CFS scheduler, -v5

i'm pleased to announce release -v5 of the CFS scheduler patchset. The 
patch against v2.6.21-rc7 and v2.6.20.7 can be downloaded from:

    http://redhat.com/~mingo/cfs-scheduler/

this CFS release mainly fixes regressions and improves interactivity:

    13 files changed, 211 insertions(+), 199 deletions(-)

the biggest user-visible change in -v5 are various interactivity 
improvements (especially under higher load) to fix reported regressions, 
and an improved way of handling nice levels. There's also a new 
sys_sched_yield_to() syscall implementation for i686 and x86_64.

All known regressions have been fixed. (knock on wood)

[ Note: while CFS's default preemption granularity is currently set to 5 
  msecs, this value does not directly transform into timeslices: for 
  example two CPU-intense tasks will have effective timeslices of 10 
  msecs with this setting. ]

Changes since -v4:

 - interactivity bugfix: fix xterm latencies and general desktop delays 
   and child task startup delays under load. (reported by Willy Tarreau 
   and Caglar Onur)

 - bugfix: the in_atomic_preempt_off() call on !PREEMPT_BKL was buggy
   and spammed the console with bogus warnings.

 - implementation fix: make the nice levels implementation
   starvation-free and smpnice-friendly. Remove the nice_offset hack.

 - feature: add initial sys_sched_yield_to() implementation. Not hooked 
   into the futex code yet, but testers are encouraged to give the 
   syscalls a try, on i686 the new syscall is __NR_yield_to==320, on 
   x86_64 it's __NR_yield_to==280. The prototype is 
   sys_sched_yield_to(pid_t), as suggested by Ulrich Drepper.

 - usability feature: add CONFIG_RENICE_X: those who dont want the 
   kernel to renice X should disable this option. (the boot option and 
   the sysctl is still available too)

 - removed my home-made "Con was right about scheduling fairness" 
   attribution to Con's scheduler interactivity work - some have 
   suggested that Con might want to see another text there. Con,
   please feel free to fill it in!

 - feature: make the CPU usage of nice levels logarithmic instead of 
   linear. This is more usable and more intuitive. (Going four nice 
   levels forward/backwards give half/twice the CPU power) [ This was
   requested a number of times in the past few years and is 
   straightforward under CFS because there nice levels are not tied to 
   any timeslice distribution mechanism. ]

 - cleanup: removed the stupid "Ingo was here" banner printk from 
   sched_init(), the -cfs EXTRAVERSION serves the purpose (of 
   identifying a booted up kernel as a CFS one) equally well.

 - various other code cleanups

As usual, any sort of feedback, bugreport, fix and suggestion is more 
than welcome,

	Ingo
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