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Date: Wed, 13 May 2009 22:55:38 +0200 (CEST)
From: Thomas Gleixner <tglx@linutronix.de>
To: Andi Kleen <andi@firstfloor.org>
Subject: Re: question about softirqs
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Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>, netdev@vger.kernel.org,
	Steven Rostedt <rostedt@goodmis.org>, linuxppc-dev@ozlabs.org,
	paulus@samba.org, Ingo Molnar <mingo@elte.hu>,
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On Wed, 13 May 2009, Andi Kleen wrote:
> On Wed, May 13, 2009 at 01:44:59PM -0600, Chris Friesen wrote:
> > Andi Kleen wrote:
> > > On Wed, May 13, 2009 at 01:04:09PM -0600, Chris Friesen wrote:
> > >> Andi Kleen wrote:
> > >>
> > >>> network packets are normally processed by the network packet interrupt's
> > >>> softirq or alternatively in the NAPI poll loop.
> > >> If we have a high priority task, ksoftirqd may not get a chance to run.
> > > 
> > > In this case the next interrupt will also process them. It will just
> > > go more slowly because interrupts limit the work compared to ksoftirqd.
> > 
> > I realize that they will eventually get processed.  My point is that the
> > documentation (in-kernel, online, and in various books) says that
> > softirqs will be processed _on the return from a syscall_. 
> 
> They are. The documentation is correct. 

No, the documentation is wrong for the case that the task, which
raised the softirq and therefor woke up ksoftirqd, has a higher
priority than ksoftirqd. In that case the kernel does _NOT_ schedule
ksoftirqd in the return from syscall path.

And that's all what Chris is pointing out.

Thanks,

	tglx