linux-kernel - Re: udelay and timers

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Date:	Fri, 26 Sep 2008 11:42:33 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Elias Oltmanns <eo@...ensachen.de>
cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org
Subject: Re: udelay and timers

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:
> 
> ...
>         mod_timer(&timera, jiffies + msecs_to_jiffies(10000));
>         mod_timer(&timerb, jiffies + HZ/50);
> ...
> 
> The callbacks for those timers are defined as follows:
> 
> void timera.function(unsigned long data)
> {
>         udelay(120);
>         mod_timer(&timera, jiffies + msecs_to_jiffies(10000));
> }
> 
> void timerb.function(unsigned long data)
> {
> ...
>         mod_timer(&timerb, jiffies + HZ/50);
> }
> 
> 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 ?

Thanks,

	tglx
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