X-Git-Url: https://codewiz.org/gitweb?a=blobdiff_plain;f=bertos%2Fkern%2Fpreempt.c;h=f77dd85eff1c632bbd73adf1053b350140cfc1ca;hb=938512200c9fbd8e08c88dbdddbdc1f030d3b663;hp=7ecd481d0cdf57c6d4c3679fc4ccfa43f5cc794e;hpb=cfa8814ca45e8bae9b36713bc534ecb581834016;p=bertos.git diff --git a/bertos/kern/preempt.c b/bertos/kern/preempt.c index 7ecd481d..f77dd85e 100644 --- a/bertos/kern/preempt.c +++ b/bertos/kern/preempt.c @@ -27,151 +27,176 @@ * the GNU General Public License. * * Copyright 2008 Bernie Innocenti + * Copyright 2009 Andrea Righi * --> * * \brief Simple preemptive multitasking scheduler. * - * \version $Id: proc.c 1616 2008-08-10 19:41:26Z bernie $ + * 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. + * * \author Bernie Innocenti + * \author Andrea Righi */ +#include "cfg/cfg_proc.h" + #include "proc_p.h" #include "proc.h" #include #include #include // CPU_IDLE -#include +#include // IRQ_DISABLE()... +#include #include +#include // CONFIG_DEPEND() +// Check config dependencies +CONFIG_DEPEND(CONFIG_KERN_PREEMPT, CONFIG_KERN); +MOD_DEFINE(preempt) -Timer preempt_timer; +/* Global preemption nesting counter */ +cpu_atomic_t preempt_count; -/** - * Disable preemptive task switching. - * - * 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(). - * - * 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. - * - * \sa proc_permit() +/* + * 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. */ -void proc_forbid(void) -{ - /* No need to protect against interrupts here. */ - ++CurrentProcess->forbid_cnt; -} +int _proc_quantum; /** - * Re-enable preemptive task switching. + * Define function prototypes exported outside. * - * \sa proc_forbid() + * Required to silent gcc "no previous prototype" warnings. */ -void proc_permit(void) -{ - /* No need to protect against interrupts here. */ - --CurrentProcess->forbid_cnt; -} +void preempt_yield(void); +int preempt_needPreempt(void); +void preempt_preempt(void); +void preempt_switch(void); +void preempt_init(void); - -void proc_preempt(void) +/** + * Call the scheduler and eventually replace the current running process. + */ +static void preempt_schedule(void) { - IRQ_DISABLE; - - LIST_ASSERT_VALID(&ProcReadyList); - CurrentProcess = (struct Process *)list_remHead(&ProcReadyList); - LIST_ASSERT_VALID(&ProcReadyList); - ASSERT2(CurrentProcess, "no idle proc?"); - - IRQ_ENABLE; - - TRACEMSG("new proc: %p:%s", CurrentProcess, CurrentProcess ? CurrentProcess->monitor.name : "---"); - monitor_report(); + _proc_quantum = CONFIG_KERN_QUANTUM; + proc_schedule(); } -void proc_preempt_timer(UNUSED_ARG(void *, param)) +/** + * Check if we need to schedule another task + */ +int preempt_needPreempt(void) { - IRQ_DISABLE; -/* - if (!CurrentProcess->forbid_cnt) - { - TRACEMSG("preempting %p:%s", CurrentProcess, CurrentProcess->monitor.name); - LIST_ASSERT_VALID(&ProcReadyList); - SCHED_ENQUEUE(CurrentProcess); - proc_preempt(); - } -*/ - IRQ_ENABLE; - - timer_setDelay(&preempt_timer, CONFIG_KERN_QUANTUM); - timer_add(&preempt_timer); + if (UNLIKELY(current_process == NULL)) + return 0; + if (!proc_preemptAllowed()) + return 0; + return _proc_quantum ? prio_next() > prio_curr() : + prio_next() >= prio_curr(); } -void proc_schedule(void) +/** + * Preempt the current task. + */ +void preempt_preempt(void) { - TRACE; - - // Will invoke proc_preempt() in interrupt context - kill(0, SIGUSR1); + 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(); } -void proc_yield(void) +/** + * 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 preempt_switch(void) { - TRACE; - - ASSERT_IRQ_ENABLED(); - IRQ_DISABLE; - SCHED_ENQUEUE(CurrentProcess); - LIST_ASSERT_VALID(&ProcReadyList); - proc_schedule(); - IRQ_ENABLE; -} + ASSERT(proc_preemptAllowed()); + IRQ_ASSERT_ENABLED(); -void proc_entry(void (*user_entry)(void)) -{ - user_entry(); - proc_exit(); + ATOMIC(preempt_schedule()); } - -static cpustack_t idle_stack[CONFIG_PROC_DEFSTACKSIZE / sizeof(cpustack_t)]; - -/* - * The idle process - * - * This process never dies and never sleeps. It's also quite apathic - * and a bit antisocial. - * - * Having an idle process costs some stack space, but simplifies the - * interrupt-driven preemption logic because there is always a user - * context to which we can return. +/** + * Voluntarily release the CPU. */ -static NORETURN void idle(void) +void preempt_yield(void) { - for (;;) - { - TRACE; - monitor_report(); - proc_yield(); // FIXME: CPU_IDLE - } + /* + * 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(); + + ATOMIC( + SCHED_ENQUEUE(current_process); + preempt_schedule(); + ); } void preempt_init(void) { - MOD_CHECK(irq); - MOD_CHECK(timer); - - irq_register(SIGUSR1, proc_preempt); - - timer_setSoftint(&preempt_timer, proc_preempt_timer, NULL); - timer_setDelay(&preempt_timer, CONFIG_KERN_QUANTUM); - timer_add(&preempt_timer); - - proc_new(idle, NULL, sizeof(idle_stack), idle_stack); + MOD_INIT(preempt); }