netdev - Re: wake_up() takes long time to return

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Date:	Mon, 22 Jan 2007 14:56:13 +0530
From:	"kalash nainwal" <nirvana.code@...il.com>
To:	"Arjan van de Ven" <arjan@...radead.org>
Cc:	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: wake_up() takes long time to return

On 1/20/07, Arjan van de Ven <arjan@...radead.org> wrote:
> On Sat, 2007-01-20 at 15:54 +0530, kalash nainwal wrote:
> > 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.
>
>
> Hi,
>
> unfortunately you didn't provide your driver code or a link to it, so
> people who want to help you would have to guess in the dark... could you
> reply to this email with the pointer to the code?
>
> Greetings,
>    Arjan van de Ven
> --
> if you want to mail me at work (you don't), use arjan (at) linux.intel.com
> Test the interaction between Linux and your BIOS via http://www.linuxfirmwarekit.org
>
>

Hi Arjan,

I won't pretend I'm working on an open-source driver. I personally
would be more than happy to share the driver code, but doing so would
probably cost me my job :)

and so...I won't expect anyone to help me with my code either.

Just wanted to know if wake_up is known to take this long to return?
(some known linux quirk may be?) If so then under what conditions? or
it _definitely_ would be my code only that's screwing up?

I'm using do_gettimeofday() before and after wake_up() to measure this time.

Thanks and regards,
-Kalash
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