linux-kernel - udelay and timers

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Message-ID: <87skrnz6o0.fsf@denkblock.local>
Date:	Fri, 26 Sep 2008 09:51:27 +0200
From:	Elias Oltmanns <eo@...ensachen.de>
To:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>
Cc:	linux-kernel@...r.kernel.org
Subject: udelay and timers

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?

Thank you very much for your assistance,

Elias
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