Re: question about softirqs

From: "Chris Friesen" <cfriesen@nortel.com>
To: Andi Kleen <andi@firstfloor.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>,
	netdev@vger.kernel.org, Ingo Molnar <mingo@elte.hu>,
	Steven Rostedt <rostedt@goodmis.org>,
	linuxppc-dev@ozlabs.org, paulus@samba.org,
	Thomas Gleixner <tglx@linutronix.de>,
	David Miller <davem@davemloft.net>
Subject: Re: question about softirqs
Date: Wed, 13 May 2009 09:05:01 -0600	[thread overview]
Message-ID: <4A0AE19D.9040509@nortel.com> (raw)
In-Reply-To: <87my9hkrmw.fsf@basil.nowhere.org>

Andi Kleen wrote:
> Thomas Gleixner <tglx@linutronix.de> writes:

>>Err, no. Chris is completely correct:
>>
>>        if (!in_interrupt())
>>		wakeup_softirqd();
> 
> Yes you have to wake it up just in case, but it doesn't normally
> process the data because a normal softirq comes in faster. It's
> just a safety policy. 

What about the scenario I raised earlier, where we have incoming network
packets, no hardware interrupts coming in other than the timer tick, and
a high-priority userspace app is spinning on recvmsg() with MSG_DONTWAIT
set?

As far as I can tell, in this scenario softirqs may not get processed on
return from a syscall (contradicting the documentation).  In the worst
case, they may not get processed until the next timer tick.

Chris