Avoid the sequential consistent thread fence and instead use the
inter-processor interrupt to set the thread dispatch necessary
indicator.
Diffstat (limited to '')
-rw-r--r-- | cpukit/score/include/rtems/score/percpu.h | 19 | ||||
-rw-r--r-- | cpukit/score/include/rtems/score/smpimpl.h | 6 | ||||
-rw-r--r-- | cpukit/score/include/rtems/score/threadimpl.h | 45 |
3 files changed, 29 insertions, 41 deletions
diff --git a/cpukit/score/include/rtems/score/percpu.h b/cpukit/score/include/rtems/score/percpu.h index f1dad90ccc..806c290b7c 100644 --- a/cpukit/score/include/rtems/score/percpu.h +++ b/cpukit/score/include/rtems/score/percpu.h @@ -279,9 +279,11 @@ typedef struct Per_CPU_Control { * @brief This is the heir thread for this processor. * * This field is not protected by a lock. The only writer after multitasking - * start is the scheduler owning this processor. This processor will set the - * dispatch necessary indicator to false, before it reads the heir. This - * field is used in combination with the dispatch necessary indicator. + * start is the scheduler owning this processor. It is assumed that stores + * to pointers are atomic on all supported SMP architectures. The CPU port + * specific code (inter-processor interrupt handling and + * _CPU_SMP_Send_interrupt()) must guarantee that this processor observes the + * last value written. * * A thread can be a heir on at most one processor in the system. * @@ -290,16 +292,15 @@ typedef struct Per_CPU_Control { struct _Thread_Control *heir; /** - * @brief This is set to true when this processor needs to run the + * @brief This is set to true when this processor needs to run the thread * dispatcher. * * It is volatile since interrupts may alter this flag. * - * This field is not protected by a lock. There are two writers after - * multitasking start. The scheduler owning this processor sets this - * indicator to true, after it updated the heir field. This processor sets - * this indicator to false, before it reads the heir. This field is used in - * combination with the heir field. + * This field is not protected by a lock and must be accessed only by this + * processor. Code (e.g. scheduler and post-switch action requests) running + * on another processors must use an inter-processor interrupt to set the + * thread dispatch necessary indicator to true. * * @see _Thread_Get_heir_and_make_it_executing(). */ diff --git a/cpukit/score/include/rtems/score/smpimpl.h b/cpukit/score/include/rtems/score/smpimpl.h index 97c78b02b0..3167e82a82 100644 --- a/cpukit/score/include/rtems/score/smpimpl.h +++ b/cpukit/score/include/rtems/score/smpimpl.h @@ -146,6 +146,12 @@ static inline void _SMP_Inter_processor_interrupt_handler( void ) { Per_CPU_Control *cpu_self = _Per_CPU_Get(); + /* + * In the common case the inter-processor interrupt is issued to carry out a + * thread dispatch. + */ + cpu_self->dispatch_necessary = true; + if ( _Atomic_Load_ulong( &cpu_self->message, ATOMIC_ORDER_RELAXED ) != 0 ) { unsigned long message = _Atomic_Exchange_ulong( &cpu_self->message, diff --git a/cpukit/score/include/rtems/score/threadimpl.h b/cpukit/score/include/rtems/score/threadimpl.h index 68c26c388f..7412bd9633 100644 --- a/cpukit/score/include/rtems/score/threadimpl.h +++ b/cpukit/score/include/rtems/score/threadimpl.h @@ -11,7 +11,7 @@ * COPYRIGHT (c) 1989-2008. * On-Line Applications Research Corporation (OAR). * - * Copyright (c) 2014 embedded brains GmbH. + * Copyright (c) 2014-2015 embedded brains GmbH. * * The license and distribution terms for this file may be * found in the file LICENSE in this distribution or at @@ -793,7 +793,8 @@ RTEMS_INLINE_ROUTINE Thread_Control *_Thread_Internal_allocate( void ) /** * @brief Gets the heir of the processor and makes it executing. * - * The thread dispatch necessary indicator is cleared as a side-effect. + * Must be called with interrupts disabled. The thread dispatch necessary + * indicator is cleared as a side-effect. * * @return The heir thread. * @@ -806,18 +807,8 @@ RTEMS_INLINE_ROUTINE Thread_Control *_Thread_Get_heir_and_make_it_executing( { Thread_Control *heir; - cpu_self->dispatch_necessary = false; - -#if defined( RTEMS_SMP ) - /* - * It is critical that we first update the dispatch necessary and then the - * read the heir so that we don't miss an update by - * _Thread_Dispatch_update_heir(). - */ - _Atomic_Fence( ATOMIC_ORDER_SEQ_CST ); -#endif - heir = cpu_self->heir; + cpu_self->dispatch_necessary = false; cpu_self->executing = heir; return heir; @@ -832,23 +823,10 @@ RTEMS_INLINE_ROUTINE void _Thread_Dispatch_update_heir( { cpu_for_heir->heir = heir; - /* - * It is critical that we first update the heir and then the dispatch - * necessary so that _Thread_Get_heir_and_make_it_executing() cannot miss an - * update. - */ - _Atomic_Fence( ATOMIC_ORDER_SEQ_CST ); - - /* - * Only update the dispatch necessary indicator if not already set to - * avoid superfluous inter-processor interrupts. - */ - if ( !cpu_for_heir->dispatch_necessary ) { - cpu_for_heir->dispatch_necessary = true; - - if ( cpu_for_heir != cpu_self ) { - _Per_CPU_Send_interrupt( cpu_for_heir ); - } + if ( cpu_for_heir == cpu_self ) { + cpu_self->dispatch_necessary = true; + } else { + _Per_CPU_Send_interrupt( cpu_for_heir ); } } #endif @@ -930,12 +908,15 @@ RTEMS_INLINE_ROUTINE void _Thread_Add_post_switch_action( ISR_Level level; cpu_of_thread = _Thread_Action_ISR_disable_and_acquire( thread, &level ); - cpu_of_thread->dispatch_necessary = true; #if defined(RTEMS_SMP) - if ( _Per_CPU_Get() != cpu_of_thread ) { + if ( _Per_CPU_Get() == cpu_of_thread ) { + cpu_of_thread->dispatch_necessary = true; + } else { _Per_CPU_Send_interrupt( cpu_of_thread ); } +#else + cpu_of_thread->dispatch_necessary = true; #endif _Chain_Append_if_is_off_chain_unprotected( |