netdev - Re: question about softirqs

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Message-ID: <alpine.LFD.2.00.0905131612180.3561@localhost.localdomain>
Date:	Wed, 13 May 2009 16:17:08 +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:

> > "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
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