Messages in this thread | | | From | Elias Oltmanns <> | Subject | udelay and timers | Date | Fri, 26 Sep 2008 09:51:27 +0200 |
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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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