Re: question about softirqs

[Thomas Gleixner <tglx@linutronix.de>]

On Wed, 13 May 2009, Andi Kleen wrote:

> > "If a soft irq is raised in process context, raise_softirq() in
> > kernel/softirq.c calls wakeup_softirqd() to make sure that ksoftirqd
> 
> softirqd is only used when the softirq runs for too long or when
> there are no suitable irq exits for a long time.
> 
> In normal situations (not excessive time in softirq) they don't
> do anything. 

Err, no. Chris is completely correct:

        if (!in_interrupt())
		wakeup_softirqd();

We can not rely on irqs coming in when the softirq is raised from
thread context. An irq_exit might be faster to process it than the
scheduler can schedule ksoftirqd in, but ksoftirqd is woken and runs
nevertheless. If it finds a softirq pending then it processes them in
it's context and irq_exit calls to softirq are returning right away.

Thanks,

	tglx