* the GNU General Public License.
*
* Copyright 2008 Bernie Innocenti <bernie@codewiz.org>
+ * Copyright 2009 Andrea Righi <arighi@develer.com>
* -->
*
* \brief Simple preemptive multitasking scheduler.
*
- * All voluntary and preemptive context switching happens on exit from
- * a common interrupt (signal) dispatcher. Preemption on quantum timeout
- * is regulated by a soft-timer. Other kinds of preemption could happen
- * if an interrupt sends a signal to a higher priority process (but this
- * is still unimplemented).
+ * 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.
*
- * In the POSIX implementaiton, context switching is done by the portable
- * SVR4 swapcontext() facility.
+ * 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.
*
- * \version $Id$
* \author Bernie Innocenti <bernie@codewiz.org>
+ * \author Andrea Righi <arighi@develer.com>
*/
#include "cfg/cfg_proc.h"
#include "proc_p.h"
#include "proc.h"
-#include "idle.h"
#include <kern/irq.h>
#include <kern/monitor.h>
+#include <kern/idle.h> // idle_proc
#include <cpu/frame.h> // CPU_IDLE
#include <cpu/irq.h> // IRQ_DISABLE()...
-#include <drv/timer.h>
+#include <cfg/log.h>
#include <cfg/module.h>
#include <cfg/depend.h> // CONFIG_DEPEND()
// Check config dependencies
-CONFIG_DEPEND(CONFIG_KERN_PREEMPT, CONFIG_KERN && CONFIG_TIMER_EVENTS && CONFIG_KERN_IRQ);
+CONFIG_DEPEND(CONFIG_KERN_PREEMPT, CONFIG_KERN);
MOD_DEFINE(preempt)
-/// Global preemption disabling nesting counter
-cpu_atomic_t _preempt_forbid_cnt;
+/**
+ * CPU dependent context switching routines.
+ *
+ * Saving and restoring the context on the stack is done by a CPU-dependent
+ * support routine which usually needs to be written in assembly.
+ */
+EXTERN_C void asm_switch_context(cpu_stack_t **new_sp, cpu_stack_t **save_sp);
-static Timer preempt_timer;
+/* 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;
-void proc_schedule(void)
+/**
+ * Call the scheduler and eventually replace the current running process.
+ */
+static void proc_schedule(void)
{
- IRQ_DISABLE;
-
- ASSERT(proc_preemptAllowed());
- LIST_ASSERT_VALID(&ProcReadyList);
- CurrentProcess = (struct Process *)list_remHead(&ProcReadyList);
- ASSERT2(CurrentProcess, "no idle proc?");
-
- IRQ_ENABLE;
+ Process *old_process = current_process;
+
+ IRQ_ASSERT_DISABLED();
+
+ /* Poll on the ready queue for the first ready process */
+ LIST_ASSERT_VALID(&proc_ready_list);
+ current_process = (Process *)list_remHead(&proc_ready_list);
+ if (UNLIKELY(!current_process))
+ current_process = idle_proc;
+ _proc_quantum = CONFIG_KERN_QUANTUM;
+ /*
+ * Optimization: don't switch contexts when the active process has not
+ * changed.
+ */
+ if (LIKELY(old_process != current_process))
+ {
+ cpu_stack_t *dummy;
+
+ /*
+ * Save context of old process and switch to new process. 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.
+ *
+ * \todo Instead of physically clearing the process at exit
+ * time, a zombie list should be created.
+ */
+ asm_switch_context(¤t_process->stack,
+ old_process ? &old_process->stack : &dummy);
+ }
- TRACEMSG("launching %p:%s", CurrentProcess, proc_currentName());
+ /* This RET resumes the execution on the new process */
+ LOG_INFO("resuming %p:%s\n", current_process, proc_currentName());
}
-void proc_preempt(UNUSED_ARG(void *, param))
+/**
+ * Check if we need to schedule another task
+ */
+int proc_needPreempt(void)
{
- if (proc_preemptAllowed())
- {
- IRQ_DISABLE;
-
- #if CONFIG_KERN_PRI
- Process *rival = (Process *)LIST_HEAD(&ProcReadyList);
- if (rival && rival->link.pri >= CurrentProcess->link.pri)
- {
- #endif
-
- TRACEMSG("preempting %p:%s", CurrentProcess, proc_currentName());
-
-// FIXME: this still breaks havoc, probably because of some reentrancy issue
-#if 0
- SCHED_ENQUEUE(CurrentProcess);
- proc_schedule();
-#endif
- #if CONFIG_KERN_PRI
- }
- #endif
-
- IRQ_ENABLE;
- }
+ if (UNLIKELY(current_process == NULL))
+ return 0;
+ if (!proc_preemptAllowed())
+ return 0;
+ return _proc_quantum ? prio_next() > prio_curr() :
+ prio_next() >= prio_curr();
+}
- timer_setDelay(&preempt_timer, CONFIG_KERN_QUANTUM);
- timer_add(&preempt_timer);
+/**
+ * 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 */
+ if (current_process != idle_proc)
+ SCHED_ENQUEUE(current_process);
+ proc_schedule();
}
+/**
+ * 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)
{
- ATOMIC(LIST_ASSERT_VALID(&ProcReadyList));
- TRACEMSG("%p:%s", CurrentProcess, proc_currentName());
- ATOMIC(LIST_ASSERT_VALID(&ProcReadyList));
-
- /* Sleeping with IRQs disabled or preemption forbidden is illegal */
- IRQ_ASSERT_ENABLED();
ASSERT(proc_preemptAllowed());
- // Will invoke proc_switch() in interrupt context
- kill(0, SIGUSR1);
+ ATOMIC(proc_schedule());
}
+/**
+ * Voluntarily release the CPU.
+ */
void proc_yield(void)
{
- TRACEMSG("%p:%s", CurrentProcess, proc_currentName());
-
- IRQ_DISABLE;
- SCHED_ENQUEUE(CurrentProcess);
- IRQ_ENABLE;
-
- proc_switch();
-}
+ /*
+ * Voluntary preemption while preemption is disabled is considered
+ * illegal, as not very useful in practice.
+ *
+ * ASSERT if it happens.
+ */
+ ASSERT(proc_preemptAllowed());
-void proc_entry(void (*user_entry)(void))
-{
- user_entry();
- proc_exit();
+ ATOMIC(
+ SCHED_ENQUEUE(current_process);
+ proc_schedule();
+ );
}
void preempt_init(void)
{
- MOD_CHECK(irq);
- MOD_CHECK(timer);
-
- irq_register(SIGUSR1, proc_schedule);
-
- timer_setSoftint(&preempt_timer, proc_preempt, NULL);
- timer_setDelay(&preempt_timer, CONFIG_KERN_QUANTUM);
- timer_add(&preempt_timer);
-
idle_init();
-
MOD_INIT(preempt);
}
projects / bertos.git / blobdiff