Hi Amith, Answer for your second question: need_resched is not something specific to a particular process and is not a part of task_struct, its global to kernel and is set by timer interrupt whenever the timer interrupt finds that the time allocated to current process (represented by 'current' pointer) has expired. Your first question is my question also, How the returning code of any interrupt finds whether the control is returning to user space or kernel space? As far as I think, it must be checking if the return address falls in kernel address range or in user address range. Can someone give the detailed insight about this? Regards, Gaurav Dhiman. -----Original Message----- From: kernelnewbies-bounce@nl.linux.org [mailto:kernelnewbies-bounce@nl.linux.org] On Behalf Of amith Sent: Friday, June 25, 2004 12:07 PM To: Robert Love Cc: Gaurav Dhiman; KERNEL; Nagaraj S Subject: Re: Premptive vs non-premptive kernel. Robert Love wrote: >A couple corrections: > >The timer interrupt does the timeslice check as you describe in the same >manner, whether or not CONFIG_PREEMPT is enabled. It simply sets the >need_resched flag. > >The actual preemption occurs when the timer interrupt returns (it >actually occurs when any interrupt returns). If need_resched is set, >the interrupt handler will invoke the scheduler to choose a new task. > > "if need_resched is set" - when executing in kernel mode , if an interrupt occurs, the ISR is called , once ISR is executed , where is need_resched set ?, and how does the ISR know the process context where the CPU was executing earlier i.e to set need_resched in the corresponding task_struct ? > "the interrupt handler will invoke the scheduler to choose a new task" does the timer ISR have the process context of the process which got interrupted , if no , then how is need_reched set for the interrupted process ? cheers, Amith -- Kernelnewbies: Help each other learn about the Linux kernel. Archive: http://mail.nl.linux.org/kernelnewbies/ FAQ: http://kernelnewbies.org/faq/ -- Kernelnewbies: Help each other learn about the Linux kernel. Archive: http://mail.nl.linux.org/kernelnewbies/ FAQ: http://kernelnewbies.org/faq/
