What does the kernel message `CPU0 attaching NULL sched-domain.` mean?

Question

Question: What does this dmesg mean? Is it bad? There is a two year old post about how this could imply degraded I/O performance, could this be/is this still true??

Background: I have been experiencing degraded I/O but I attribute this to using the built in RAID on my motherboard as opposed to a dedicated card. I've been tracking down a few random kernel messages and now I've switched to my desktop -- more out of curiosity than anything. The box has an Intel Core i7 930 processor. The following messages appear with some regularity. I'm running openPBS, if that matters. But these messages appear six times within the past day and a half since the system was rebooted.

The output of uname -a is:

Linux ###### 2.6.32-26-generic #48-Ubuntu SMP Wed Nov 24 10:14:11 UTC 2010 x86_64 GNU/Linux

And the messages from dmesg are:

[  185.609169] CPU0 attaching NULL sched-domain.
[  185.609173] CPU1 attaching NULL sched-domain.
[  185.609175] CPU2 attaching NULL sched-domain.
[  185.609176] CPU3 attaching NULL sched-domain.
[  185.609178] CPU4 attaching NULL sched-domain.
[  185.609179] CPU5 attaching NULL sched-domain.
[  185.609180] CPU6 attaching NULL sched-domain.
[  185.609182] CPU7 attaching NULL sched-domain.
[  185.756764] CPU0 attaching sched-domain:
[  185.756766]  domain 0: span 0,7 level SIBLING
[  185.756768]   groups: 0 (cpu_power = 589) 7 (cpu_power = 589)
[  185.756772]   domain 1: span 0-7 level MC
[  185.756773]    groups: 0,7 (cpu_power = 1178) 1,4 (cpu_power = 1178) 2,5 (cpu_power = 1178) 3,6 (cpu_power = 1178)
[  185.756780] CPU1 attaching sched-domain:
[  185.756781]  domain 0: span 1,4 level SIBLING
[  185.756782]   groups: 1 (cpu_power = 589) 4 (cpu_power = 589)
[  185.756786]   domain 1: span 0-7 level MC
[  185.756787]    groups: 1,4 (cpu_power = 1178) 2,5 (cpu_power = 1178) 3,6 (cpu_power = 1178) 0,7 (cpu_power = 1178)
[  185.756793] CPU2 attaching sched-domain:
[  185.756794]  domain 0: span 2,5 level SIBLING
[  185.756795]   groups: 2 (cpu_power = 589) 5 (cpu_power = 589)
[  185.756799]   domain 1: span 0-7 level MC
[  185.756800]    groups: 2,5 (cpu_power = 1178) 3,6 (cpu_power = 1178) 0,7 (cpu_power = 1178) 1,4 (cpu_power = 1178)
[  185.756806] CPU3 attaching sched-domain:
[  185.756807]  domain 0: span 3,6 level SIBLING
[  185.756808]   groups: 3 (cpu_power = 589) 6 (cpu_power = 589)
[  185.756812]   domain 1: span 0-7 level MC
[  185.756813]    groups: 3,6 (cpu_power = 1178) 0,7 (cpu_power = 1178) 1,4 (cpu_power = 1178) 2,5 (cpu_power = 1178)
[  185.756819] CPU4 attaching sched-domain:
[  185.756820]  domain 0: span 1,4 level SIBLING
[  185.756821]   groups: 4 (cpu_power = 589) 1 (cpu_power = 589)
[  185.756825]   domain 1: span 0-7 level MC
[  185.756826]    groups: 1,4 (cpu_power = 1178) 2,5 (cpu_power = 1178) 3,6 (cpu_power = 1178) 0,7 (cpu_power = 1178)
[  185.756832] CPU5 attaching sched-domain:
[  185.756833]  domain 0: span 2,5 level SIBLING
[  185.756834]   groups: 5 (cpu_power = 589) 2 (cpu_power = 589)
[  185.756838]   domain 1: span 0-7 level MC
[  185.756839]    groups: 2,5 (cpu_power = 1178) 3,6 (cpu_power = 1178) 0,7 (cpu_power = 1178) 1,4 (cpu_power = 1178)
[  185.756845] CPU6 attaching sched-domain:
[  185.756846]  domain 0: span 3,6 level SIBLING
[  185.756847]   groups: 6 (cpu_power = 589) 3 (cpu_power = 589)
[  185.756850]   domain 1: span 0-7 level MC
[  185.756852]    groups: 3,6 (cpu_power = 1178) 0,7 (cpu_power = 1178) 1,4 (cpu_power = 1178) 2,5 (cpu_power = 1178)
[  185.756858] CPU7 attaching sched-domain:
[  185.756859]  domain 0: span 0,7 level SIBLING
[  185.756860]   groups: 7 (cpu_power = 589) 0 (cpu_power = 589)
[  185.756863]   domain 1: span 0-7 level MC
[  185.756865]    groups: 0,7 (cpu_power = 1178) 1,4 (cpu_power = 1178) 2,5 (cpu_power = 1178) 3,6 (cpu_power = 1178)

Answer 1

The message is just telling you that the CPU core is being initialised to a null domain. You can turn the messaged off if you like, but it's doing no harm to your I/O.

You could use iostat to find problems in your IO and what's going on as in that bug.

If you can, install ubuntu-desktop package, log out and log into gnome and see if you suffer the same problem. This will separate out kio and kthread issues from system wide io issues. Run tests on both.

Answer 2

The same message appeared in my logs after explicitly disabling and enabling a CPU core with:

echo 0 > /sys/devices/system/cpu/cpu6/online
echo 1 > /sys/devices/system/cpu/cpu6/online

My suggestion is, if you get this message, try cycling through disabling and enabling the CPU core as above.