Thanks for the explanation. What is this preempt_count variable for? Also please let me know in which file can I find this interrupt returning code. Gaurav. -----Original Message----- From: Robert Love [mailto:rml@ximian.com] Sent: Friday, June 25, 2004 8:13 PM To: Dhiman, Gaurav Cc: amith; KERNEL Subject: RE: Premptive vs non-premptive kernel. On Fri, 2004-06-25 at 15:53 +0530, Dhiman, Gaurav wrote: > I have one doubt, as you said that timer interrupt sets the need_resched > if the time allocated has expired and while returning from any interrupt > the the scheduler will be called if need_resched is set. My question is, > will the scheduler be invoked by any other interrupts, other than timer > interrupt? I think after setting the need_resched when timer interrupt > will be returning it will clear the need_resched and will call the > scheduler, so all other interrupts occurring after that need not to > worry about need_resched and should not check it, as scheduler has > already been called by timer interrupt. Why are we checking the > need_resched on return of every interrupt as in all cases it will only > be handled by timer interrupt? Two reasons. Primarily, because need_resched is set by things other than scheduler_tick(). Different places in the scheduler set it, so we need to check for it in a more common path. Sticking it at the end of the return-from-interrupt code is simple. Second, because kernel preemptions (preempting when the interrupt handler would of returned to kernel space) can only occur when preempt_count==0. If it is nonzero, then we cannot preempt. So we check for the state "need_resched!=0 and preempt_count==0" at the end of all interrupts. Robert Love -- Kernelnewbies: Help each other learn about the Linux kernel. Archive: http://mail.nl.linux.org/kernelnewbies/ FAQ: http://kernelnewbies.org/faq/
