Re: Add prefetch switch stack hook in scheduler function

[Keith Owens <kaos@ocs.com.au>]

On Fri, 29 Jul 2005 09:04:48 +0200, 
Ingo Molnar <mingo@elte.hu> wrote:
>ok, how about the additional patch below? Does this do the trick on 
>ia64? It makes complete sense on every architecture to prefetch from 
>below the current kernel stack, in the expectation of the next task 
>touching the stack. The only difference is that for ia64 the 'expected 
>minimum stack footprint' is larger, due to the switch_stack.
>...
>Index: linux/kernel/sched.c
>=================================>--- linux.orig/kernel/sched.c
>+++ linux/kernel/sched.c
>@@ -2869,7 +2869,14 @@ go_idle:
> 	 * its thread_info, its kernel stack and mm:
> 	 */
> 	prefetch(next->thread_info);
>-	prefetch(kernel_stack(next));
>+	/*
>+	 * Prefetch (at least) a cacheline below the current
>+	 * kernel stack (in expectation of any new task touching
>+	 * the stack at least minimally), and a cacheline above
>+	 * the stack:
>+	 */
>+	prefetch_range(kernel_stack(next) - MIN_KERNEL_STACK_FOOTPRINT,
>+		       MIN_KERNEL_STACK_FOOTPRINT + L1_CACHE_BYTES);
> 	prefetch(next->mm);
> 
> 	if (!rt_task(next) && next->activated > 0) {

Surely the prefetch range has to depend on which direction the stack
grows.  For stacks that grow down, we want esp/ksp upwards,

prefetch_range(kernel_stack(next),
	MIN_KERNEL_STACK_FOOTPRINT + L1_CACHE_BYTES);

For stacks that grow up, we want esp/ksp downwards

prefetch_range(kernel_stack(next) - MIN_KERNEL_STACK_FOOTPRINT,
	MIN_KERNEL_STACK_FOOTPRINT + L1_CACHE_BYTES);

BTW, for ia64 you may as well prefetch pt_regs, that is also quite
large.

#define MIN_KERNEL_STACK_FOOTPRINT (IA64_SWITCH_STACK_SIZE + IA64_PT_REGS_SIZE)