学步园

不知谁对? 望高人指点一二. 谢谢

我的理解是, 时钟中断处理函数(do_timer())没有进行实质性的中断服务,

只是设置TIMER_BH, 在Timer Bottom Half 被运行时再作真正的服务.

而Timer Bottom Half 被调用只在这三个时机:

* schedule()

* 系统调用返回

* slow 中断服务返回

我的问题是, 如果系统上运行了两个计算量很大的程序. 它们没有调用任何

系统调用, 那么谁会调用schedule()呢. 是不是一个程序一旦获得CPU, 只要

他不主动放弃 (不调用任何系统调用), 他便不会被调度出去.

我一直以为时钟中断处理函数会作调度工作, 但是看了书和源代码后, 发现

恰恰相反, 是schedule()激活在Timer Bottom Half. 如果真是如此,

Linux的抢占式体现在哪里呢?

我觉得我是哪儿搞错了,很是迷惑. 请大家指点.

多谢.

arch/i386/kernel/entry.S:




ENTRY(system_call)


	pushl ％eax			# save orig_eax


	SAVE_ALL


	GET_CURRENT(％ebx)


	cmpl $(NR_syscalls),％eax


	jae badsys


	testb $0x20,flags(％ebx)		# PF_TRACESYS


	jne tracesys


	call *SYMBOL_NAME(sys_call_table)(,％eax,4)


	movl ％eax,EAX(％esp)		# save the return value


	ALIGN


	.globl ret_from_sys_call


	.globl ret_from_intr


ret_from_sys_call:


	movl SYMBOL_NAME(bh_mask),％eax


	andl SYMBOL_NAME(bh_active),％eax


	jne handle_bottom_half

ret_with_reschedule:


	cmpl $0,need_resched(％ebx)	# need_resched可被pdate_process_times()所置位


	jne reschedule



	cmpl $0,sigpending(％ebx)


	jne signal_return


restore_all:


	RESTORE_ALL




	ALIGN


signal_return:


	sti				# we can get here from an interrupt handler


	testl $(VM_MASK),EFLAGS(％esp)


	movl ％esp,％eax


	jne v86_signal_return


	xorl ％edx,％edx


	call SYMBOL_NAME(do_signal)


	jmp restore_all




	ALIGN


v86_signal_return:


	call SYMBOL_NAME(save_v86_state)


	movl ％eax,％esp


	xorl ％edx,％edx


	call SYMBOL_NAME(do_signal)


	jmp restore_all




	ALIGN


tracesys:


	movl $-ENOSYS,EAX(％esp)


	call SYMBOL_NAME(syscall_trace)


	movl ORIG_EAX(％esp),％eax


        cmpl $(NR_syscalls),％eax 


        jae 1f 


	call *SYMBOL_NAME(sys_call_table)(,％eax,4)


	movl ％eax,EAX(％esp)		# save the return value


1:	call SYMBOL_NAME(syscall_trace)


	jmp ret_from_sys_call


badsys:


	movl $-ENOSYS,EAX(％esp)


	jmp ret_from_sys_call




	ALIGN


ret_from_exception:


	movl SYMBOL_NAME(bh_mask),％eax


	andl SYMBOL_NAME(bh_active),％eax


	jne handle_bottom_half


	ALIGN




ret_from_intr:				# 硬件中断的返回点


	GET_CURRENT(％ebx)


	movl EFLAGS(％esp),％eax		# mix EFLAGS and CS


	movb CS(％esp),％al


	testl $(VM_MASK | 3),％eax	# return to VM86 mode or non-supervisor?


	jne ret_with_reschedule		# 如果是VM86态或用户态就转ret_with_reschedule


	jmp restore_all





	ALIGN


handle_bottom_half:


	call SYMBOL_NAME(do_bottom_half)


	jmp ret_from_intr




	ALIGN

reschedule:


	call SYMBOL_NAME(schedule)    # test


	jmp ret_from_sys_call



---------------------------------------


kernel/sched.c:




update_process_times()被Timer Bottom Half所调用




static void update_process_times(unsigned long ticks, unsigned long system)


{


/*


 * SMP does this on a per-CPU basis elsewhere


 */


#ifndef  __SMP__


	struct task_struct * p = current;


	unsigned long user = ticks - system;


	if (p->pid) {


		p->counter -= ticks;


		if (p->counter < 0) {


			p->counter = 0;

p->need_resched = 1;



		}


		if (p->priority < DEF_PRIORITY)


			kstat.cpu_nice += user;


		else


			kstat.cpu_user += user;


		kstat.cpu_system += system;


	}


	update_one_process(p, ticks, user, system, 0);


#endif


}