question about softirqs
Chris Friesen
cfriesen at nortel.com
Sat May 9 10:28:58 EST 2009
Paul Mackerras wrote:
> If a soft irq is raised in process context, raise_softirq() in
> kernel/softirq.c calls wakeup_softirqd() to make sure that ksoftirqd
> runs soon to process the soft irq. So what would happen is that we
> would see the TIF_RESCHED_PENDING flag on the current task in the
> syscall exit path and call schedule() which would switch to ksoftirqd
> to process the soft irq (if it hasn't already been processed by that
> stage).
I think I see a problem with this. Suppose I have a SCHED_FIFO task
spinning on recvmsg() with MSG_DONTWAIT set (and maybe doing other stuff
if there are no messages). Under the scenario you described, schedule()
would re-run the spinning task, no? This could prevent any incoming
packets from actually 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.
Chris
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