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#define DECREMENTER_EXCEPTION						      \	START_EXCEPTION(Decrementer)					      \	NORMAL_EXCEPTION_PROLOG;					      \	lis     r0,TSR_DIS@h;           /* Setup the DEC interrupt mask */    \	mtspr   SPRN_TSR,r0;		/* Clear the DEC interrupt */	      \	addi    r3,r1,STACK_FRAME_OVERHEAD;				      \	EXC_XFER_LITE(0x0900, timer_interrupt)	void timer_interrupt(struct pt_regs * regs)	=>int cpu = smp_processor_id();	=>if (atomic_read(&ppc_n_lost_interrupts) != 0)//如果还有未处理的外部中断处理函数，则处理		do_IRQ(regs);	=>old_regs = set_irq_regs(regs);//保存寄存器现场，注意old_regs是指针，指向现场全局寄存器的指针		=>struct pt_regs *old_regs, **pp_regs = &__get_cpu_var(__irq_regs);//保存在__irq_regs全局变量里面，每个核一个		old_regs = *pp_regs;		*pp_regs = new_regs;		return old_regs	=>irq_enter();//中断计数加一，进入中断上下文	=>while ((ticks = tb_ticks_since(per_cpu(last_jiffy, cpu))) >= tb_ticks_per_jiffy)		per_cpu(last_jiffy, cpu) += tb_ticks_per_jiffy;		account_process_time(regs);//#define account_process_time(regs)	update_process_times(user_mode(regs))			=>scheduler_tick();//选择合适的进程，用于时钟中断返回继续调度				=>curr->sched_class->task_tick(rq, curr);//.task_tick		= task_tick_fair					=>struct sched_entity *se = &curr->se;					=>for_each_sched_entity(se) {						cfs_rq = cfs_rq_of(se);						entity_tick(cfs_rq, se);                            =>update_curr(cfs_rq);                            =>if (cfs_rq->nr_running > 1 || !sched_feat(WAKEUP_PREEMPT))		                        check_preempt_tick(cfs_rq, curr);                                =>ideal_runtime = sched_slice(cfs_rq, curr);//计算理想调度时间                                =>delta_exec = curr->sum_exec_runtime - curr->prev_sum_exec_runtime;	                            =>if (delta_exec > ideal_runtime) {//实际时间大于理想时间，则调度		                            resched_task(rq_of(cfs_rq)->curr);		                            /*		                             * The current task ran long enough, ensure it doesn't get		                             * re-elected due to buddy favours.		                             */		                            clear_buddies(cfs_rq, curr);		                            return;	                            }                    }		if (cpu != boot_cpuid)			continue;		tb_next_jiffy = tb_last_jiffy + tb_ticks_per_jiffy;		if (per_cpu(last_jiffy, cpu) >= tb_next_jiffy) {			tb_last_jiffy = tb_next_jiffy;			do_timer(1);			timer_recalc_offset(tb_last_jiffy);			timer_check_rtc();		}	=>next_dec = tb_ticks_per_jiffy - ticks;	=>set_dec(next_dec);//设置DEC寄存器，用于触发下一次的时钟中断	=>irq_exit();	=>set_irq_regs(old_regs);	void timer_interrupt(struct pt_regs * regs)	=>struct decrementer_clock *decrementer =  &__get_cpu_var(decrementers);	=>set_dec(DECREMENTER_MAX);//重新设置最大值		=>mtspr(SPRN_DEC, val);	=>__get_cpu_var(irq_stat).timer_irqs++;	=>old_regs = set_irq_regs(regs);//保存寄存器现场，注意old_regs是指针，指向现场全局寄存器的指针		=>struct pt_regs *old_regs;		old_regs = __this_cpu_read(__irq_regs);//保存在__irq_regs全局变量里面，每个核一个		__this_cpu_write(__irq_regs, new_regs);		return old_regs;	=>irq_enter();//中断计数加一，进入中断上下文		desc->handle_irq = handle_level_irq;//Mpc8xx_pic.c (c:\linux\linux-2.6.23\arch\powerpc\sysdev):	=>desc->chip->ack(irq);/* Start handling the irq */	=>desc->status |= IRQ_INPROGRESS;/* Mark the IRQ currently in progress.*/	=>do//可能好几个中断都上报		desc->status &= ~IRQ_PENDING;		action_ret = handle_IRQ_event(irq, action);			=>do//依次查看是否共享中断				ret = action->handler(irq, action->dev_id);				retval |= ret;				action = action->next;			=>while (action);	=>while ((desc->status & (IRQ_PENDING | IRQ_DISABLED)) == IRQ_PENDING);

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