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

[Ahmed S. Darwish]

Hi all,

Avoid duplicating __tasklet_schedule() and __tasklet_hi_schedule()
code in tasklet_action() and tasklet_hi_action() respectively.

Signed-off-by: Ahmed S. Darwish <darwi...@gmail.com>
---

This also saves a few bytes of image space:

text data bss dec hex filename
3632 12 324 3968 f80 softirq.o.before
3552 12 324 3888 f30 softirq.o.after

diff --git a/kernel/softirq.c b/kernel/softirq.c
index 5b3aea5..3068dc3 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -414,11 +414,8 @@ static void tasklet_action(struct softirq_action *a)
tasklet_unlock(t);
}

- 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);
}
}

@@ -447,11 +444,8 @@ static void tasklet_hi_action(struct softirq_action *a)
tasklet_unlock(t);
}

- local_irq_disable();
- t->next = __get_cpu_var(tasklet_hi_vec).list;
- __get_cpu_var(tasklet_hi_vec).list = t;
- __raise_softirq_irqoff(HI_SOFTIRQ);
- local_irq_enable();
+ /* We were not lucky enough to run, reschedule. */
+ __tasklet_hi_schedule(t);
}
}

Regards,

--
Ahmed S. Darwish
Homepage: http://darwish.07.googlepages.com
Blog: http://darwish-07.blogspot.com

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[Ingo Molnar]

* Ahmed S. Darwish <darwi...@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.

Ingo

[Ahmed S. Darwish]

On Tue, Feb 19, 2008 at 04:52:52PM +0100, Ingo Molnar wrote:
>
> * Ahmed S. Darwish <darwi...@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 ;)).

Regards

--
Ahmed S. Darwish
Homepage: http://darwish.07.googlepages.com
Blog: http://darwish-07.blogspot.com

--

[Ingo Molnar]

* Ahmed S. Darwish <darwi...@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.

Ingo

[Ahmed S. Darwish]

On Wed, Feb 20, 2008 at 11:41:13AM +0100, Ingo Molnar wrote:
>
> * Ahmed S. Darwish <darwi...@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

--
Ahmed S. Darwish
Homepage: http://darwish.07.googlepages.com
Blog: http://darwish-07.blogspot.com

--

[Dmitry Adamushko]

Look at irq_exit() in softirq.c.

The common sequence is ... -> do_IRQ() --> irq_exit() --> invoke_softirq()

--
Best regards,
Dmitry Adamushko

[Ahmed S. Darwish]

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

[btw, your MUA broke the CC list]

--
Ahmed S. Darwish
Homepage: http://darwish.07.googlepages.com
Blog: http://darwish-07.blogspot.com

--