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><H1
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><H1
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><A
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><A
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NAME="KERNEL-SCHEDCONTROL">Scheduler Control</H1
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NAME="KERNEL-SCHEDCONTROL">Scheduler Control</H1
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><DIV
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><DIV
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CLASS="REFNAMEDIV"
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CLASS="REFNAMEDIV"
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><A
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><A
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NAME="AEN1784"
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NAME="AEN1784"
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></A
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></A
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><H2
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><H2
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>Name</H2
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>Name</H2
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>cyg_scheduler_start, cyg_scheduler_lock, cyg_scheduler_unlock, cyg_scheduler_safe_lock, cyg_scheduler_read_lock -- Control the state of the scheduler</DIV
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>cyg_scheduler_start, cyg_scheduler_lock, cyg_scheduler_unlock, cyg_scheduler_safe_lock, cyg_scheduler_read_lock -- Control the state of the scheduler</DIV
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><DIV
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><DIV
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CLASS="REFSYNOPSISDIV"
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CLASS="REFSYNOPSISDIV"
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><A
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><A
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NAME="AEN1791"><H2
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NAME="AEN1791"><H2
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>Synopsis</H2
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>Synopsis</H2
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><DIV
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><DIV
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CLASS="FUNCSYNOPSIS"
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CLASS="FUNCSYNOPSIS"
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><A
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><TD
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><PRE
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><PRE
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CLASS="FUNCSYNOPSISINFO"
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CLASS="FUNCSYNOPSISINFO"
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>#include <cyg/kernel/kapi.h>
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>#include <cyg/kernel/kapi.h>
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</PRE
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</PRE
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></TD
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></TD
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></TR
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><P
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><P
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><CODE
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><CODE
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><CODE
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><CODE
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CLASS="FUNCDEF"
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CLASS="FUNCDEF"
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>void cyg_scheduler_start</CODE
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>void cyg_scheduler_start</CODE
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>(void);</CODE
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>(void);</CODE
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></P
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></P
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><P
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><P
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><CODE
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><CODE
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><CODE
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><CODE
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CLASS="FUNCDEF"
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CLASS="FUNCDEF"
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>void cyg_scheduler_lock</CODE
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>void cyg_scheduler_lock</CODE
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>(void);</CODE
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>(void);</CODE
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></P
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></P
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><P
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><P
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><CODE
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><CODE
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><CODE
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><CODE
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CLASS="FUNCDEF"
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CLASS="FUNCDEF"
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>void cyg_scheduler_unlock</CODE
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>void cyg_scheduler_unlock</CODE
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>(void);</CODE
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>(void);</CODE
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></P
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></P
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><P
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><P
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><CODE
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><CODE
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><CODE
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><CODE
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CLASS="FUNCDEF"
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CLASS="FUNCDEF"
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>cyg_ucount32 cyg_scheduler_read_lock</CODE
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>cyg_ucount32 cyg_scheduler_read_lock</CODE
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>(void);</CODE
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>(void);</CODE
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></P
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></P
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><P
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><P
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></P
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></P
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></DIV
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></DIV
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></DIV
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></DIV
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><DIV
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><DIV
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CLASS="REFSECT1"
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CLASS="REFSECT1"
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><A
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><A
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NAME="KERNEL-SCHEDCONTROL-DESCRIPTION"
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NAME="KERNEL-SCHEDCONTROL-DESCRIPTION"
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></A
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></A
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><H2
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><H2
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>Description</H2
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>Description</H2
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><P
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><P
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><TT
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><TT
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CLASS="FUNCTION"
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CLASS="FUNCTION"
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>cyg_scheduler_start</TT
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>cyg_scheduler_start</TT
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> should only be called once,
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> should only be called once,
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to mark the end of system initialization. In typical configurations it
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to mark the end of system initialization. In typical configurations it
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is called automatically by the system startup, but some applications
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is called automatically by the system startup, but some applications
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may bypass the standard startup in which case
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may bypass the standard startup in which case
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<TT
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<TT
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CLASS="FUNCTION"
|
CLASS="FUNCTION"
|
>cyg_scheduler_start</TT
|
>cyg_scheduler_start</TT
|
> will have to be called
|
> will have to be called
|
explicitly. The call will enable system interrupts, allowing I/O
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explicitly. The call will enable system interrupts, allowing I/O
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operations to commence. Then the scheduler will be invoked and control
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operations to commence. Then the scheduler will be invoked and control
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will be transferred to the highest priority runnable thread. The call
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will be transferred to the highest priority runnable thread. The call
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will never return.
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will never return.
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</P
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</P
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><P
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><P
|
>The various data structures inside the eCos kernel must be protected
|
>The various data structures inside the eCos kernel must be protected
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against concurrent updates. Consider a call to
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against concurrent updates. Consider a call to
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<TT
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<TT
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CLASS="FUNCTION"
|
CLASS="FUNCTION"
|
>cyg_semaphore_post</TT
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>cyg_semaphore_post</TT
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> which causes a thread to be
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> which causes a thread to be
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woken up: the semaphore data structure must be updated to remove the
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woken up: the semaphore data structure must be updated to remove the
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thread from its queue; the scheduler data structure must also be
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thread from its queue; the scheduler data structure must also be
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updated to mark the thread as runnable; it is possible that the newly
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updated to mark the thread as runnable; it is possible that the newly
|
runnable thread has a higher priority than the current one, in which
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runnable thread has a higher priority than the current one, in which
|
case preemption is required. If in the middle of the semaphore post
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case preemption is required. If in the middle of the semaphore post
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call an interrupt occurred and the interrupt handler tried to
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call an interrupt occurred and the interrupt handler tried to
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manipulate the same data structures, for example by making another
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manipulate the same data structures, for example by making another
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thread runnable, then it is likely that the structures will be left in
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thread runnable, then it is likely that the structures will be left in
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an inconsistent state and the system will fail.
|
an inconsistent state and the system will fail.
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</P
|
</P
|
><P
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><P
|
>To prevent such problems the kernel contains a special lock known as
|
>To prevent such problems the kernel contains a special lock known as
|
the scheduler lock. A typical kernel function such as
|
the scheduler lock. A typical kernel function such as
|
<TT
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<TT
|
CLASS="FUNCTION"
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CLASS="FUNCTION"
|
>cyg_semaphore_post</TT
|
>cyg_semaphore_post</TT
|
> will claim the scheduler lock,
|
> will claim the scheduler lock,
|
do all its manipulation of kernel data structures, and then release
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do all its manipulation of kernel data structures, and then release
|
the scheduler lock. The current thread cannot be preempted while it
|
the scheduler lock. The current thread cannot be preempted while it
|
holds the scheduler lock. If an interrupt occurs and a DSR is supposed
|
holds the scheduler lock. If an interrupt occurs and a DSR is supposed
|
to run to signal that some event has occurred, that DSR is postponed
|
to run to signal that some event has occurred, that DSR is postponed
|
until the scheduler unlock operation. This prevents concurrent updates
|
until the scheduler unlock operation. This prevents concurrent updates
|
of kernel data structures.
|
of kernel data structures.
|
</P
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</P
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><P
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><P
|
>The kernel exports three routines for manipulating the scheduler lock.
|
>The kernel exports three routines for manipulating the scheduler lock.
|
<TT
|
<TT
|
CLASS="FUNCTION"
|
CLASS="FUNCTION"
|
>cyg_scheduler_lock</TT
|
>cyg_scheduler_lock</TT
|
> can be called to claim the
|
> can be called to claim the
|
lock. On return it is guaranteed that the current thread will not be
|
lock. On return it is guaranteed that the current thread will not be
|
preempted, and that no other code is manipulating any kernel data
|
preempted, and that no other code is manipulating any kernel data
|
structures. <TT
|
structures. <TT
|
CLASS="FUNCTION"
|
CLASS="FUNCTION"
|
>cyg_scheduler_unlock</TT
|
>cyg_scheduler_unlock</TT
|
> can be used to
|
> can be used to
|
release the lock, which may cause the current thread to be preempted.
|
release the lock, which may cause the current thread to be preempted.
|
<TT
|
<TT
|
CLASS="FUNCTION"
|
CLASS="FUNCTION"
|
>cyg_scheduler_read_lock</TT
|
>cyg_scheduler_read_lock</TT
|
> can be used to query the
|
> can be used to query the
|
current state of the scheduler lock. This function should never be
|
current state of the scheduler lock. This function should never be
|
needed because well-written code should always know whether or not the
|
needed because well-written code should always know whether or not the
|
scheduler is currently locked, but may prove useful during debugging.
|
scheduler is currently locked, but may prove useful during debugging.
|
</P
|
</P
|
><P
|
><P
|
>The implementation of the scheduler lock involves a simple counter.
|
>The implementation of the scheduler lock involves a simple counter.
|
Code can call <TT
|
Code can call <TT
|
CLASS="FUNCTION"
|
CLASS="FUNCTION"
|
>cyg_scheduler_lock</TT
|
>cyg_scheduler_lock</TT
|
> multiple times,
|
> multiple times,
|
causing the counter to be incremented each time, as long as
|
causing the counter to be incremented each time, as long as
|
<TT
|
<TT
|
CLASS="FUNCTION"
|
CLASS="FUNCTION"
|
>cyg_scheduler_unlock</TT
|
>cyg_scheduler_unlock</TT
|
> is called the same number of
|
> is called the same number of
|
times. This behaviour is different from mutexes where an attempt by a
|
times. This behaviour is different from mutexes where an attempt by a
|
thread to lock a mutex multiple times will result in deadlock or an
|
thread to lock a mutex multiple times will result in deadlock or an
|
assertion failure.
|
assertion failure.
|
</P
|
</P
|
><P
|
><P
|
>Typical application code should not use the scheduler lock. Instead
|
>Typical application code should not use the scheduler lock. Instead
|
other synchronization primitives such as mutexes and semaphores should
|
other synchronization primitives such as mutexes and semaphores should
|
be used. While the scheduler is locked the current thread cannot be
|
be used. While the scheduler is locked the current thread cannot be
|
preempted, so any higher priority threads will not be able to run.
|
preempted, so any higher priority threads will not be able to run.
|
Also no DSRs can run, so device drivers may not be able to service
|
Also no DSRs can run, so device drivers may not be able to service
|
I/O requests. However there is one situation where locking the
|
I/O requests. However there is one situation where locking the
|
scheduler is appropriate: if some data structure needs to be shared
|
scheduler is appropriate: if some data structure needs to be shared
|
between an application thread and a DSR associated with some interrupt
|
between an application thread and a DSR associated with some interrupt
|
source, the thread can use the scheduler lock to prevent concurrent
|
source, the thread can use the scheduler lock to prevent concurrent
|
invocations of the DSR and then safely manipulate the structure. It is
|
invocations of the DSR and then safely manipulate the structure. It is
|
desirable that the scheduler lock is held for only a short period of
|
desirable that the scheduler lock is held for only a short period of
|
time, typically some tens of instructions. In exceptional cases there
|
time, typically some tens of instructions. In exceptional cases there
|
may also be some performance-critical code where it is more
|
may also be some performance-critical code where it is more
|
appropriate to use the scheduler lock rather than a mutex, because the
|
appropriate to use the scheduler lock rather than a mutex, because the
|
former is more efficient.
|
former is more efficient.
|
</P
|
</P
|
></DIV
|
></DIV
|
><DIV
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><DIV
|
CLASS="REFSECT1"
|
CLASS="REFSECT1"
|
><A
|
><A
|
NAME="KERNEL-SCHEDCONTROL-CONTEXT"
|
NAME="KERNEL-SCHEDCONTROL-CONTEXT"
|
></A
|
></A
|
><H2
|
><H2
|
>Valid contexts</H2
|
>Valid contexts</H2
|
><P
|
><P
|
><TT
|
><TT
|
CLASS="FUNCTION"
|
CLASS="FUNCTION"
|
>cyg_scheduler_start</TT
|
>cyg_scheduler_start</TT
|
> can only be called during
|
> can only be called during
|
system initialization, since it marks the end of that phase. The
|
system initialization, since it marks the end of that phase. The
|
remaining functions may be called from thread or DSR context. Locking
|
remaining functions may be called from thread or DSR context. Locking
|
the scheduler from inside the DSR has no practical effect because the
|
the scheduler from inside the DSR has no practical effect because the
|
lock is claimed automatically by the interrupt subsystem before
|
lock is claimed automatically by the interrupt subsystem before
|
running DSRs, but allows functions to be shared between normal thread
|
running DSRs, but allows functions to be shared between normal thread
|
code and DSRs.
|
code and DSRs.
|
</P
|
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