netdev - Re: question about softirqs

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Message-ID: <20090512081237.GA16403@elte.hu>
Date:	Tue, 12 May 2009 10:12:37 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Chris Friesen <cfriesen@...tel.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>
Cc:	David Miller <davem@...emloft.net>, linuxppc-dev@...abs.org,
	paulus@...ba.org, netdev@...r.kernel.org
Subject: Re: question about softirqs


* Chris Friesen <cfriesen@...tel.com> wrote:

> This started out as a thread on the ppc list, but on the 
> suggestion of DaveM and Paul Mackerras I'm expanding the receiver 
> list a bit.
> 
> Currently, if a softirq is raised in process context the 
> TIF_RESCHED_PENDING flag gets set and on return to userspace we 
> run the scheduler, expecting it to switch to ksoftirqd to handle 
> the softirqd processing.
> 
> I think I see a possible problem with this. Suppose I have a 
> SCHED_FIFO task spinning on recvmsg() with MSG_DONTWAIT set. Under 
> the scenario above, schedule() would re-run the spinning task 
> rather than ksoftirqd, thus preventing any incoming packets from 
> being sent up the stack until we get a real hardware 
> interrupt--which could be a whole jiffy if interrupt mitigation is 
> enabled in the net device.

TIF_RESCHED_PENDING will not be set if a SCHED_FIFO task wakes up a 
SCHED_OTHER ksoftirqd task. But starvation of ksoftirqd processing 
will occur.

> DaveM pointed out that if we're doing transmits we're likely to 
> hit local_bh_enable(), which would process the softirq work.  
> However, I think we may still have a problem in the above rx-only 
> scenario--or is it too contrived to matter?

This could occur, and the problem is really that task priorities do 
not extend across softirq work processing.

This could occur in ordinary SCHED_OTHER tasks as well, if the 
softirq is bounced to ksoftirqd - which it only should be if there's 
serious softirq overload - or, as you describe it above, if the 
softirq is raised in process context:

        if (!in_interrupt())
                wakeup_softirqd();

that's not really clean. We look into eliminating process context 
use of raise_softirq_irqsoff(). Such code sequence:

	local_irq_save(flags);
	...
	raise_softirq_irqsoff(nr);
	...
	local_irq_restore(flags);

should be converted to something like:

	local_irq_save(flags);
	...
	raise_softirq_irqsoff(nr);
	...
	local_irq_restore(flags);
	recheck_softirqs();

If someone does not do proper local_bh_disable()/enable() sequences 
for micro-optimization reasons, then push the check to after the 
critcal section - and dont cause extra reschedules by waking up 
ksoftirqd. raise_softirq_irqsoff() will also be faster.

	Ingo
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