2. Arm体系架构中进程切换过程 在之后的文章里，可能会有很大部分的篇幅是介绍内核如何调度和管理进程。学习了解这部分内容，很多时候是和task struct，run queue，schedule entity等数据结构打交道。关于linux进程调度的文章网上已经有很多，大多数都比较详细的介绍调度算法。调度算法的内容说白了就是在什么时间，内核会用什么样的进程替换目前正在运行的进程。而最后进程间的切换是如何完成的，介绍的文章较少。在此，我们在修炼高深内功之前，必须了解一下这个内功中的内功，打通一下任通二脉。下面介绍一下进程切换的过程。
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/*
* context_switch - switch to the new MM and the new
* thread's register state.
*/
static inline void
context_switch(struct rq *rq, struct task_struct *prev,
struct task_struct *next)
{
struct mm_struct *mm, *oldmm;
prepare_task_switch(rq, prev, next);
trace_sched_switch(rq, prev, next);
mm = next->mm;
oldmm = prev->active_mm;
/*
* For paravirt, this is coupled with an exit in switch_to to
* combine the page table reload and the switch backend into
* one hypercall.
*/
arch_enter_lazy_cpu_mode();
if (unlikely(!mm))
{
next->active_mm = oldmm;
atomic_inc(&oldmm->mm_count);
enter_lazy_tlb(oldmm, next);
}
else
switch_mm(oldmm, mm, next);
if (unlikely(!prev->mm))
{
prev->active_mm = NULL;
rq->prev_mm = oldmm;
}
/*
* Since the runqueue lock will be released by the next
* task (which is an invalid locking op but in the case
* of the scheduler it's an obvious special-case), so we
* do an early lockdep release here:
*/
#ifndef __ARCH_WANT_UNLOCKED_CTXSW
spin_release(&rq->lock.dep_map, 1, _THIS_IP_);
#endif
/* Here we just switch the register state and the stack. */
switch_to(prev, next, prev);
barrier();
/*
* this_rq must be evaluated again because prev may have moved
* CPUs since it called schedule(), thus the 'rq' on its stack
* frame will be invalid.
*/
finish_task_switch(this_rq(), prev);
}
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context_switch函数就是在进程调度过程中完成进程切换的函数，prev是被切换掉的进程，而next是切换到的进程。
代码首先判断切换掉的进程有没有用户空间，如果mm为空，表示next指向的进程是一个内核线程，根本就没有用户空间。这个时候，next进程根本就不会去访问用户空间的地址，因此，不需要更换MMU的页目录表。enter_lazy_tlb函数对于ARM架构就是一个空实现。如果next是个普通的用户进程，就执行switch_mm执行更换MMU的页目录表。
下面代码就是更换MMU页目录表的过程，其实很简单，就是操作设置新的页目录表的基地址到cp15协处理中。每个进程的页目录表基地址保存在mm_struct中的pgd中。
| /*
* cpu_arm920_switch_mm(pgd)
*
* Set the translation base pointer to be as described by pgd.
*
* pgd: new page tables
*/
.align 5
ENTRY(cpu_arm920_switch_mm)
#ifdef CONFIG_MMU
mov ip, #0
#ifdef CONFIG_CPU_DCACHE_WRITETHROUGH
mcr p15, 0, ip, c7, c6, 0 @ invalidate D cache
#else
@ && 'Clean & Invalidate whole DCache'
@ && Re-written to use Index Ops.
@ && Uses registers r1, r3 and ip
mov r1, #(CACHE_DSEGMENTS - 1) << 5 @ 8 segments
1: orr r3, r1, #(CACHE_DENTRIES - 1) << 26 @ 64 entries
2: mcr p15, 0, r3, c7, c14, 2 @ clean & invalidate D index
subs r3, r3, #1 << 26
bcs 2b @ entries 63 to 0
subs r1, r1, #1 << 5
bcs 1b @ segments 7 to 0
#endif
mcr p15, 0, ip, c7, c5, 0 @ invalidate I cache
mcr p15, 0, ip, c7, c10, 4 @ drain WB
mcr p15, 0, r0, c2, c0, 0 @ load page table pointer
mcr p15, 0, ip, c8, c7, 0 @ invalidate I & D TLBs
#endif
mov pc, lr |
完成了页目录表的替换，再次回到context_switch函数中来，最后要完成进程切换，即切换到next进程的执行流程上去。
这个切换过程也比较简单，首先保存当前cpu现场，然后设置了一下cp15协处理器的domain寄存器，之后恢复next进程的cpu现场。切换到了next进程。
/*
* switch_to(prev, next) should switch from task `prev' to `next'
* `prev' will never be the same as `next'. schedule() itself
* contains the memory barrier to tell GCC not to cache `current'.
*/
extern struct task_struct *__switch_to(struct task_struct *, struct thread_info *, struct thread_info *);
#define switch_to(prev,next,last) \
do { \
last = __switch_to(prev,task_thread_info(prev), task_thread_info(next)); \
} while (0)
ENTRY(__switch_to)
add ip, r1, #TI_CPU_SAVE
ldr r3, [r2, #TI_TP_VALUE]
stmia ip!, {r4 - sl, fp, sp, lr} @ Store most regs on stack
#ifdef CONFIG_MMU
ldr r6, [r2, #TI_CPU_DOMAIN]
#endif
#ifdef CONFIG_MMU
mcr p15, 0, r6, c3, c0, 0 @ Set domain register
#endif
mov r5, r0
add r4, r2, #TI_CPU_SAVE
ldr r0, =thread_notify_head
mov r1, #THREAD_NOTIFY_SWITCH
bl atomic_notifier_call_chain
mov r0, r5
ldmia r4, {r4 - sl, fp, sp, pc} @ Load all regs saved previously
ENDPROC(__switch_to)
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