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 <kern/idle.h> // idle_proc
95 #include <cpu/frame.h> // CPU_IDLE
96 #include <cpu/irq.h> // IRQ_DISABLE()...
98 #include <cfg/module.h>
99 #include <cfg/depend.h> // CONFIG_DEPEND()
101 // Check config dependencies
102 CONFIG_DEPEND(CONFIG_KERN_PREEMPT, CONFIG_KERN);
107 * CPU dependent context switching routines.
109 * Saving and restoring the context on the stack is done by a CPU-dependent
110 * support routine which usually needs to be written in assembly.
112 EXTERN_C void asm_switch_context(cpu_stack_t **new_sp, cpu_stack_t **save_sp);
114 /* Global preemption nesting counter */
115 cpu_atomic_t preempt_count;
118 * The time sharing interval: when a process is scheduled on a CPU it gets an
119 * amount of CONFIG_KERN_QUANTUM clock ticks. When these ticks expires and
120 * preemption is enabled a new process is selected to run.
125 * Define function prototypes exported outside.
127 * Required to silent gcc "no previous prototype" warnings.
129 void preempt_yield(void);
130 int preempt_needPreempt(void);
131 void preempt_preempt(void);
132 void preempt_switch(void);
133 void preempt_init(void);
136 * Call the scheduler and eventually replace the current running process.
138 static void preempt_schedule(void)
140 Process *old_process = current_process;
142 IRQ_ASSERT_DISABLED();
144 /* Poll on the ready queue for the first ready process */
145 LIST_ASSERT_VALID(&proc_ready_list);
146 current_process = (Process *)list_remHead(&proc_ready_list);
147 if (UNLIKELY(!current_process))
148 current_process = idle_proc;
149 _proc_quantum = CONFIG_KERN_QUANTUM;
151 * Optimization: don't switch contexts when the active process has not
154 if (LIKELY(old_process != current_process))
159 * Save context of old process and switch to new process. If
160 * there is no old process, we save the old stack pointer into
161 * a dummy variable that we ignore. In fact, this happens only
162 * when the old process has just exited.
164 * \todo Instead of physically clearing the process at exit
165 * time, a zombie list should be created.
167 asm_switch_context(¤t_process->stack,
168 old_process ? &old_process->stack : &dummy);
171 /* This RET resumes the execution on the new process */
172 LOG_INFO("resuming %p:%s\n", current_process, proc_currentName());
176 * Check if we need to schedule another task
178 int preempt_needPreempt(void)
180 if (UNLIKELY(current_process == NULL))
182 if (!proc_preemptAllowed())
184 return _proc_quantum ? prio_next() > prio_curr() :
185 prio_next() >= prio_curr();
189 * Preempt the current task.
191 void preempt_preempt(void)
193 IRQ_ASSERT_DISABLED();
194 ASSERT(current_process);
196 /* Perform the kernel preemption */
197 LOG_INFO("preempting %p:%s\n", current_process, proc_currentName());
198 /* We are inside a IRQ context, so ATOMIC is not needed here */
199 if (current_process != idle_proc)
200 SCHED_ENQUEUE(current_process);
205 * Give the control of the CPU to another process.
207 * \note Assume the current process has been already added to a wait queue.
209 * \warning This should be considered an internal kernel function, even if it
210 * is allowed, usage from application code is strongly discouraged.
212 void preempt_switch(void)
214 ASSERT(proc_preemptAllowed());
215 IRQ_ASSERT_ENABLED();
217 ATOMIC(preempt_schedule());
221 * Voluntarily release the CPU.
223 void preempt_yield(void)
226 * Voluntary preemption while preemption is disabled is considered
227 * illegal, as not very useful in practice.
229 * ASSERT if it happens.
231 ASSERT(proc_preemptAllowed());
232 IRQ_ASSERT_ENABLED();
235 SCHED_ENQUEUE(current_process);
240 void preempt_init(void)