Re: [PATCH] -mm: Small schedule() optimization

[Con Kolivas]

cc'ed Ingo since he's maintainer.

On Thursday 09 March 2006 04:54, Andreas Mohr wrote:
> Hello all,
>
> I found that there's a possible small optimization right at the very
> beginning of schedule():
>
>         if (likely(!current->exit_state)) {
>                 if (unlikely(in_atomic())) {
>
> can be reversed into
>
>         if (unlikely(in_atomic())) {
>                 if (likely(!current->exit_state)) {
>
> This is a Good Thing since it avoids having to evaluate both checks,
> and both use current_thread_info() which has an inherent AGI stall risk on
> x86 CPUs if it cannot be inter-mingled with other unrelated opcodes.
>
> I'm a bit puzzled that this has not been done like that before.
> Probably since the exit_state check got added as an after-thought...
> Or did I miss some important reason here? (branch prediction??)

This looks good. See below.

> Patch against 2.6.16-rc5-mm3.
>
> Thanks!
>
> Signed-off-by: Andreas Mohr <andi@xxxxxxxx>
>
>
> --- linux-2.6.16-rc5-mm3/kernel/sched.c.orig	2006-03-08 18:36:58.000000000
> +0100 +++ linux-2.6.16-rc5-mm3/kernel/sched.c	2006-03-08 18:39:55.000000000
> +0100 @@ -3022,8 +3022,8 @@
>  	 * schedule() atomically, we ignore that path for now.
>  	 * Otherwise, whine if we are scheduling when we should not be.
>  	 */
> -	if (likely(!current->exit_state)) {
> -		if (unlikely(in_atomic())) {
> +	if (unlikely(in_atomic())) {
> +		if (likely(!current->exit_state)) {

I suspect that once we're in_atomic() then we're no longer likely to 
be !current->exit_state

Probably better to just
	if (unlikely(in_atomic())) {
		if (!current->exit_state) {

Ingo?

Cheers,
Con
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