On two different servers (with Ubuntu 12.04LTS AMD64) I have seen the following behaviour:

op - 10:50:05 up 305 days, 21:17,  1 user,  load average: 1.94, 2.52, 2.97
Tasks: 141 total,   2 running, 139 sleeping,   0 stopped,   0 zombie
Cpu(s): 41.5%us,  6.5%sy,  0.0%ni, 51.8%id,  0.0%wa,  0.2%hi,  0.1%si,  0.0%st
Mem:   8178432k total,  5753740k used,  2424692k free,   159480k buffers
Swap: 15625208k total,        0k used, 15625208k free,  4905292k cached

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND                                                                               
    1 root      20   0 23928 2072 1216 S    0  0.0   0:56.42 init                                                                                  
    2 root      20   0     0    0    0 S    0  0.0   0:00.01 kthreadd                                                                              
    3 root      RT   0     0    0    0 S    0  0.0   0:01.23 migration/0                                                                           
    4 root      20   0     0    0    0 S    0  0.0   2:39.82 ksoftirqd/0                                                                           
    5 root      RT   0     0    0    0 S    0  0.0   0:00.00 watchdog/0                                                                            
    6 root      RT   0     0    0    0 S    0  0.0   0:02.99 migration/1                                                                           
    7 root      20   0     0    0    0 S    0  0.0   2:32.15 ksoftirqd/1                                                                           
    8 root      RT   0     0    0    0 S    0  0.0   0:00.00 watchdog/1                                                                            
    9 root      RT   0     0    0    0 S    0  0.0   0:11.67 migration/2                                                                           
   10 root      20   0     0    0    0 S    0  0.0  29:00.34 ksoftirqd/2  

The server is working fine, but top shows all processes as using 0% CPU. A reboot fixed this on an earlier machine, but I haven't yet tried it on this one.

I have tried top several times, and so am sure that I haven't accidentally pressed '<' or '>' to sort by a different column. Sorting the process list by all of the available columns, stills shows 0% CPU for all displayed processes.

What is going on? If this a kernel bug?

Update: If I use top -p <PID> for a know, busy process, top still displays 0% CPU for that process.