True, I should have said it is not concurrently stopping all the threads. If you actually need the register state of a thread that is currently running on another processor it may be stopped to get it (though if you ask for the state of a thread that is not currently executing you may get its saved register state out of the kernel without having to actually pause its execution, of course being able to tell what is and is not actually running on other CPUs at that granularity is not really feasible).