Messages in this thread | | | Date | Fri, 26 Sep 2008 11:42:33 +0200 (CEST) | From | Thomas Gleixner <> | Subject | Re: udelay and timers |
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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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