Should I enable SMP on heterogeneous multi-threaded CPU's?

I'm building the Linux kernel for a big.LITTLE board and I've been wondering about the CONFIG_SMP option, which enables the kernel's Symmetric-processing support. Linux's documentation says this should be enabled on Multi-Threaded processors, but I wonder if Symmetric Multi processing wouldn't only work properly on processors that are actually symmetric. I understand what SMP is, but I haven't found any hint or documentation saying anything about it's use on Linux built for ARM's big.LITTLE.

Yes, if you want to use more than a single core you have to enable CONFIG_SMP. This in itself will make all cores (both big and little ones) available to the kernel.

Then, you have two options (I'm assuming you are using the mainline Linux kernel or something not excessively different from it, e.g. not an Android kernel):

1. If you also enable CONFIG_BL_SWITCHER (-> Kernel Features -> big.LITTLE support -> big.LITTLE switcher support) and CONFIG_ARM_BIG_LITTLE_CPUFREQ (-> CPU Power Management -> CPU Frequency scaling -> CPU Frequency scaling -> Generic ARM big LITTLE CPUfreq driver), each big core in your SoC will be paired to a little core, and only one of the cores in each pair will be active at any given time, depending on the CPU load. So basically the number of logical cores will be half the number of physical cores, and each logical core will combine one physical big core and one physical little core (unless the total number of big cores differs from the number of little cores, in which case there will be non-paired physical cores that are also logical cores). For each logical core, switching between the big and little physical core will be managed by the cpufreq governor and will be conceptually equivalent to CPU frequency switching.
2. If you don't enable the above two configuration options, then all physical cores will be available as logical cores, can be active at the same time and are treated by the scheduler as if they were identical.

The first option is more suited if you are aiming at low power consumption, while the second option allows you to get the most out of the CPU.

This will change when Heterogeneous Multi-Processing (HMP) support is integrated in the mainline kernel.