* invalidate any other reasons why the executable file might be covered by
* the GNU General Public License.
*
- * Copyright 2001, 2004 Develer S.r.l. (http://www.develer.com/)
- * Copyright 1999, 2000, 2001, 2008 Bernie Innocenti <bernie@codewiz.org>
+ * \brief Simple preemptive multitasking scheduler.
+ *
+ * Preemption is explicitly regulated at the exit of each interrupt service
+ * routine (ISR). Each task obtains a time quantum as soon as it is scheduled
+ * on the CPU and its quantum is decremented at each clock tick. The frequency
+ * of the timer determines the system tick granularity and CONFIG_KERN_QUANTUM
+ * the time sharing interval.
+ *
+ * When the quantum expires the handler proc_needPreempt() checks if the
+ * preemption is enabled and in this case proc_schedule() is called, that
+ * possibly replaces the current running thread with a different one.
+ *
+ * The preemption can be disabled or enabled via proc_forbid() and
+ * proc_permit() primitives. This is implemented using a global atomic counter.
+ * When the counter is greater than 0 the task cannot be preempted; only when
+ * the counter reaches 0 the task can be preempted again.
+ *
+ * Preemption-disabled sections may be nested. The preemption will be
+ * re-enabled when the outermost preemption-disabled section completes.
+ *
+ * The voluntary preemption still happens via proc_switch() or proc_yield().
+ * The first one assumes the current process has been already added to a
+ * private wait queue (e.g., on a semaphore or a signal), while the second one
+ * takes care of adding the process into the ready queue.
+ *
+ * Context switch is done by CPU-dependent support routines. In case of a
+ * voluntary preemption the context switch routine must take care of
+ * saving/restoring only the callee-save registers (the voluntary-preemption is
+ * actually a function call). The kernel-preemption always happens inside a
+ * signal/interrupt context and it must take care of saving all registers. For
+ * this, in the entry point of each ISR the caller-save registers must be
+ * saved. In the ISR exit point, if the context switch must happen, we switch
+ * to user-context and call the same voluntary context switch routine that take
+ * care of saving/restoring also the callee-save registers. On resume from the
+ * switch, the interrupt exit point moves back to interrupt-context, resumes
+ * the caller-save registers (saved in the ISR entry point) and return from the
+ * interrupt-context.
+ *
+ * \note Thread priority (if enabled by CONFIG_KERN_PRI) defines the order in
+ * the \p proc_ready_list and the capability to deschedule a running process. A
+ * low-priority thread can't preempt a high-priority thread.
+ *
+ * A high-priority process can preempt a low-priority process immediately (it
+ * will be descheduled and replaced in the interrupt exit point). Processes
+ * running at the same priority can be descheduled when they expire the time
+ * quantum.
+ *
+ * \note Sleeping while preemption is disabled fallbacks to a busy-wait sleep.
+ * Voluntary preemption when preemption is disabled raises a kernel bug.
+ *
* -->
*
- * \brief Simple cooperative multitasking scheduler.
+ * \brief Simple cooperative and preemptive multitasking scheduler.
*
* \author Bernie Innocenti <bernie@codewiz.org>
* \author Stefano Fedrigo <aleph@develer.com>
+ * \author Andrea Righi <arighi@develer.com>
*/
#include "proc_p.h"
#endif /* CONFIG_KERN_HEAP */
+/*
+ * Check if the process context switch can be performed directly by the
+ * architecture-dependent asm_switch_context() or if it must be delayed
+ * because we're in the middle of an ISR.
+ *
+ * Return true if asm_switch_context() can be executed, false
+ * otherwise.
+ *
+ * NOTE: if an architecture does not implement IRQ_RUNNING() this function
+ * always returns true.
+ */
+#define CONTEXT_SWITCH_FROM_ISR() (!IRQ_RUNNING())
+
+/*
+ * Save context of old process and switch to new process.
+ */
+static void proc_context_switch(Process *next, Process *prev)
+{
+ cpu_stack_t *dummy;
+
+ if (UNLIKELY(next == prev))
+ return;
+ /*
+ * If there is no old process, we save the old stack pointer into a
+ * dummy variable that we ignore. In fact, this happens only when the
+ * old process has just exited.
+ */
+ asm_switch_context(&next->stack, prev ? &prev->stack : &dummy);
+}
+
static void proc_initStruct(Process *proc)
{
/* Avoid warning for unused argument. */
/**
* Call the scheduler and eventually replace the current running process.
*/
-void proc_schedule(void)
+static void proc_schedule(void)
{
Process *old_process = current_process;
IRQ_DISABLE;
}
if (CONTEXT_SWITCH_FROM_ISR())
- proc_switchTo(current_process, old_process);
+ proc_context_switch(current_process, old_process);
/* This RET resumes the execution on the new process */
LOG_INFO("resuming %p:%s\n", current_process, proc_currentName());
}
+
+#if CONFIG_KERN_PREEMPT
+/* Global preemption nesting counter */
+cpu_atomic_t preempt_count;
+
+/*
+ * The time sharing interval: when a process is scheduled on a CPU it gets an
+ * amount of CONFIG_KERN_QUANTUM clock ticks. When these ticks expires and
+ * preemption is enabled a new process is selected to run.
+ */
+int _proc_quantum;
+
+/**
+ * Check if we need to schedule another task
+ */
+bool proc_needPreempt(void)
+{
+ if (UNLIKELY(current_process == NULL))
+ return false;
+ if (!proc_preemptAllowed())
+ return false;
+ if (LIST_EMPTY(&proc_ready_list))
+ return false;
+ return preempt_quantum() ? prio_next() > prio_curr() :
+ prio_next() >= prio_curr();
+}
+
+/**
+ * Preempt the current task.
+ */
+void proc_preempt(void)
+{
+ IRQ_ASSERT_DISABLED();
+ ASSERT(current_process);
+
+ /* Perform the kernel preemption */
+ LOG_INFO("preempting %p:%s\n", current_process, proc_currentName());
+ /* We are inside a IRQ context, so ATOMIC is not needed here */
+ SCHED_ENQUEUE(current_process);
+ preempt_reset_quantum();
+ proc_schedule();
+}
+#endif /* CONFIG_KERN_PREEMPT */
+
+/* Immediately switch to a particular process */
+static void proc_switchTo(Process *proc)
+{
+ Process *old_process = current_process;
+
+ SCHED_ENQUEUE(current_process);
+ preempt_reset_quantum();
+ current_process = proc;
+ proc_context_switch(current_process, old_process);
+}
+
+/**
+ * Give the control of the CPU to another process.
+ *
+ * \note Assume the current process has been already added to a wait queue.
+ *
+ * \warning This should be considered an internal kernel function, even if it
+ * is allowed, usage from application code is strongly discouraged.
+ */
+void proc_switch(void)
+{
+ ASSERT(proc_preemptAllowed());
+ ATOMIC(
+ preempt_reset_quantum();
+ proc_schedule();
+ );
+}
+
+/**
+ * Immediately wakeup a process, dispatching it to the CPU.
+ */
+void proc_wakeup(Process *proc)
+{
+ ASSERT(proc_preemptAllowed());
+ ASSERT(current_process);
+ IRQ_ASSERT_DISABLED();
+
+ if (prio_proc(proc) >= prio_curr())
+ proc_switchTo(proc);
+ else
+ SCHED_ENQUEUE_HEAD(proc);
+}
+
+/**
+ * Voluntarily release the CPU.
+ */
+void proc_yield(void)
+{
+ Process *proc;
+
+ /*
+ * Voluntary preemption while preemption is disabled is considered
+ * illegal, as not very useful in practice.
+ *
+ * ASSERT if it happens.
+ */
+ ASSERT(proc_preemptAllowed());
+ IRQ_ASSERT_ENABLED();
+
+ IRQ_DISABLE;
+ proc = (struct Process *)list_remHead(&proc_ready_list);
+ if (proc)
+ proc_switchTo(proc);
+ IRQ_ENABLE;
+}
projects / bertos.git / blobdiff