Re: [PATCH] Tasklets: Avoid duplicating __tasklet_{,hi_}schedule() code

Ahmed S. Darwish
2008-02-20T13:40:05Z

On Wed, Feb 20, 2008 at 11:41:13AM +0100, Ingo Molnar wrote:
>
> * Ahmed S. Darwish <darwish.07@gmail.com> wrote:
>
> > > > - local_irq_disable();
> > > > - t->next = __get_cpu_var(tasklet_vec).list;
> > > > - __get_cpu_var(tasklet_vec).list = t;
> > > > - __raise_softirq_irqoff(TASKLET_SOFTIRQ);
> > > > - local_irq_enable();
> > > > + /* We were not lucky enough to run, reschedule. */
> > > > + __tasklet_schedule(t);
> > >
> > > i think there's a subtle difference that you missed: this one does
> > > __raise_softirq_irqoff(), while __tasklet_schedule() does a
> > > raise_softirq_irqoff(). (note the lack of undescores)
> > >
> > > the reason is to avoid infinitely self-activating tasklets.
> >
> > Indeed, thanks a lot for the explanation. (maybe it's time to check
> > for new eyeglasses ;)).
>
> nah, it's rather subtle and the code looked good to me at first but i
> remembered that there was some small detail here to watch out for.
>
> i really dont like tasklets due to their many, arbitrary scheduling
> limitations, we should really use the "turn tasklets into kthreads"
> patch i posted last year.
>

While we are at it, there's a small question that is bothering me
for a while (and I'm really thankful for help).

I keep reading that softirqs (and naturally, tasklets) got executed
in interrupt context at the return from hardirq code path.

Checking entry_32.S, I find no mentioning of softirqs on the return
path (beginning from ret_from_intr: to restore_all: )

The only invocation I'm able to find is from local_bh_enable() and
from ksoftirqd/n threads (by calling do_softirq()). AFAIK, both
invocations occur in a _nont-interrupt_ context (exception context).

So, where does the interrupt-context tasklets invocation really
occur ?

Thanks