I noticed large latency spikes in 2.6.28 (and 2.6.27.10) which would make the system unresponsive for minutes at a time (alt+Fn switching wouldn't even work) when the system is under load. I could trigger the latency by running a large make -j 3. Sometimes it would trigger in a few seconds other times it would take a few minutes.Please disregard this. I just had a latency spike again, so this is not the change I'm looking for... I'll need to look for what is causing it again, apparently not only the make -j 3 I thought, and do another test.
I bisected it in 2.6.27.10 to d9a888b061f55534016d2d86a21639948312a117 -> Revert "sched_clock: prevent scd->clock from moving backwards" (ca7e716c7833aeaeb8fedd6d004c5f5d5e14d325 in 2.6.28) and reverting it in 2.6.28 make the latency spike go away. At least, so far no spike in over 24 hours.