Interactivity degrades with CONFIG_[FAIR]_GROUP_SCHED set in 2.6.24 and later

[Joe Peterson <joe@skyrush.com>]

First of all, I am very excited to see the CFS in the kernel.
Interactivity is really great; much better than before.

When going from kernel 2.6.23 to 2.6.24, however, I noticed that
interactivity (namely the smoothness of mouse motion) was noticeably
less smooth under load.  The most obvious case was while pulling down a
Subversion repository; the mouse would freeze for quite long periods
while being moved (i.e. a large fraction of a second), which never
happens under 2.6.23.  I was using ZFS-FUSE on the partition I was
writing to, and it uses a lot of CPU at times, but 2.6.23 seems to
handle it better.

After reading many posts on similar sounding issues, I decided to try
getting later versions of the kernel: 2.6.25-rc5 and the latest 2.6.25
from the git archive (as of yesterday, 3/16) in case recent patches
addressed this.  They both still exibit the issue, however.

I then tried compiling without CONFIG_GROUP_SCHED (which also turned off
CONFIG_FAIR_GROUP_SCHED and a number of other options), and the behavior
seems to be back to the (better) 2.6.23 performance, at least has far as
what I can tell from my tests.

After playing around with the later kernels with CONFIG_GROUP_SCHED
enabled, I noticed a number of random times the mouse was not smooth and
seemed to not be getting an "interactive" amount of CPU time, but I have
a couple of tasks that repeat the problem reliably, so if there is any
debug information I can provide, please let me know, and I'll be glad to
help out in any way I can.

					Thanks, Joe