For most people, of course, it is working just fine. There do appear to be some problems, however, for those who run systems under very high load. The SCSI scanning order has changed, though, for some users with multiple adaptors, leading to an unwelcome renaming of the SCSI devices on the system. The 2.4.0 kernel was about as stable as it could have been, really. Every stable kernel series has taken a few releases to truly stabilize, and 2.4 is no exception.
Some rough edges remain, but it's getting there.
Fixing the scheduler . It all started with a posting from Fabio Riccardi regarding some performance problems he has been having. It seems he's working on some improvements to Apache, and he has found that system performance drops badly when he starts running over 1000 processes. The nature of the problem is quite well known. The Linux kernel maintains a single queue of all the processes that would like to be running in the CPU at any time.