Linux进程调度

进程的调度时机与进程的切换

操作系统原理中介绍了大量进程调度算法，这些算法从实现的角度看仅仅是从运行队列中选择一个新进程，选择的过程中运用了不同的策略而已。

对于理解操作系统的工作机制，反而是进程的调度时机与进程的切换机制更为关键。

进程调度的时机

中断处理过程（包括时钟中断、I/O中断、系统调用和异常）中，直接调用schedule()，或者返回用户态时根据need_resched标记调用schedule()；

内核线程可以直接调用schedule()进行进程切换，也可以在中断处理过程中进行调度，也就是说内核线程作为一类的特殊的进程可以主动调度，也可以被动调度；

用户态进程无法实现主动调度，仅能通过陷入内核态后的某个时机点进行调度，即在中断处理过程中进行调度。

进程的切换

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，与中断时保存现场是不同的，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；

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（注意中断也要保存硬件上下文只是保存的方法不同）

schedule()函数选择一个新的进程来运行，并调用context_switch进行上下文的切换，这个宏调用switch_to来进行关键上下文切换

next = pick_next_task(rq, prev);//进程调度算法都封装这个函数内部

context_switch(rq, prev, next);//进程上下文切换

switch_to利用了prev和next两个参数：prev指向当前进程，next指向被调度的进程

schedule()函数源码如下：

/*
* __schedule() is the main scheduler function.
*
* The main means of driving the scheduler and thus entering this function are:
*
*   1. Explicit blocking: mutex, semaphore, waitqueue, etc.
*
*   2. TIF_NEED_RESCHED flag is checked on interrupt and userspace return
*      paths. For example, see arch/x86/entry_64.S.
*
*      To drive preemption between tasks, the scheduler sets the flag in timer
*      interrupt handler scheduler_tick().
*
*   3. Wakeups don't really cause entry into schedule(). They add a
*      task to the run-queue and that's it.
*
*      Now, if the new task added to the run-queue preempts the current
*      task, then the wakeup sets TIF_NEED_RESCHED and schedule() gets
*      called on the nearest possible occasion:
*
*       - If the kernel is preemptible (CONFIG_PREEMPT=y):
*
*         - in syscall or exception context, at the next outmost
*           preempt_enable(). (this might be as soon as the wake_up()'s
*           spin_unlock()!)
*
*         - in IRQ context, return from interrupt-handler to
*           preemptible context
*
*       - If the kernel is not preemptible (CONFIG_PREEMPT is not set)
*         then at the next:
*
*          - cond_resched() call
*          - explicit schedule() call
*          - return from syscall or exception to user-space
*          - return from interrupt-handler to user-space
*/
static void __sched __schedule(void)
{
struct task_struct *prev, *next;
unsigned long *switch_count;
struct rq *rq;
int cpu;

need_resched:
preempt_disable();
cpu = smp_processor_id();
rq = cpu_rq(cpu);
rcu_note_context_switch(cpu);
prev = rq->curr;

schedule_debug(prev);

if (sched_feat(HRTICK))
hrtick_clear(rq);

/*
* Make sure that signal_pending_state()->signal_pending() below
* can't be reordered with __set_current_state(TASK_INTERRUPTIBLE)
* done by the caller to avoid the race with signal_wake_up().
*/
smp_mb__before_spinlock();
raw_spin_lock_irq(&rq->lock);

switch_count = &prev->nivcsw;
if (prev->state && !(preempt_count() & PREEMPT_ACTIVE)) {
if (unlikely(signal_pending_state(prev->state, prev))) {
prev->state = TASK_RUNNING;
} else {
deactivate_task(rq, prev, DEQUEUE_SLEEP);
prev->on_rq = 0;

/*
* If a worker went to sleep, notify and ask workqueue
* whether it wants to wake up a task to maintain
* concurrency.
*/
if (prev->flags & PF_WQ_WORKER) {
struct task_struct *to_wakeup;

to_wakeup = wq_worker_sleeping(prev, cpu);
if (to_wakeup)
try_to_wake_up_local(to_wakeup);
}
}
switch_count = &prev->nvcsw;
}

if (task_on_rq_queued(prev) || rq->skip_clock_update < 0)
update_rq_clock(rq);

next = pick_next_task(rq, prev);//从等待队列中挑选出下一个等待调度的进程
clear_tsk_need_resched(prev);
clear_preempt_need_resched();
rq->skip_clock_update = 0;

if (likely(prev != next)) {
rq->nr_switches++;
rq->curr = next;
++*switch_count;

context_switch(rq, prev, next); /* unlocks the rq ，进行进程上下文的切换*/
/*
* The context switch have flipped the stack from under us
* and restored the local variables which were saved when
* this task called schedule() in the past. prev == current
* is still correct, but it can be moved to another cpu/rq.
*/
cpu = smp_processor_id();
rq = cpu_rq(cpu);
} else
raw_spin_unlock_irq(&rq->lock);

post_schedule(rq);

sched_preempt_enable_no_resched();
if (need_resched())
goto need_resched;
}

context_switch代码如下：

/*
* 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);

mm = next->mm;//mm指向了next进程的内存空间
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_start_context_switch(prev);

if (!mm) {
next->active_mm = oldmm;
atomic_inc(&oldmm->mm_count);
enter_lazy_tlb(oldmm, next);
} else
switch_mm(oldmm, mm, next);//进程内存空间切换

if (!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:
*/
spin_release(&rq->lock.dep_map, 1, _THIS_IP_);

context_tracking_task_switch(prev, next);
/* Here we just switch the register state and the stack. */
switch_to(prev, next, prev);//进程切换的宏，切换寄存器、栈空间

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);
}

swith_to宏的代码如下，这是一段汇编代码，源码中已经注释的很明确了，直接看注释

#define switch_to(prev, next, last)					\
do {									\
/*								\
* Context-switching clobbers all registers, so we clobber	\
* them explicitly, via unused output variables.		\
* (EAX and EBP is not listed because EBP is saved/restored	\
* explicitly for wchan access and EAX is the return value of	\
* __switch_to())						\
*/								\
unsigned long ebx, ecx, edx, esi, edi;				\
\
asm volatile("pushfl\n\t"		/* save    flags */	\
"pushl %%ebp\n\t"		/* save    EBP   */	\
"movl %%esp,%[prev_sp]\n\t"	/* save    ESP   */ \
"movl %[next_sp],%%esp\n\t"	/* restore ESP   */ \
"movl $1f,%[prev_ip]\n\t"	/* save    EIP   */	\
"pushl %[next_ip]\n\t"	/* restore EIP   */	\
__switch_canary					\
"jmp __switch_to\n"	/* regparm call  */	\
"1:\t"						\
"popl %%ebp\n\t"		/* restore EBP   */	\
"popfl\n"			/* restore flags */	\
\
/* output parameters */				\
: [prev_sp] "=m" (prev->thread.sp),		\
[prev_ip] "=m" (prev->thread.ip),		\
"=a" (last),					\
\
/* clobbered output registers: */		\
"=b" (ebx), "=c" (ecx), "=d" (edx),		\
"=S" (esi), "=D" (edi)				\
\
__switch_canary_oparam				\
\
/* input parameters: */				\
: [next_sp]  "m" (next->thread.sp),		\
[next_ip]  "m" (next->thread.ip),		\
\
/* regparm parameters for __switch_to(): */	\
[prev]     "a" (prev),				\
[next]     "d" (next)				\
\
__switch_canary_iparam				\
\
: /* reloaded segment registers */			\
"memory");					\
} while (0)