netdev - Re: question about softirqs

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Message-ID: <4A08C62F.1050105@nortel.com>
Date:	Mon, 11 May 2009 18:43:27 -0600
From:	"Chris Friesen" <cfriesen@...tel.com>
To:	David Miller <davem@...emloft.net>
CC:	linuxppc-dev@...abs.org, paulus@...ba.org, netdev@...r.kernel.org,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: question about softirqs

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.

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?

Thanks,

Chris
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