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



Question:

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".

EDIT: Complete dmesg output is at http://pastebin.com/meg4kVK3

If I boot OSX, a similar utility (iStat Menus) shows both CPUs active, so I assume the hardware is not to blame.

How can I get Xubuntu to use both cores?

179:[    0.070855] CPU0: Intel(R) Core(TM)2 Duo CPU     P7350  @ 2.00GHz stepping 0a    180-[    0.072003] Performance Events: PEBS fmt0+, Core2 events, Intel PMU driver.    --    189-[    0.072003] Booting Node   0, Processors  #1 Ok.    190:[    0.072003] smpboot cpu 1: start_ip = 9a000    191:[    5.260093] CPU1: Not responding.    192:[    5.260158] Brought up 1 CPUs    193-[    5.260163] Total of 1 processors activated (3979.99 BogoMIPS).  

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