Question

I’m running 2 threads ( assume they are pthreads for the moment) . Thread_1() makes a user-defined API call which ultimately does some work in the kernel . Thread_2() is totally in user-space.

My question is : Can Thread_2() start executing by pre-empting Thread_1() while the API call is in progress , the control is somewhere inside the kernel ? If not , why , and if I want this scenario to occur ( for any reasons ) , what do I have to do ?

Answer 1

If you are asking whether a blocking kernel call like an fread() which requires disk IO can be pre-empted, then yes.

More specifically a blocking call will basically put Thread_1 to sleep while waiting for whatever it’s waiting for. If Thread_1 is asleep then Thread_2 will be scheduled to run (unless there’s something of higher priority waiting to run).

Edit: If you want a way to be “fairly confident” that Thread_1 is performing a blocking call, make Thread_2 lower priority than Thread_1 (so that it generally doesn’t run unless Thread_1 is blocked) and when it runs, it elevates its priority to a higher level than Thread_1 until the hardware interrupt has been delivered, at which point it lowers its priority and calls sched_yield().