On Mon, Oct 12, 2009 at 12:24 PM, Joel Fernandes <agnel.joel@xxxxxxxxx> wrote:
> Hi Friends,
>
> I have a question about tasklets:
>
> I understand that they are used to defer work during interrupt
> handling. What I don't understand is - how do they handle queuing of
> deferred work?
via softirq mechanism: look at kernel/softirq.c:
55 static struct softirq_action softirq_vec[NR_SOFTIRQS]
__cacheline_aligned_in_smp;
56
57 static DEFINE_PER_CPU(struct task_struct *, ksoftirqd);
58
59 char *softirq_to_name[NR_SOFTIRQS] = {
60 "HI", "TIMER", "NET_TX", "NET_RX", "BLOCK", "BLOCK_IOPOLL",
61 "TASKLET", "SCHED", "HRTIMER", "RCU"
62 };
>From above, we can see that tasklet is just but one of the softirq
(software interrupt) todo list. Tasklet are implement by "HI" for
HI_SOFTIRQ (high priority tasklet) and TASKLET. To indicate a
software interrupt occurring u used raise_softirq() - then the action
will be executed later by softirqd (after wakeup_softirqd() is
called).
The loop to go through each item on the tasklet todo list is here:
398
399 static void tasklet_action(struct softirq_action *a)
400 {
401 struct tasklet_struct *list;
402
408
409 while (list) {
414 if (tasklet_trylock(t)) {
415 if (!atomic_read(&t->count)) {
416 if
(!test_and_clear_bit(TASKLET_STATE_SCHED, &t->state))
417 BUG();
418 t->func(t->data);
419 tasklet_unlock(t);
420 continue;
421 }
422 tasklet_unlock(t);
423 }
> Specially with tasklets, if a tasklet is re-scheduled before the
> previous scheduling of the same tasklet has a chance to run, then the
> tasklet is executed only once - not twice. But what if the work that
> the tasklet was supposed to do in these 2 instances were different and
> that its function was supposed to be passed 2 different structures?
> Shouldn't the tasklet be executed twice with these 2 structures?
>
> I know that work queues are there for that, but how do device drivers
> that use tasklets cope with the above?
>
> Thanks,
> -Joel
--
Regards,
Peter Teoh
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