udelay and timers

[Elias Oltmanns <eo@nebensachen.de>]

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