Re: top stack (l)users for 2.5.69

[Roland Dreier <roland@topspin.com>]

>>>>> "Richard" == Richard B Johnson <root@chaos.analogic.com> writes:

    Richard> On Wed, 7 May 2003, Roland Dreier wrote: The kernel
    Richard> stack, at least for ix86, is only one, set upon startup
    Richard> at 8192 bytes above a label called init_task_unit. The
    Richard> kernel must have a separate stack and, contrary to what
    Richard> I've been reading on this list, it can't have more kernel
    Richard> stacks than CPUs and, I don't see a separate stack
    Richard> allocated for different CPUs.

    Roland> This is total nonsense.  Please don't confuse matters by
    Roland> spreading misinformation like this.  Every task has a
    Roland> separate (8K) kernel stack.  Look at the implementation of
    Roland> do_fork() and in particular alloc_task_struct().

    Roland> If there were only one kernel stack, what do you think
    Roland> would happen if a process went to sleep in kernel code?

    Richard> No, No. That is a process stack. Every process has it's
    Richard> own, entirely seperate stack. This stack is used only in
    Richard> user mode. The kernel has it's own stack. Every time you
    Richard> switch to kernel mode either by calling the kernel or by
    Richard> a hardware interrupt, the kernel's stack is used.

Again, this is nonsense and misinformation.  Look at do_fork() and
alloc_task_struct().  Do you see how alloc_task_struct() is just
defined to be __get_free_pages(GFP_KERNEL,1) for i386?  Do you
understand that that just allocates two pages (8K) of kernel memory?
Do you see that it is never mapped into userspace, and that anyway
a userspace process can use far more than 8K of stack?

That 8K of memory is used for the kernel stack for a particular
process.  When a process makes a system call, that specific stack is
used as the kernel stack.

    Richard> When a task sleeps, it sleeps in kernel mode. The kernel
    Richard> schedules other tasks until the sleeper has been
    Richard> satisfied either by time or by event.

Right.  Now think about where the kernel stack for the process that is
sleeping in the kernel is kept.

 - Roland