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

[James Morse]

Hi Reinette,

On 01/03/2025 02:28, Reinette Chatre wrote:
> On 2/28/25 11:51 AM, James Morse wrote:
>> 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?

Ah, I misread this as one of the functions marked resctrl_arch_.


>> 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.)?

And the realloc threshold, yes. These are the things that would need further abstraction
to allow the filesystem to be a module that isn't loaded. But these would all be changes
to the existing behaviour.
This one is just putting the definition in a header.


>> 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.


Thanks,

James