Does linux schedule a process or a thread?

The Linux scheduler (on recent Linux kernels, e.g. 3.0 at least) is scheduling schedulable tasks or simply tasks.

A task may be :

• a single-threaded process (e.g. created by fork without any thread library)
• any thread inside a multi-threaded process (including its main thread), in particular Posix threads (pthreads)
• kernel tasks, which are started internally in the kernel and stay in kernel land (e.g. kworker, nfsiod, kjournald , kauditd, kswapd etc etc…)

In other words, threads inside multi-threaded processes are scheduled like non-threaded -i.e. single threaded- processes.

The low-level clone(2) syscall creates user-land schedulable tasks (and can be used both for creating fork-ed process or for implementation of thread libraries, like pthread). Unless you are a low-level thread library implementor, you don’t want to use clone directly.

AFAIK, for multi-threaded processes, the kernel is (almost) not scheduling the process, but each individual thread inside (including the main thread).

^{Actually, there is some notion of thread groups and affinity in the scheduling, but I don’t know them well}

These days, processors have generally more than one core, and each core is running a task (at some given instant) so you do have several tasks running in parallel.

CPU quantum times are given to tasks, not to processes