Anything in square brackets in top/ps is a kernel thread.

mmcqd is the MMC queue thread, managing reads/writes to the MMC - there'll be a separate thread per device (eMMC & SD card).

kmmcd is the MMC core workqueue thread, so there'll just be the one of those. It'll do whatever pre-processing of the read/write request is needed before handing it on to the queue thread.

kondemand is the On Demand CPU governor, managing switching between different clock speeds as required.

kswapd is the thread managing read/write to swap.

If you're seeing high CPU usage on all these, it sounds like your N900 is thrashing the swap space. Check what programs you have running and how much memory they're consuming.