I have been testing the 2.6.9-rc1, and 2.6.9-rc2 kernel patches over
the past couple days and have been having some issues with
hyperthreading (SMT) turned on.
This problem first exhibited itself when I was testing 2.6.9-rc2-mm2-love2. I noticed the following quirks that ONLY show
themselves with hyperthreading enabled on my 3.0C Pentium 4.
Random HARD LOCKS. No messages from the kernel. Just a good swift hard lock.
Hard locks when mounting two cdrom drives in quick succession.
Turning off hyperthreading solves these issues. Going back to 2.6.8.1
solves these issues.
I then tried 2.6.9-rc1 with no mm or love patches. I had the exact same issues.
Today I downloaded the prepatch to 2.6.9-rc2 and applied it to clean
2.6.8 source. The issues are still there.
I hope someone is paying attention to the way scheduler tweaks and
changes are affecting SMT enabled kernels. I don't think anyone wants
to disable features of their hardware in order to run an optimized
scheduler.