Re: [PATCH] micro optimization in kernel/params.c; don't call to_module_kobject before we really have to.

[Brian Gerst <bgerst@didntduck.org>]

Jesper Juhl wrote:
> In kernel/params.c::module_attr_show and 
> kernel/params.c::module_attr_store we call to_module_kobject() and save 
> the result in a local variable right before a conditional statement that 
> does not depend on the call to to_module_kobject() and may cause the 
> function to return. If the function returns before we use the result of 
> to_module_kobject() then the call is just a waste of time. 
> The patch moves the call to to_module_kobject() down just before it is 
> actually used, thus we save a call to the function in a few cases. I doubt 
> this is in any way measurable, but I see no reason to not move the call - 
> it should be an infinitesimal improvement with no ill sideeffects.
> Please consider applying.
> 
> 
> Signed-off-by: Jesper Juhl <juhl-lkml@dif.dk>
> 
> diff -up linux-2.6.11-rc2-bk7-orig/kernel/params.c linux-2.6.11-rc2-bk7/kernel/params.c
> --- linux-2.6.11-rc2-bk7-orig/kernel/params.c	2005-01-29 23:54:53.000000000 +0100
> +++ linux-2.6.11-rc2-bk7/kernel/params.c	2005-01-30 00:27:08.000000000 +0100
> @@ -625,11 +625,12 @@ static ssize_t module_attr_show(struct k
>  	int ret;
>  
>  	attribute = to_module_attr(attr);
> -	mk = to_module_kobject(kobj);
>  
>  	if (!attribute->show)
>  		return -EPERM;
>  
> +	mk = to_module_kobject(kobj);
> +
>  	if (!try_module_get(mk->mod))
>  		return -ENODEV;
>  
> @@ -649,11 +650,12 @@ static ssize_t module_attr_store(struct 
>  	int ret;
>  
>  	attribute = to_module_attr(attr);
> -	mk = to_module_kobject(kobj);
>  
>  	if (!attribute->store)
>  		return -EPERM;
>  
> +	mk = to_module_kobject(kobj);
> +
>  	if (!try_module_get(mk->mod))
>  		return -ENODEV;
>  
> 
> 
> 

I'd bet that the compiler already reorders the assignment since there is 
no side effect to to_module_kobject().  Even with a patch like this you 
can't control exactly when the assignment is done.  There may not even 
be an assignment, since mk is a constant offset of kobj.

--
				Brian Gerst