Hi Pradeep, On Wed, Apr 15, 2009 at 6:59 AM, pradeep singh <pradeep.rautela@xxxxxxxxx> wrote: > On Wed, Apr 15, 2009 at 12:20 PM, Peter Teoh <htmldeveloper@xxxxxxxxx> wrote: > [...] >>> What I know is that there are only two types of contextes: process, and >>> interrupt. Hardware interrupt handler and deferrable functions(including >>> softirq and tasklete) run in interrupt context. >>> >> >> well....hardware interrupt context is when the hardware IRQ flag is >> still set, which will be turned off (via hardware - not software) when >> u do a iret. >> >> software interrupt context is a kernel processing loop trying to clear >> up all the remaining activities outstanding from hardware interrupt >> context. >> >> and process context is when the kernel switch over to execute with >> full pagetable addrespace ownership (see below). >> >> In Intel manual there is something called "software >> interrupts".....nothing to do with anything mentioned here. >> >>>> >>>> read this for further details (from LDDv3): >>>> >>>> http://lwn.net/images/pdf/LDD3/ch07.pdf >>>> >>>> within it explained what is a kernel thread. So basically while >>>> running process A in kernel mode, it can be intercepted by any kernel >>>> threads, and these kernel threads therefore "DOES NOT HAVE PROCESS >>>> CONTEXT", as they are not related to the currently running process A, >>> If these threads does not have process context, then what context are they in? >>> hardware interrupt context? software interrupt context? Or you'll create another >>> new context named "thread context"? >>> >> >> Firstly, it is related to Intel hardware MMU/pagetable/CR3 mechanism. >> In Linux kernel design, there is no change in the pagetables when >> transition from kernel to user space (and vice versa) are made - >> because of performance cost (called lazy-TLB). It is switch only >> when there is a process switch - by changing the CR3. >> >> "No process context" actually means that the taskstruct's >> mm_struct->mm is NULL. > > Hi Peter, I wonder saying no process context will be appropriate here? > They do have a process context because scheduler does not differentiate between > kernel threads and other processes AFAIK. right? > Yes, u are right, kernel threads and processes are all schedulable. Let me quote another source for explanation: http://www.linuxquestions.org/linux/articles/Technical/Linux_Kernel_Thread "KERNEL THREADS Kernels can attain parallelism by spawning some operations in the background. The kernel achieves this by delegating these operations to the kernel threads. A kernel thread is a process that exists only in kernel space and does not have access to user address space i.e. mm pointer is NULL. Being an integral part of a kernel and running in a kernel address space, they have an access to kernel data structures. A Kernel thread does not have a privilege over other processes and hence they are also as schedulable and pre-emptable as any other process." read the article......very readable. thanks. -- Regards, Peter Teoh -- To unsubscribe from this list: send an email with "unsubscribe kernelnewbies" to ecartis@xxxxxxxxxxxx Please read the FAQ at http://kernelnewbies.org/FAQ
