Re: [PATCH v5 4/5] selftests/resctrl: Use bandwidth values for AMD MBA schemata

[Reinette Chatre <reinette.chatre@intel.com>]

Hi Babu,

On 11/6/26 1:25 PM, Babu Moger wrote:
> On 11/4/26 22:16, Reinette Chatre wrote:
>> On 11/4/26 8:05 PM, Swapnil Sapkal wrote:
>>> On 29-10-2026 00:15, Reinette Chatre wrote:
>>>> On 10/22/26 6:25 AM, Swapnil Sapkal wrote:
>>>>
>>>>> diff --git a/tools/testing/selftests/resctrl/mba_test.c b/tools/testing/selftests/resctrl/mba_test.c
>>>>> index 17ae73d3476c..947735e26099 100644
>>>>> --- a/tools/testing/selftests/resctrl/mba_test.c
>>>>> +++ b/tools/testing/selftests/resctrl/mba_test.c
>>>>> @@ -13,10 +13,51 @@
>>>>>    #define RESULT_FILE_NAME    "result_mba"
>>>>>    #define NUM_OF_RUNS        5
>>>>>    #define MAX_DIFF_PERCENT    15
>>>>> +
>>>>> +/* Intel MBA is specified as a percentage: 10%, 20% ... 100%. */
>>>>
>>>> The percentage based interface is the generic interface and not Intel specific.
>>>> You can drop "Intel" from references to this interface.
>>>>
>>>
>>> Sure, will do.
>>>
>>>>>    #define ALLOCATION_MAX        100
>>>>>    #define ALLOCATION_MIN        10
>>>>>    #define ALLOCATION_STEP        10
>>>>>    +/*
>>>>> + * AMD MBA is not a percentage but an absolute memory bandwidth value. The
>>>>> + * test sweeps the bandwidth from 4 GB/s to 40 GB/s in steps of 4 GB/s,
>>>>> + * which the hardware represents in units of 1/8 GB/s (i.e. 4 GB/s -> 32,
>>>>> + * 8 GB/s -> 64, ... 40 GB/s -> 320).
>>>>> + */
>>>>> +#define AMD_ALLOCATION_MAX    320
>>>>
>>>> The AMD maximum is enumerated from hardware. How can it be ensured this is
>>>> the appropriate maximum for all AMD systems?
>>>>
>>>
>>> As Babu mentioned, it's difficult for a single-threaded workload to
>>> reach the hardware-reported value. For that reason, I used these
>>> values to verify whether throttling takes effect.
>>
>> Just to confirm, my question was not about needing this value to be the actual
>> maximum of a system. Instead, I just wanted to confirm that this "320" value that
>> is hardcoded here for all AMD systems is a _valid_ value. Since the actual maximum
>> value is enumerated from  hardware it is not obvious to me that all systems will support
>> this hardcoded maximum.
>>
> 
> The value 320 corresponds to 40 GB/s. I have tested this on several systems and was able to achieve the expected 40 GB/s bandwidth. The tests did not fail.
> 
> There are some older systems where the test is not expected to run because AMD UMC support is not available.
> 
> If you are concerned, since we are no longer depending on the maximum value, we can be conservative and start with the following adjusted values:
> 
> #define AMD_ALLOCATION_MIN    32
> #define AMD_ALLOCATION_STEP    16
> 
> This allows the value to reach 32 + 16 × 10(num_allocs) = 192, which corresponds to approximately 24 GB/s.
> 
> Hope this answers your question.
> 
I seem to have trouble articulating my question. Let me try to ask a different way.

Consider the scenario where:
	#define AMD_ALLOCATION_MAX    320

If the below (kernel code) snippet is run on all AMD systems on which the
test could run:
	u32 eax, ebx, ecx, edx;
	cpuid_count(0x80000020, 1, &eax, &ebx, &ecx, &edx);

Is it possible for eax to ever contain 8 or less?

Reinette