X-Git-Url: https://codewiz.org/gitweb?a=blobdiff_plain;f=bertos%2Fkern%2Fpreempt.c;h=bdb6d3c9e716b543529b464167b8ecc04a6a0751;hb=d1b1e979862ca866eaee324e0363d57bf5a0e13b;hp=f068a8a82c24d314d159e84a045422de18b55475;hpb=87302355709b6d5ac0592024b84207ba86baa82e;p=bertos.git diff --git a/bertos/kern/preempt.c b/bertos/kern/preempt.c index f068a8a8..bdb6d3c9 100644 --- a/bertos/kern/preempt.c +++ b/bertos/kern/preempt.c @@ -26,55 +26,207 @@ * 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 + * Copyright 2008 Bernie Innocenti + * Copyright 2009 Andrea Righi * --> * - * \brief Simple realtime multitasking scheduler. - * Context switching is only done cooperatively. + * \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 preempt_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: proc.c 1616 2008-08-10 19:41:26Z bernie $ * \author Bernie Innocenti - * \author Stefano Fedrigo + * \author Andrea Righi */ +#include "cfg/cfg_proc.h" + #include "proc_p.h" #include "proc.h" +#include +#include +#include // CPU_IDLE +#include // IRQ_DISABLE()... +#include +#include +#include // CONFIG_DEPEND() + +// Check config dependencies +CONFIG_DEPEND(CONFIG_KERN_PREEMPT, CONFIG_KERN); + +MOD_DEFINE(preempt) + +/* Global preemption nesting counter */ +cpu_atomic_t preempt_count; /* - * The time sharing scheduler forces a task switch when the current - * process has exhausted its quantum. + * 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. */ -uint16_t Quantum; +int _proc_quantum; /** - * Disable preemptive task switching. + * Define function prototypes exported outside. * - * The scheduler maintains a per-process nesting counter. Task switching is - * effectively re-enabled only when the number of calls to proc_permit() - * matches the number of calls to proc_forbid(). + * Required to silent gcc "no previous prototype" warnings. + */ +void preempt_yield(void); +int preempt_needPreempt(void); +void preempt_preempt(void); +void preempt_switch(void); +void preempt_wakeup(Process *proc); +void preempt_init(void); + +static void preempt_switchTo(Process *proc) +{ + Process *old_process = current_process; + + SCHED_ENQUEUE(current_process); + _proc_quantum = CONFIG_KERN_QUANTUM; + current_process = proc; + proc_switchTo(current_process, old_process); +} + +/** + * Call the scheduler and eventually replace the current running process. + */ +static void preempt_schedule(void) +{ + _proc_quantum = CONFIG_KERN_QUANTUM; + proc_schedule(); +} + +/** + * Check if we need to schedule another task + */ +int preempt_needPreempt(void) +{ + if (UNLIKELY(current_process == NULL)) + return 0; + if (!proc_preemptAllowed()) + return 0; + if (LIST_EMPTY(&proc_ready_list)) + return 0; + return _proc_quantum ? prio_next() > prio_curr() : + prio_next() >= prio_curr(); +} + +/** + * Preempt the current task. + */ +void preempt_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_schedule(); +} + +/** + * Give the control of the CPU to another process. * - * Calling functions that could sleep while task switching is disabled - * is dangerous, although supported. Preemptive task switching is - * resumed while the process is sleeping and disabled again as soon as - * it wakes up again. + * \note Assume the current process has been already added to a wait queue. * - * \sa proc_permit() + * \warning This should be considered an internal kernel function, even if it + * is allowed, usage from application code is strongly discouraged. + */ +void preempt_switch(void) +{ + ASSERT(proc_preemptAllowed()); + + ATOMIC(preempt_schedule()); +} + +/** + * Immediately wakeup a process, dispatching it to the CPU. */ -void proc_forbid(void) +void preempt_wakeup(Process *proc) { - /* No need to protect against interrupts here. */ - ++CurrentProcess->forbid_cnt; + ASSERT(proc_preemptAllowed()); + ASSERT(current_process); + IRQ_ASSERT_DISABLED(); + + if (prio_proc(proc) >= prio_curr()) + preempt_switchTo(proc); + else + SCHED_ENQUEUE_HEAD(proc); } /** - * Re-enable preemptive task switching. - * - * \sa proc_forbid() + * Voluntarily release the CPU. */ -void proc_permit(void) +void preempt_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) + preempt_switchTo(proc); + IRQ_ENABLE; +} + +void preempt_init(void) { - /* No need to protect against interrupts here. */ - --CurrentProcess->forbid_cnt; + MOD_INIT(preempt); }