netdev - Re: question about softirqs

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Date:	Wed, 13 May 2009 22:55:38 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Andi Kleen <andi@...stfloor.org>
cc:	Chris Friesen <cfriesen@...tel.com>, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Steven Rostedt <rostedt@...dmis.org>,
	David Miller <davem@...emloft.net>, linuxppc-dev@...abs.org,
	paulus@...ba.org, netdev@...r.kernel.org
Subject: Re: question about softirqs

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
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