linux-kernel - Re: udelay and timers

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Message-ID: <87prmowsxx.fsf@denkblock.local>
Date:	Sun, 28 Sep 2008 10:55:22 +0200
From:	Elias Oltmanns <eo@...ensachen.de>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org
Subject: Re: udelay and timers

[Apologies for the delayed response, I have just temporary internet
access right now.]

Thomas Gleixner <tglx@...utronix.de> wrote:
> On Fri, 26 Sep 2008, Elias Oltmanns wrote:
>
>> Hi all,
>> 
>> finally, I have managed to identify the cause of some odd symptoms on my
>> system, but I need your help to understand what is really going on and
>> what should be done to fix things. My problem is this: I have written a
>> small test module which, in due course, does the following:
[...]
>> Now, I can observe that because of the call to udelay() in the callback
>> of timera, timerb rather frequently fires *much* too early, i.e. after
>> less than 1 msec rather than 20 msecs. This means that an udelay in a
>> timer callback heavily affects the precision of other timers running at
>> the time. The effect if particularly grave on a tickless system, but
>> even when NO_HZ was not set, I have observed this behaviour.
>> 
>> As I understand, udelay() is meant to be usable in softirq context. What
>> can I do to find out what exactly causes the problem?
>
> I have no idea what's going wrong. Can you please provide the full
> source of your test module ?

Even though I still don't understand exactly what is going on myself, I
have, after some more testing, come to the conclusion that the ath5k
driver is to blame for messing up softirq handling. I have to talk to
the wireless people in order to get this sorted out. Sorry for the
noise.

Regards,

Elias
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