Why is only one CPU detected on a Macbook with a dual-core processor?

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http://askubuntu.com – I have a January, 2009 Macbook5,2. It has an Intel Core 2 Duo 2Ghz processor. Since installing Xubuntu 12.04, it runs very slowly. The CPU gadget in the top panel shows only 1 dark column to the left of the graph (apparently 1 per CPU/core, as my desktop machine appropriately shows 4 columns). "I could swear" (though I can't prove it now), that when I had 11.10 installed the machine was faster and the CPU gadget showed 2 columns. The dmesg output (see below) complains "CPU 1 not responding". If I boot OSX, a similar utility (iStat Menus) shows both CPUs active, so I assume the hardware is n (HowTos)