4 * This file is part of BeRTOS.
6 * Bertos is free software; you can redistribute it and/or modify
7 * it under the terms of the GNU General Public License as published by
8 * the Free Software Foundation; either version 2 of the License, or
9 * (at your option) any later version.
11 * This program is distributed in the hope that it will be useful,
12 * but WITHOUT ANY WARRANTY; without even the implied warranty of
13 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
14 * GNU General Public License for more details.
16 * You should have received a copy of the GNU General Public License
17 * along with this program; if not, write to the Free Software
18 * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
20 * As a special exception, you may use this file as part of a free software
21 * library without restriction. Specifically, if other files instantiate
22 * templates or use macros or inline functions from this file, or you compile
23 * this file and link it with other files to produce an executable, this
24 * file does not by itself cause the resulting executable to be covered by
25 * the GNU General Public License. This exception does not however
26 * invalidate any other reasons why the executable file might be covered by
27 * the GNU General Public License.
29 * Copyright 2008 Bernie Innocenti <bernie@codewiz.org>
30 * Copyright 2009 Andrea Righi <arighi@develer.com>
33 * \brief Simple preemptive multitasking scheduler.
35 * Preemption is explicitly regulated at the exit of each interrupt service
36 * routine (ISR). Each task obtains a time quantum as soon as it is scheduled
37 * on the CPU and its quantum is decremented at each clock tick. The frequency
38 * of the timer determines the system tick granularity and CONFIG_KERN_QUANTUM
39 * the time sharing interval.
41 * When the quantum expires the handler proc_needPreempt() checks if the
42 * preemption is enabled and in this case preempt_schedule() is called, that
43 * possibly replaces the current running thread with a different one.
45 * The preemption can be disabled or enabled via proc_forbid() and
46 * proc_permit() primitives. This is implemented using a global atomic counter.
47 * When the counter is greater than 0 the task cannot be preempted; only when
48 * the counter reaches 0 the task can be preempted again.
50 * Preemption-disabled sections may be nested. The preemption will be
51 * re-enabled when the outermost preemption-disabled section completes.
53 * The voluntary preemption still happens via proc_switch() or proc_yield().
54 * The first one assumes the current process has been already added to a
55 * private wait queue (e.g., on a semaphore or a signal), while the second one
56 * takes care of adding the process into the ready queue.
58 * Context switch is done by CPU-dependent support routines. In case of a
59 * voluntary preemption the context switch routine must take care of
60 * saving/restoring only the callee-save registers (the voluntary-preemption is
61 * actually a function call). The kernel-preemption always happens inside a
62 * signal/interrupt context and it must take care of saving all registers. For
63 * this, in the entry point of each ISR the caller-save registers must be
64 * saved. In the ISR exit point, if the context switch must happen, we switch
65 * to user-context and call the same voluntary context switch routine that take
66 * care of saving/restoring also the callee-save registers. On resume from the
67 * switch, the interrupt exit point moves back to interrupt-context, resumes
68 * the caller-save registers (saved in the ISR entry point) and return from the
71 * \note Thread priority (if enabled by CONFIG_KERN_PRI) defines the order in
72 * the \p proc_ready_list and the capability to deschedule a running process. A
73 * low-priority thread can't preempt a high-priority thread.
75 * A high-priority process can preempt a low-priority process immediately (it
76 * will be descheduled and replaced in the interrupt exit point). Processes
77 * running at the same priority can be descheduled when they expire the time
80 * \note Sleeping while preemption is disabled fallbacks to a busy-wait sleep.
81 * Voluntary preemption when preemption is disabled raises a kernel bug.
83 * \author Bernie Innocenti <bernie@codewiz.org>
84 * \author Andrea Righi <arighi@develer.com>
87 #include "cfg/cfg_proc.h"
93 #include <kern/monitor.h>
94 #include <cpu/frame.h> // CPU_IDLE
95 #include <cpu/irq.h> // IRQ_DISABLE()...
97 #include <cfg/module.h>
98 #include <cfg/depend.h> // CONFIG_DEPEND()
100 // Check config dependencies
101 CONFIG_DEPEND(CONFIG_KERN_PREEMPT, CONFIG_KERN);
105 /* Global preemption nesting counter */
106 cpu_atomic_t preempt_count;
109 * The time sharing interval: when a process is scheduled on a CPU it gets an
110 * amount of CONFIG_KERN_QUANTUM clock ticks. When these ticks expires and
111 * preemption is enabled a new process is selected to run.
116 * Define function prototypes exported outside.
118 * Required to silent gcc "no previous prototype" warnings.
120 void preempt_yield(void);
121 int preempt_needPreempt(void);
122 void preempt_preempt(void);
123 void preempt_switch(void);
124 void preempt_init(void);
127 * Call the scheduler and eventually replace the current running process.
129 static void preempt_schedule(void)
131 _proc_quantum = CONFIG_KERN_QUANTUM;
136 * Check if we need to schedule another task
138 int preempt_needPreempt(void)
140 if (UNLIKELY(current_process == NULL))
142 if (!proc_preemptAllowed())
144 return _proc_quantum ? prio_next() > prio_curr() :
145 prio_next() >= prio_curr();
149 * Preempt the current task.
151 void preempt_preempt(void)
153 IRQ_ASSERT_DISABLED();
154 ASSERT(current_process);
156 /* Perform the kernel preemption */
157 LOG_INFO("preempting %p:%s\n", current_process, proc_currentName());
158 /* We are inside a IRQ context, so ATOMIC is not needed here */
159 SCHED_ENQUEUE(current_process);
164 * Give the control of the CPU to another process.
166 * \note Assume the current process has been already added to a wait queue.
168 * \warning This should be considered an internal kernel function, even if it
169 * is allowed, usage from application code is strongly discouraged.
171 void preempt_switch(void)
173 ASSERT(proc_preemptAllowed());
174 IRQ_ASSERT_ENABLED();
176 ATOMIC(preempt_schedule());
180 * Voluntarily release the CPU.
182 void preempt_yield(void)
185 * Voluntary preemption while preemption is disabled is considered
186 * illegal, as not very useful in practice.
188 * ASSERT if it happens.
190 ASSERT(proc_preemptAllowed());
191 IRQ_ASSERT_ENABLED();
194 SCHED_ENQUEUE(current_process);
199 void preempt_init(void)