When a process A running, and to run another process B. I need scheduler to schedule.
But scheduler itself is a process, so to schedule the scheduler i need one more scheduler? and this goes on ?.
No, it doesn't happen that way, otherwise it will be vicious cycle.
We take support from hardware here. A Hardware timer runs along with process, when this timer expires, it generates a hardware interrupt. The interrupt handle routine pauses execution of current process and schedules the scheduler. Now scheduler decides which process to schedule next.