I found that ksoftirqd and MATLAB in my machines comsume extremely high CPU load even they were idle.
The affected machines all run on Centos 6 and 6.2.
The solution is simple, just run:
# date -s "`date`"
on the affected machine and they got back normal.
Reference:
http://marc.info/?l=linux-kernel&m=134113389621450&w=2
Updates:
When I read the system log, the kenrel indeed acknowledge the occurrence of lead second.
I think the problem is from Java, which Matlab runs on it.