OK, backing out that patch seems to fix it. Thanks Andy ;-)
This is a shot in the dark. We haven't confirmed 1. there is a problem 2. that this is the problem nor 3. that this patch will fix the problem. I say we wait for the results of 1. If the improved smp nice handling patch ends up being responsible then it should not be merged upstream, and then this patch can be tested on top.
Martin I know your work move has made it not your responsibility to test backing out this change, but are you aware of anything being done to test this hypothesis?