linux-kernel - Re: [PATCH] Tasklets: Avoid duplicating __tasklet_{,hi

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Message-ID: <20080220133718.GA8205@ubuntu>
Date:	Wed, 20 Feb 2008 15:37:18 +0200
From:	"Ahmed S. Darwish" <darwish.07@...il.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Tasklets: Avoid duplicating __tasklet_{,hi_}schedule()
	code

On Wed, Feb 20, 2008 at 11:41:13AM +0100, Ingo Molnar wrote:
> 
> * Ahmed S. Darwish <darwish.07@...il.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

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