stable backport: sched: Fix migration thread runtime bogosity

[Steven Rostedt]

Greg, Ben,

Can you add this commit to the stable branches. Without it, the
migration thread's accounting is just totally screwed up:

By running a simple shell while loop along with a ps loop, top shows:

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND            
   17 root      RT   0     0    0    0 S 9999.0  0.0   9:27.50 migration/3    
   13 root      RT   0     0    0    0 S 128.2  0.0   9:50.23 migration/2       
 2805 root      20   0  105m 1904 1416 S 13.9  0.1   0:06.69 bash               
 4090 root      20   0  105m 1904 1416 S  2.0  0.1   0:00.90 bash               
 2773 root      20   0 85484 3372 2620 S  0.3  0.2   0:00.03 sshd               


For 3.4, the commit can be directly cherry picked:

commit 8f6189684eb4e85e6c593cd710693f09c944450a
Author: Mike Galbraith <mgalbraith@xxxxxxx>
Date:   Sat Aug 4 05:44:14 2012 +0200

    sched: Fix migration thread runtime bogosity

Below is a backport for 3.2.

Thanks! 

-- Steve