Re: [PATCH v6 29/42] x86/resctrl: Move get_config_index() to a header

[Reinette Chatre]

Hi James,

On 2/28/25 11:51 AM, James Morse wrote:
> Hi Reinette,
> 
> On 20/02/2025 01:27, Reinette Chatre wrote:
>> On 2/7/25 10:18 AM, James Morse wrote:
>>> get_config_index() is used by the architecture specific code to map a
>>> CLOSID+type pair to an index in the configuration arrays.
>>>
>>> MPAM needs to do this too to preserve the ABI to user-space, there is
>>> no reason to do it differently.
>>>
>>> Move the helper to a header file.
> 
>>> --- a/include/linux/resctrl.h
>>> +++ b/include/linux/resctrl.h
>>> @@ -384,6 +384,21 @@ void resctrl_arch_mon_event_config_write(void *config_info);
>>>   */
>>>  void resctrl_arch_mon_event_config_read(void *config_info);
>>>  
>>> +/* For use by arch code to remap resctrl's smaller CDP CLOSID range */
>>> +static inline u32 resctrl_get_config_index(u32 closid,
>>> +					   enum resctrl_conf_type type)
>>> +{
>>> +	switch (type) {
>>> +	default:
>>> +	case CDP_NONE:
>>> +		return closid;
>>> +	case CDP_CODE:
>>> +		return closid * 2 + 1;
>>> +	case CDP_DATA:
>>> +		return closid * 2;
>>> +	}
>>> +}
>>> +
> 
>> Could you please add the motivation for the use of an inline function?
> 
> Putting this in the header file means it isn't duplicated, so its behaviour can't become

I am not following this. How would making this part of a .c file of fs/resctrl with just
the prototype in include/linux/resctrl.h result in this function being duplicated?

> different. If its in a header file, it has to be marked inline otherwise every C file that
> includes it gets a copy that probably isn't used, and upsets the linker.
> 
> Calling from the arch code into the filesystem prevents the arch code from being
> standalone. This is a useful direction of travel because it allows fs/resctrl to one
> day become a module

Don't we have this already with all the needed CPU and domain management (
resctrl_online_ctrl_domain(), resctrl_online_mon_domain(), resctrl_online_cpu(),
resctrl_offline_cpu(), etc.)?

> 
> Today, the compiler is choosing to inline this:
> | x86_64-linux-objdump -d ctrlmondata.o | grep resctrl_get_config_index | wc -l
> | 0
> 
> This kind of arithmetic for an array lookup is the kind of thing its good to give the
> compiler full visibility of as its good fodder for constant folding.
> 
> For so few call sites, I don't think this is really worth thinking about.
> Forcing this call out of line makes the kernel text bigger, but only by 32 bytes.
> 
> 
> I've expanded the last paragraph of the commit message to read:
> | Move the helper to a header file to allow all architectures that either
> | use or emulate CDP to use the same pattern of CLOSID values. Moving
> | this to a header file means it must be marked inline, which matches
> | the existing compiler choice for this static function.
> 
> 

Reinette