标签：schedule linux 进程 操作系统 上下文 内核堆栈
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对于现代操作系统，多任务是必备的，在linux系统下，进程会不断的被内核调度，从X进程切换为Y进程，以实现用户所见到的多任务状态，下面我们就看一看这样的过程，分析一下内核如何对进程调度，以及进程间如何切换。
内核使用schedule()函数实现进程的调度，而通常的用户进程要无法主动调度这个函数，只能通过中断处理过程（包括时钟中断、I/O中断、系统调用和异常）在某个合适的时机点被动调度；对于现代操作系统，还有内核线程，而内核线程是可以直接调度schedule函数的，只有内核态，当然也可以象用户态进程一样在中断处理过程中被动调度。
为了控制进程的执行，内核必须有能力挂起正在CPU上执行的进程，并恢复以前挂起的某个进程的执行，这叫做进程切换、任务切换、上下文切换；挂起正在CPU上执行的进程，与中断时保存现场不同的，中断前后是在同一个进程上下文中，只是由用户态转向内核态执行；而进程切换是在两个进程之间进行转换，切换前后的上下文是在不同的进程空间。进程上下文包含了进程执行需要的所有信息：用户地址空间：包括程序代码，数据，用户堆栈等；控制信息：进程描述符，内核堆栈等；硬件上下文。
下面将进程切换的关键代码摘录如下：
1、schedule函数
asmlinkage __visible void __sched schedule(void)
{
struct task_struct *tsk = current;
sched_submit_work(tsk);
__schedule();
}2、__schedule()函数
2770static void __sched __schedule(void)
2771{
2772 struct task_struct *prev, *next;
2773 unsigned long *switch_count;
2774 struct rq *rq;
2775 int cpu;
2776
2777need_resched:
2778 preempt_disable();
2779 cpu = smp_processor_id();
2780 rq = cpu_rq(cpu);
2781 rcu_note_context_switch(cpu);
2782 prev = rq->curr;
2783
2784 schedule_debug(prev);
2785
2786 if (sched_feat(HRTICK))
2787 hrtick_clear(rq);
2788
2789 /*
2790 * Make sure that signal_pending_state()->signal_pending() below
2791 * can‘t be reordered with __set_current_state(TASK_INTERRUPTIBLE)
2792 * done by the caller to avoid the race with signal_wake_up().
2793 */
2794 smp_mb__before_spinlock();
2795 raw_spin_lock_irq(&rq->lock);
2796
2797 switch_count = &prev->nivcsw;
2798 if (prev->state && !(preempt_count() & PREEMPT_ACTIVE)) {
2799 if (unlikely(signal_pending_state(prev->state, prev))) {
2800 prev->state = TASK_RUNNING;
2801 } else {
2802 deactivate_task(rq, prev, DEQUEUE_SLEEP);
2803 prev->on_rq = 0;
2804
2805 /*
2806 * If a worker went to sleep, notify and ask workqueue
2807 * whether it wants to wake up a task to maintain
2808 * concurrency.
2809 */
2810 if (prev->flags & PF_WQ_WORKER) {
2811 struct task_struct *to_wakeup;
2812
2813 to_wakeup = wq_worker_sleeping(prev, cpu);
2814 if (to_wakeup)
2815 try_to_wake_up_local(to_wakeup);
2816 }
2817 }
2818 switch_count = &prev->nvcsw;
2819 }
2820
2821 if (task_on_rq_queued(prev) || rq->skip_clock_update < 0)
2822 update_rq_clock(rq);
2823
2824 next = pick_next_task(rq, prev);
2825 clear_tsk_need_resched(prev);
2826 clear_preempt_need_resched();
2827 rq->skip_clock_update = 0;
2828
2829 if (likely(prev != next)) {
2830 rq->nr_switches++;
2831 rq->curr = next;
2832 ++*switch_count;
2833
2834 context_switch(rq, prev, next); /* unlocks the rq */
2835 /*
2836 * The context switch have flipped the stack from under us
2837 * and restored the local variables which were saved when
2838 * this task called schedule() in the past. prev == current
2839 * is still correct, but it can be moved to another cpu/rq.
2840 */
2841 cpu = smp_processor_id();
2842 rq = cpu_rq(cpu);
2843 } else
2844 raw_spin_unlock_irq(&rq->lock);
2845
2846 post_schedule(rq);
2847
2848 sched_preempt_enable_no_resched();
2849 if (need_resched())
2850 goto need_resched;
2851}其中关键语句：
struct task_struct *prev, *next; next = pick_next_task(rq, prev); //进程调度算法 context_switch(rq, prev, next); /* unlocks the rq */ //进程上下文切换
3、context_switch函数
2332 * context_switch - switch to the new MM and the new
2333 * thread‘s register state.
2334 */
2335static inline void
2336context_switch(struct rq *rq, struct task_struct *prev,
2337 struct task_struct *next)
2338{
2339 struct mm_struct *mm, *oldmm;
2340
2341 prepare_task_switch(rq, prev, next);
2342
2343 mm = next->mm;
2344 oldmm = prev->active_mm;
2345 /*
2346 * For paravirt, this is coupled with an exit in switch_to to
2347 * combine the page table reload and the switch backend into
2348 * one hypercall.
2349 */
2350 arch_start_context_switch(prev);
2351
2352 if (!mm) {
2353 next->active_mm = oldmm;
2354 atomic_inc(&oldmm->mm_count);
2355 enter_lazy_tlb(oldmm, next);
2356 } else
2357 switch_mm(oldmm, mm, next);
2358
2359 if (!prev->mm) {
2360 prev->active_mm = NULL;
2361 rq->prev_mm = oldmm;
2362 }
2363 /*
2364 * Since the runqueue lock will be released by the next
2365 * task (which is an invalid locking op but in the case
2366 * of the scheduler it‘s an obvious special-case), so we
2367 * do an early lockdep release here:
2368 */
2369 spin_release(&rq->lock.dep_map, 1, _THIS_IP_);
2370
2371 context_tracking_task_switch(prev, next);
2372 /* Here we just switch the register state and the stack. */
2373 switch_to(prev, next, prev);
2374
2375 barrier();
2376 /*
2377 * this_rq must be evaluated again because prev may have moved
2378 * CPUs since it called schedule(), thus the ‘rq‘ on its stack
2379 * frame will be invalid.
2380 */
2381 finish_task_switch(this_rq(), prev);
2382} 4、switch_to宏定义了一段内联汇编代码
31#define switch_to(prev, next, last) 32do { 33 /* 34 * Context-switching clobbers all registers, so we clobber 35 * them explicitly, via unused output variables. 36 * (EAX and EBP is not listed because EBP is saved/restored 37 * explicitly for wchan access and EAX is the return value of 38 * __switch_to()) 39 */ 40 unsigned long ebx, ecx, edx, esi, edi; 41 42 asm volatile("pushfl\n\t" /* save flags */ 43 "pushl %%ebp\n\t" /* save EBP */ 44 "movl %%esp,%[prev_sp]\n\t" /* save ESP */ 45 "movl %[next_sp],%%esp\n\t" /* restore ESP */ 46 "movl $1f,%[prev_ip]\n\t" /* save EIP */ 47 "pushl %[next_ip]\n\t" /* restore EIP */ 48 __switch_canary 49 "jmp __switch_to\n" /* regparm call */ 50 "1:\t" 51 "popl %%ebp\n\t" /* restore EBP */ 52 "popfl\n" /* restore flags */ 53 54 /* output parameters */ 55 : [prev_sp] "=m" (prev->thread.sp), 56 [prev_ip] "=m" (prev->thread.ip), 57 "=a" (last), 58 59 /* clobbered output registers: */ 60 "=b" (ebx), "=c" (ecx), "=d" (edx), 61 "=S" (esi), "=D" (edi) 62 63 __switch_canary_oparam 64 65 /* input parameters: */ 66 : [next_sp] "m" (next->thread.sp), 67 [next_ip] "m" (next->thread.ip), 68 69 /* regparm parameters for __switch_to(): */ 70 [prev] "a" (prev), 71 [next] "d" (next) 72 73 __switch_canary_iparam 74 75 : /* reloaded segment registers */ 76 "memory"); 77} while (0) 通过以上代码，我们可以看到，当cpu由正在运行的X进程切换到Y进程的大致步骤，其中X，Y是哪一个进程是由调度算法决定的。
进程X正在中运行->发生中断->进行中断处理（保存当前的eflag，eip，esp；加载内核中特定的eflag，eip，esp）->执行SAVE ALL->中断处理过程中或中断返回前调用了schedule(),switch_to实现关键的进程上下文切换->开始从标号1之后运行用户态进程Y->restore all->iret从内核堆栈中返回eflag，eip，esp->继续执行Y进程。对于前面提到的内核线程，以及系统中的特殊调用fork和execve会有些特殊，但大致原则是相同的。
标签：schedule linux 进程 操作系统 上下文 内核堆栈
原文地址：http://swordautumn.blog.51cto.com/1485402/1636681
