Re: [RFC PATCH 05/31] x86/resctrl: Parse ACPI CMRC table

["Chen, Yu C" <yu.c.chen@intel.com>]

Hi Tony,

On 8/5/2026 2:13 AM, Luck, Tony wrote:
> On Tue, Aug 04, 2026 at 10:19:26AM -0700, Luck, Tony wrote:
>> On Mon, Aug 03, 2026 at 12:03:48AM +0800, Chen Yu wrote:
> 
>>>   
>>> +/* Scale to bytes for the monitoring counters when ERDT is enabled. */
>>> +static int erdt_scale;
>>
>> Single value for scale? See below.
> 
> Now I've read ahead to patch 9 and see how this is used to set
> resctrl_rmid_realloc_threshold. Needs better comments here and
> for above erdt_get_scale()

OK, will add comment here.

resctrl_rmid_realloc_threshold is a single global value,
and resctrl_arch_round_mon_val() takes no domain argument, so a
single scale has to be derived from the per-domain cmrc->up_scale.
max() was chosen because the rounding is a floor: a larger scale
yields a slightly lower threshold, i.e. an RMID has to drop to
a slightly lower occupancy before it is reused.

>>
>>> +
>>>   int erdt_get_max_rmid(void)
>>>   {
>>>   	return erdt_max_rmid;
>>>   }
>>>   
>>> +int erdt_get_scale(void)
>>> +{
>>> +	return erdt_scale;
>>> +}
>>> +
>>> +	erdt_scale = max_t(int, erdt_scale, cmrc->up_scale);
>>
>> I'd expect that on a machine all CMRC tables would report the same
>> up_scale factor. But RDT architecture allows them to be different. Two
>> ways to handle this:
>>
>> 1) Check that they are all the same. Complain if they are different and
>> don't enable cache occupancy events.
>>
>> 2) Save the value for each domain and use that value to upscale when
>> reporting to user.
> 
> I see in patch 9 that you do use this option "2" with "cmrc->up_scale".
> 
>> Picking the max value doesn't feel like a good answer.
> 
> Given how this is actually used, seems like the best option.

OK, the erdt_mon_read() reads via the per domain cmrc->up_scale.

thanks,
Chenyu

>>
>>> +
>>> +	return 0;
>>> +}
>>
> -Tony