On 2011-05-30 09:03, Pavel Machek wrote: > On Sat 2011-05-28 16:32:45, Jan Kiszka wrote: >> On 2011-05-27 21:11, Gilles Chanteperdrix wrote: >>> On 05/27/2011 08:29 PM, Jonas Witt wrote: >>>> Sorry, I missed the NTP-part. I am not using NTP. Just plain timer >>>> queries on a single system. >>>> >>>> My clock source is tsc which is the same for Xenomai I suppose. >>>> >>>> I wonder how a Xenomai task, even if it occupies 50% or even 90% of a 4 >>>> milliseconds time slice can interfere with the tsc. The tsc is not >>>> incremented via an interrupt, is it? But I do not know much about the >>>> inner workings of these functions. >>> >>> The problem is not the clocksource, the problem is the timer interrupt. >>> The kernel expects 1 timer tick every millisecond. >> >> Not on archs that are CONFIG_NO_HZ capable. > > Umm. NO_HZ is only active while system is idle. Kernel will still > expect the periodic ticks when CPU is busy.... > > (I'm not sure how the compensation works; perhaps it can compensate > even while busy..)
See update_wall_time, the !CONFIG_ARCH_USES_GETTIMEOFFSET includes no fixed tick length. Again, this is also important for Linux when running over hypervisors which tend to miss ticks on overcommitment as well. Jan
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