low-latency scheduling patch for 2.4.0

[Andrew Morton <andrewm@uow.edu.au>]

A patch against kernel 2.4.0 final which provides low-latency
scheduling is at

	http://www.uow.edu.au/~andrewm/linux/schedlat.html#downloads

Some notes:

- Worst-case scheduling latency with *very* intense workloads is now
  0.8 milliseconds on a 500MHz uniprocessor.

  For normal workloads you can expect to achieve better than 0.5
  milliseconds for ever.  For example, worst-case latency between entry
  to an interrupt routine and activation of a usermode process during a
  `make clean && make bzImage' is 0.35 milliseconds.  This is one to
  three orders of magnitude better than BeOS, MacOS and the Windowses.

- Low latency is enabled from the `Processor type and features'
  kernel configuration menu for all architectures.  It would be nice to
  hear from non-x86 users.

- The SMP problem hasn't been addressed.  Enabling low-latency for
  SMP works well under normal workloads but comes unstuck under very
  heavy workloads.  I'll be taking a further look at this.

- The supporting tools `rtc_debug' and `amlat' have been updated. 
  These are quite useful tools for providing accurate measurement of
  latencies.  They may also be used to identify the causes of poor
  latency in the kernel.

- Remaining problem areas (the Don't Do That list) is pretty small:

  - Scrolling the fb console.
  - Running hdparm.
  - Using LILO
  - Starting the X server

- Low latency will probably only be achieved when using the ext2 and
  NFS filesystems.

- If you care about latency, be *very* cautious about upgrading to
  XFree86 4.x.  I'll cover this issue in a separate email, copied
  to the XFree team.

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