wake_up() takes long time to return

["kalash nainwal" <nirvana.code@gmail.com>]

Hi there,

We've a kernel (n/w) module, which sits over ethernet. Whenever a pkt
is received (in softirq), after doing some minimal processing,
wake_up() is called to wake up another kernel thread which does rest
(bulk) of the processing.

We notice that this wake_up() call is sometimes taking as long as 48
milli-seconds to return. This happens around 10 times out of 10M. We
earlier thought its possibly because of the contention on rq->lock,
but we see the same phenomenon even on a uniprocessor box. So obviosly
thats not the case.

We can't figure out any other reason for wake_up() to take this much
time? As this call comes directly in our (receive) hotpath, we're very
concerned. Any help would be greatly appreciated.

Thanks and regards,
-Kalash