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Subject(Fwd) about timer in linux kernel.
Maybe one of the people having written the code want to explain...

Thanks, Ulrich
------- Forwarded message follows -------
From: "meng-ju" <mengju@research.att.com>
To: <Ulrich.Windl@rz.uni-regensburg.de>
Subject: about timer in linux kernel.
Date sent: Fri, 18 May 2001 16:58:55 -0700

Hi! Mr. Ulrich Windl,

I want to know how timer works in kernel.
When we call add_timer(), it will call add_timer_internal to add it to its list.
Now I am confused how the system checks if it is expired or not?
In run_timer_list(),
Why it uses tv1.vec + tv1.index to find out the expiration point while in add_timer_internal(), the expiration timer minus timer_jiffies?
I don't understand what roles jiffies, timer_jiffies and tv1.index play.
Thanks for your patient and answering.

static inline void run_timer_list(void)
{
spin_lock_irq(&timerlist_lock);
while ((long)(jiffies - timer_jiffies) >= 0) {
struct list_head *head, *curr;
if (!tv1.index) {
int n= 1;
do {
cascade_timers(tvecs[n]);
} while (tvecs[n]->index == 1 && ++n < NOOF_TVECS);
}
repeat:
head = tv1.vec + tv1.index;
curr = head->next;
if (curr != head) {
struct timer_list *timer;
void (*fn)(unsigned long);
unsigned long data;

timer = list_entry(curr, struct timer_list, list);
fn = timer->function;
data= timer->data;

detach_timer(timer);
timer->list.next = timer->list.prev = NULL;
timer_enter(timer);
spin_unlock_irq(&timerlist_lock);
fn(data);
spin_lock_irq(&timerlist_lock);
timer_exit();
goto repeat;
}
++timer_jiffies;
tv1.index = (tv1.index + 1) & TVR_MASK;
}
........

Meng-Ju

------- End of forwarded message -------
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