|

softirq->yes->handle_softirq->ret_from_intr->(mode ==

USER_MODE) ? ret_with_reschedule:RESTORE_ALL

|no
signal_pending->yes->signal_return->do_signal->restore_all->RESTORE_ALL
|no
ret_with_reschedule:need_resched->yes->reschedule->schedule->ret_from_sys_call
|no
restore_all->RESTORE_ALL

Task Queues and Bottom Halves :

Task_Queues are extended bottom halves which define functions to be either

called at specific points by using pre-defined task_queues

(tq_immediate,tq_timer,tq_disk.I dont see tq_scheduler in my 2.4s

include/tqueue.h file, where as its there in 2.2.x. and sched.c in my 2.4 source

doesnt call run_task_queue(&tq_scheduler). To understand task_queues you

should first understand bottom_halves or tasklets,and to understand them you

should have a look at linux/softirq.c. Its judicial and advisable to split the

job of the interrupt handler into 2 halves. While the interrupt handler runs by

disabling the interrupt occured,it should be fast in processing the interrupt.

Any processing of data or other stuffs can be taken up by a bottom half. For

example an interrupt handler can fetch the data,and mark up a task to be

executed at known points. What are these known points and when are they

processed ? Bottom halves are consumed in 2 places. In the ret_from_sys_call or

ret_from_exception or in schedule(), by calling do_softirq (defined

linux/kernel/softirq.c). The do_softirq goes through the list of active_softirq

(which is a bitmask of softirq_active(cpu).(check include/linux/interrupt.h,and

follow up the headers in that file.) It goes through each softirq_action

structure and calls the function registered. The tasklets

TASKLET_SOFTIRQ,HI_SOFTIRQ ,each having an instance of tasklet_struct,queue

themselves by calling either tasklet_schedule or tasklet_hi_schedule. The latter

being used by mark_bh routine which is retained for backwards compatibility. The

softirqs are initialised at system startup by calling softirq_init,which opens

up the softirqs TASKLET_SOFTIRQ and HI_SOFTIRQ. The older versions of

bottom_half,are also initialised. If one queues up tasklets by calling

tasklet_schedule, they will be consumed by calling tasklet_action defined in

softirq.c. The older ones using mark_bh are consumed by calling