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

Ahmed S. Darwish
2008-02-20T19:41:57Z

On Wed, Feb 20, 2008 at 03:20:52PM +0100, Dmitry Adamushko wrote:
> On 20/02/2008, Ahmed S. Darwish <darwish.07@gmail.com> wrote:
> > 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 ?
>
> Look at irq_exit() in softirq.c.
>
> The common sequence is ... -> do_IRQ() --> irq_exit() --> invoke_softirq()
>
>

Great, this was the last missing block in my understanding of softirqs :).
Thank you.

[btw, your MUA broke the CC list]