[LAU] Ubuntu: changing realtime priority of irqs

[Florian Schmidt]

On Saturday 21 July 2007, Atte André Jensen wrote:

> > Which kernel version is this?
>
> Homemade (with mingo rt-patch) 2.6.18-rt7
>
> 2.6.20-16-lowlatency

The lowlatency kernel of ubuntu is not a realtime kernel. And for your home 
made kernel, please let us see your .config

e.g.

~$ grep PREEMPT /boot/config-$(uname -r) 
# CONFIG_PREEMPT_NONE is not set
# CONFIG_PREEMPT_VOLUNTARY is not set
# CONFIG_PREEMPT_DESKTOP is not set
CONFIG_PREEMPT_RT=y
^^^^^^^^^^^^^^^
CONFIG_PREEMPT=y
CONFIG_PREEMPT_SOFTIRQS=y
CONFIG_PREEMPT_HARDIRQS=y
CONFIG_PREEMPT_BKL=y
CONFIG_PREEMPT_RCU=y
# CONFIG_DEBUG_PREEMPT is not set
# CONFIG_CRITICAL_PREEMPT_TIMING is not set

I underlined the relevant line there

>
> >   On Debian, I get ksoftirqd as well, one
> > per processor core.  ksoftirqd is part of the 2.6 kernel series.  I
> > imagine you're still using a 2.4 kernel on your Debian system,
>
> Nope...
>
> > or the
> > kernel is configured and built differently to mine.
>
> Might be...
>
> > It may be possible to rebuild the Ubuntu kernel you are using to match
> > what you are used to, but I don't know if that will get you what you
> > want.
>
> I grabbed a vanilla 2.6.22.1 and applied the latest patch and quick
> testing seems to suggest that the system is running at least at well as
> under debian:
>
> atte at ajstrup:~$ uname -r
> 2.6.22.1-rt4
>
> atte at ajstrup:~$ ps aux | grep IRQ
> root        36  0.0  0.0      0     0 ?        S<   18:15   0:00 [IRQ-9]
> root       268  0.0  0.0      0     0 ?        S<   18:16   0:00 [IRQ-4]
> root       292  0.0  0.0      0     0 ?        S<   18:16   0:00
> [IRQ-12]
> root       293  0.0  0.0      0     0 ?        S<   18:16   0:00 [IRQ-1]
> root      1108  0.0  0.0      0     0 ?        S<   18:16   0:00
> [IRQ-14]
> root      1139  0.0  0.0      0     0 ?        S<   18:16   0:00
> [IRQ-15]
> root      1140  0.2  0.0      0     0 ?        S<   18:16   0:02
> [IRQ-11]
> root      1936  0.0  0.0      0     0 ?        S<   18:16   0:00 [IRQ-8]
> root      2025  0.0  0.0      0     0 ?        S<   18:16   0:00 [IRQ-7]
> root      2103  0.3  0.0      0     0 ?        S<   18:16   0:03
> [IRQ-10]
> root      3432  0.0  0.0      0     0 ?        S<   18:16   0:00 [IRQ-3]
> atte      4674  0.0  0.0   2888   772 pts/2    R+   18:33   0:00 grep
> IRQ
>
> >   I've never tried setting real-time priority on kernel threads
> > like that, and I don't know if it would achieve anything.  I trust it
> > did for you?
>
> Alot!
>
> Bottom line: It seems it's still necessary to roll your own kernel to
> get optimal realtime performance under ubuntu :-(



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