Re: [Question] one question about 'current' in scheduler_tick()

[Steven Rostedt]

On Mon, 17 Oct 2005, liyu wrote:

> Hi Steve, Thanks once again :)
>
>     I searched arch/i386/kernel/irq.c, and got this line of code:
>
>     irqctx->tinfo.task = curctx->tinfo.task;
>
>     I think it just is answer that I want to get. It seem interrupt
> stack is
> created at top of the original kernel stack. Are my words right?

Not with 4KSTACK.  With 4KSTACK defined, the interrupts have their own
stack.  So lets look at how current works (for x86).

static inline struct task_struct * get_current(void)
{
	return current_thread_info()->task;
}

#define current get_current()

OK, current is really a MACRO that calls get_current, which then calls
current_thread_info to get the task structure.

Now current_thread_info is

/* how to get the thread information struct from C */
static inline struct thread_info *current_thread_info(void)
{
	struct thread_info *ti;
	__asm__("andl %%esp,%0; ":"=r" (ti) : "0" (~(THREAD_SIZE - 1)));
	return ti;
}


Where THREAD_SIZE is really the size of the kernel stack:

#ifdef CONFIG_4KSTACKS
#define THREAD_SIZE            (4096)
#else
#define THREAD_SIZE		(8192)
#endif

4K when CONFIG_4KSTACKS is defined, and 8K otherwise.  Since 4K is half of
8K (obviously) you need to be more carefull not to overflow the stack.  So
to help with this, we give interrupts there own stacks.  When
CONFIG_4KSTACKS is _not_ defined, interrupts just use the stack of the
current running thread.

So the thread_info is really just placed on the bottom (or top, depending
on your view of where things get added).  This memory used for the stack
is _always_ page aligned and always a multiple of pages. So the kernel
takes advantage of this. By placing the thread_info at the bottom of the
stack, you only need to mask off the bits on the stack pointer to find the
thread_info.

>
>     In passing, How 'current' work in this case when we disable 4KSTACK?
> I just found these code only be compiled when we define CONFIG_4KSTACK,
> but cann't found #else branch relevantly.
>

You don't need to do this copy when CONFIG_4KSTACKS is not defined,
because it is already there.  Since the thread_info is on the threads
stack, and the interrupts also use the current thread's stack (when
CONFIG_4KSTACKS is not defined), there's no need to copy.

Now when you do define CONFIG_4KSTACKS, which make the interrupts use
their own stack, you must copy the thread_info to the interrupt stack
otherwise current will not work for interrupts.

-- Steve

-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/