Hi..... > yes you are right .. i mean it's not going to be interrupted cause > it's time quantum has finished .. but interrupts coming from hardware > (for example) will interrupt the execution for sure and continue > execution after the completion of the interrupt handler as you've > noted Sometimes the word "interrupted" brings confusion when you talk about interrupts :) Interrupt (be it a software interrupt or hardware interrupt) interrupts current CPU execution path and switch to interrupt handler. After it is finished, CPU jumps back to the last interrupted code path by using information saved on kernel mode stack (which is EIP, instruction pointer) But the interrupt itself doesn't change the current running process, it's just the process's code that is interleaved by interrupt handler. > > <Raja> Who do this decrementing of the counter and setting the flag > > etc ? Is it the timer interrupt handler which runs on each tick ? > > <Raja> > yes To be precise, by the bottom half of the interrupt timer > > so it's not interrupted while it's in the kernel mode but it's > > marked for being scheduled later.. > > once the process has finished what it was doing in the kernel "a > > syscall", it'll call "ret_from_syscall" before it get back to user > > land simply the "current->need_resched" is being checked in > > "ret_from_syscall" .. if it was set then schedule is called .. else > > it returns to user land to continue executing A bit out of context from original question. Please notice that even the task is in user mode when the time slice is over, it will still be scheduled out. Why? Simply because ret_from_syscall's code segment is also called by ret_from_intr. So, the above rule is still valid even the process is in user mode. > > <Raja> I believe above description holds good for only non > > preemptive kernels. ? How does rescheduling happen in a preemptive > > kernel ? <Raja> basically, in preemptive kernel, there are more preemption point. The addition is, scheduling can happen even the process comes back to kernel mode, while in non preemptive kernel, scheduling only happens when the process wants to return to user mode only. Hope this helps regards Mulyadi -- Kernelnewbies: Help each other learn about the Linux kernel. Archive: http://mail.nl.linux.org/kernelnewbies/ FAQ: http://kernelnewbies.org/faq/
