小弟最近研究了一段时间的ARM Linux,想把进程管理方面的感受跟大家交流下，不对的地方多多指点
Process Creation and Termination
Process Scheduling and Dispatching
Process Switching
Porcess Synchronization and support for interprocess communication
Management of process control block
--------from <Operating system:internals and design principles>
进程调度
Linux2.4.x是一个基于非抢占式的多任务的分时操作系统，虽然在用户进程的调度上采用抢占式策略，但是而在内核还是采用了轮转的方法，如果有个内核态的线程恶性占有CPU不释放，那系统无法从中解脱出来，所以实时性并不是很强。这种情况有望在Linux 2.6版本中得到改善，在2.6版本中采用了抢占式的调度策略。
内核中根据任务的实时程度提供了三种调度策略：
需要说明的是，SCHED_FIFO和SCHED_RR两种调度策略之间没有优先级上的区别，主要的区别是任务的大小上。另外，task_struct结构中的policy中还包含了一个SCHED_YIELD位，置位时表示该进程主动放弃CPU。
在上述三种调度策略的基础上，进程依照优先级的高低被分别调系统。优先级是一些简单的整数，它代表了为决定应该允许哪一个进程使用CPU的资源时判断方便而赋予进程的权值——优先级越高，它得到CPU时间的机会也就越大。
在Linux中，非实时进程有两种优先级，一种是静态优先级，另一种是动态优先级。实时进程又增加了第三种优先级，实时优先级。
在每个tick到来的时候（也就是时钟中断发生），系统减小当前占有CPU的进程的counter，如果counter减小到0，则将need_resched置1，中断返回过程中进行调度。update_process_times()为时钟中断处理程序调用的一个子函数：
void update_process_times(int user_tick)
{
struct task_struct *p = current;
int cpu = smp_processor_id(), system = user_tick ^ 1;
update_one_process(p, user_tick, system, cpu);
if (p->pid) {
if (--p->counter <= 0) {
p->counter = 0;
p->need_resched = 1;
}
if (p->nice > 0)
kstat.per_cpu_nice[cpu] += user_tick;
else
kstat.per_cpu_user[cpu] += user_tick;
kstat.per_cpu_system[cpu] += system;
} else if (local_bh_count(cpu) || local_irq_count(cpu) > 1)
kstat.per_cpu_system[cpu] += system;
}
Linux中进程的调度使在schedule（）函数中实现的，该函数在下面的ARM汇编片断中被调用到：
/*
* This is the fast syscall return path. We do as little as
* possible here, and this includes saving r0 back into the SVC
* stack.
*/
ret_fast_syscall:
ldr r1, [tsk, #TSK_NEED_RESCHED]
ldr r2, [tsk, #TSK_SIGPENDING]
teq r1, #0 @ need_resched || sigpending
teqeq r2, #0
bne slow
fast_restore_user_regs
/*
* Ok, we need to do extra processing, enter the slow path.
*/
slow: str r0, [sp, #S_R0+S_OFF]! @ returned r0
b 1f
/*
* "slow" syscall return path. "why" tells us if this was a real syscall.
*/
reschedule:
bl SYMBOL_NAME(schedule)
ENTRY(ret_to_user)
ret_slow_syscall:
ldr r1, [tsk, #TSK_NEED_RESCHED]
ldr r2, [tsk, #TSK_SIGPENDING]
1: teq r1, #0 @ need_resched => schedule()
bne reschedule @如果需要重新调度则调用schedule
teq r2, #0 @ sigpending => do_signal()
blne __do_signal
restore_user_regs
而这段代码在中断返回或者系统调用返回中反复被调用到。
1． 进程状态转换时： 如进程终止，睡眠等,当进程要调用sleep（）或exit（）等函数使进程状态发生改变时，这些函数会主动调用schedule（）转入进程调度。
2． 可运行队列中增加新的进程时；
ENTRY(ret_from_fork)
bl SYMBOL_NAME(schedule_tail)
get_current_task tsk
ldr ip, [tsk, #TSK_PTRACE] @ check for syscall tracing
mov why, #1
tst ip, #PT_TRACESYS @ are we tracing syscalls?
beq ret_slow_syscall
mov r1, sp
mov r0, #1 @ trace exit [IP = 1]
bl SYMBOL_NAME(syscall_trace)
b ret_slow_syscall @跳转到上面的代码片断
3． 在时钟中断到来后：Linux初始化时，设定系统定时器的周期为10毫秒。当时钟中断发生时，时钟中断服务程序timer_interrupt立即调用时钟处理函数do_timer( )，在do_timer()会将当前进程的counter减1，如果counter为0则置need_resched标志，在从时钟中断返回的过程中会调用schedule.
4． 进程从系统调用返回到用户态时；判断need_resched标志是否置位，若是则转入执行schedule()。系统调用实际上就是通过软中断实现的，下面是ARM平台下软中断处理代码。
.align 5
ENTRY(vector_swi)
save_user_regs
zero_fp
get_scno
enable_irqs ip
str r4, [sp, #-S_OFF]! @ push fifth arg
get_current_task tsk
ldr ip, [tsk, #TSK_PTRACE] @ check for syscall tracing
bic scno, scno, #0xff000000 @ mask off SWI op-code
eor scno, scno, #OS_NUMBER << 20 @ check OS number
adr tbl, sys_call_table @ load syscall table pointer
tst ip, #PT_TRACESYS @ are we tracing syscalls?
bne __sys_trace
adrsvc al, lr, ret_fast_syscall @ 装载返回地址，用于在跳转调用后返回到
@上面的代码片断中的ret_fast_syscall
cmp scno, #NR_syscalls @ check upper syscall limit
ldrcc pc, [tbl, scno, lsl #2] @ call sys_* routine
add r1, sp, #S_OFF
2: mov why, #0 @ no longer a real syscall
cmp scno, #ARMSWI_OFFSET
eor r0, scno, #OS_NUMBER << 20 @ put OS number back
bcs SYMBOL_NAME(arm_syscall)
b SYMBOL_NAME(sys_ni_syscall) @ not private func
5． 内核处理完中断后，进程返回到用户态。
6． 进程主动调用schedule()请求进行进程调度。
schedule()函数分析：
/*
* 'schedule()' is the scheduler function. It's a very simple and nice
* scheduler: it's not perfect, but certainly works for most things.
*
* The goto is "interesting".
*
* NOTE!! Task 0 is the 'idle' task, which gets called when no other
* tasks can run. It can not be killed, and it cannot sleep. The 'state'
* information in task[0] is never used.
*/
asmlinkage void schedule(void)
{
struct schedule_data * sched_data;
struct task_struct *prev, *next, *p;
struct list_head *tmp;
int this_cpu, c;
spin_lock_prefetch(&runqueue_lock);
if (!current->active_mm) BUG();
need_resched_back:
prev = current;
this_cpu = prev->processor;
if (unlikely(in_interrupt())) {
printk("Scheduling in interrupt\n");
BUG();
}
release_kernel_lock(prev, this_cpu);
/*
* 'sched_data' is protected by the fact that we can run
* only one process per CPU.
*/
sched_data = & aligned_data[this_cpu].schedule_data;
spin_lock_irq(&runqueue_lock);
/* move an exhausted RR process to be last.. */
if (unlikely(prev->policy == SCHED_RR))
/*
* 如果采用轮转法调度，则重新检查counter是否为0, 若是则将其挂到运行队列的最后
*/
if (!prev->counter) {
prev->counter = NICE_TO_TICKS(prev->nice);
move_last_runqueue(prev);
}
switch (prev->state) {
case TASK_INTERRUPTIBLE:
/*
* 如果是TASK_INTERRUPTIBLE,并且能够唤醒它的信号已经来临,
* 则将状态置为TASK_RUNNING
*/
if (signal_pending(prev)) {
prev->state = TASK_RUNNING;
break;
}
default:
del_from_runqueue(prev);
case TASK_RUNNING:;
}
prev->need_resched = 0;
/*
* this is the scheduler proper:
*/
repeat_schedule:
/*
* Default process to select..
*/
next = idle_task(this_cpu);
c = -1000;
list_for_each(tmp, &runqueue_head) {
/*
* 遍历运行队列,查找优先级最高的进程, 优先级最高的进程将获得CPU
*/
p = list_entry(tmp, struct task_struct, run_list);
if (can_schedule(p, this_cpu)) {
/*
* goodness()中，如果是实时进程，则weight = 1000 p->rt_priority,
* 使实时进程的优先级永远比非实时进程高
*/
int weight = goodness(p, this_cpu, prev->active_mm);
if (weight > c) //注意这里是”>”而不是”>=”，如果权值相同，则先来的先上
c = weight, next = p;
}
}
/* Do we need to re-calculate counters? */
if (unlikely(!c)) {
/*
* 如果当前优先级为0,那么整个运行队列中的进程将重新计算优先权
*/
struct task_struct *p;
spin_unlock_irq(&runqueue_lock);
read_lock(&tasklist_lock);
for_each_task(p)
p->counter = (p->counter >> 1) NICE_TO_TICKS(p->nice);
read_unlock(&tasklist_lock);
spin_lock_irq(&runqueue_lock);
goto repeat_schedule;
}
/*
* from this point on nothing can prevent us from
* switching to the next task, save this fact in sched_data.
*/
sched_data->curr = next;
task_set_cpu(next, this_cpu);
spin_unlock_irq(&runqueue_lock);
if (unlikely(prev == next)) {
/* We won't go through the normal tail, so do this by hand */
prev->policy &= ~SCHED_YIELD;
goto same_process;
}
kstat.context_swtch ;
/*
* there are 3 processes which are affected by a context switch:
*
* prev == .... ==> (last => next)
*
* It's the 'much more previous' 'prev' that is on next's stack,
* but prev is set to (the just run) 'last' process by switch_to().
* This might sound slightly confusing but makes tons of sense.
*/
prepare_to_switch(); {
struct mm_struct *mm = next->mm;
struct mm_struct *oldmm = prev->active_mm;
if (!mm) { //如果是内核线程的切换，则不做页表处理
if (next->active_mm) BUG();
next->active_mm = oldmm;
atomic_inc(&oldmm->mm_count);
enter_lazy_tlb(oldmm, next, this_cpu);
} else {
if (next->active_mm != mm) BUG();
switch_mm(oldmm, mm, next, this_cpu); //如果是用户进程，切换页表
}
if (!prev->mm) {
prev->active_mm = NULL;
mmdrop(oldmm);
}
}
/*
* This just switches the register state and the stack.
*/
switch_to(prev, next, prev);
__schedule_tail(prev);
same_process:
reacquire_kernel_lock(current);
if (current->need_resched)
goto need_resched_back;
return;
}
switch_mm中是进行页表的切换，即将下一个的pgd的开始物理地址放入CP15中的C2寄存器。进程的pgd的虚拟地址存放在task_struct结构中的pgd指针中，通过__virt_to_phys宏可以转变成成物理地址。
static inline void
switch_mm(struct mm_struct *prev, struct mm_struct *next,
struct task_struct *tsk, unsigned int cpu)
{
if (prev != next)
cpu_switch_mm(next->pgd, tsk);
}
#define cpu_switch_mm(pgd,tsk) cpu_set_pgd(__virt_to_phys((unsigned long)(pgd)))
#define cpu_get_pgd() \
({ \
unsigned long pg; \
__asm__("mrc p15, 0, %0, c2, c0, 0" \
: "=r" (pg)); \
pg &= ~0x3fff; \
(pgd_t *)phys_to_virt(pg); \
})
switch_to()完成进程上下文的切换，通过调用汇编函数__switch_to完成，其实现比较简单，也就是保存prev进程的上下文信息，该上下文信息由context_save_struct结构描述，包括主要的寄存器，然后将next的上下文信息读出，信息保存在task_struct中的thread.save中TSS_SAVE标识了thread.save在task_struct中的位置。
/*
* Register switch for ARMv3 and ARMv4 processors
* r0 = previous, r1 = next, return previous.
* previous and next are guaranteed not to be the same.
*/
ENTRY(__switch_to)
stmfd sp!, {r4 - sl, fp, lr} @ Store most regs on stack
mrs ip, cpsr
str ip, [sp, #-4]! @ Save cpsr_SVC
str sp, [r0, #TSS_SAVE] @ Save sp_SVC
ldr sp, [r1, #TSS_SAVE] @ Get saved sp_SVC
ldr r2, [r1, #TSS_DOMAIN]
ldr ip, [sp], #4
mcr p15, 0, r2, c3, c0 @ Set domain register
msr spsr, ip @ Save tasks CPSR into SPSR for this return
ldmfd sp!, {r4 - sl, fp, pc}^ @ Load all regs saved previously
struct context_save_struct {
unsigned long cpsr;
unsigned long r4;
unsigned long r5;
unsigned long r6;
unsigned long r7;
unsigned long r8;
unsigned long r9;
unsigned long sl;
unsigned long fp;
unsigned long pc;
};
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