Re: [PATCH v3 3/9] arm_mpam: Disable Narrow-PARTID when MBA lacks support

[Zeng Heng <zengheng4@huawei.com>]

Hi Shaopeng,

On 2026/4/11 14:50, Zeng Heng wrote:
> 
> 
> On 2026/4/10 9:07, Shaopeng Tan (Fujitsu) wrote:
>> Hello Zeng Heng,
>>
>>> MPAM supports mixed systems with MSCs that may or may not implement
>>> Narrow-PARTID. However, when the MBA MSC uses percentage-based
>>> throttling (non-bitmap partition control) and lacks Narrow-PARTID 
>>> support,
>>> resctrl cannot correctly apply control group configurations across 
>>> multiple
>>> PARTIDs.
>>>
>>> Since there is no straightforward way to program compatible control 
>>> values in
>>> this scenario, disable Narrow-PARTID system-wide when detected. The
>>> detection occurs at initialization time on the first call to 
>>> get_num_reqpartid()
>>> from mpam_resctrl_pick_counters(), which is guaranteed to occur after
>>> mpam_resctrl_pick_mba() has set up the MBA resource class.
>>>
>>> If MBA MSCs lack Narrow-PARTID support, get_num_reqpartid() falls 
>>> back to
>>> returning the number of internal PARTIDs (mpam_intpartid_max).
>>>
>>> Signed-off-by: Zeng Heng <zengheng4@huawei.com>
>>> ---
>>>   drivers/resctrl/mpam_resctrl.c | 38
>>> +++++++++++++++++++++++++++++++++-
>>>   1 file changed, 37 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/resctrl/mpam_resctrl.c 
>>> b/drivers/resctrl/mpam_resctrl.c
>>> index 222ea1d199e1..1b18c095cfce 100644
>>> --- a/drivers/resctrl/mpam_resctrl.c
>>> +++ b/drivers/resctrl/mpam_resctrl.c
>>> @@ -240,9 +240,45 @@ u32 resctrl_arch_get_num_closid(struct rdt_resource
>>> *ignored)
>>>       return mpam_intpartid_max + 1;
>>>   }
>>>
>>> +/*
>>> + * Determine the effective number of PARTIDs available for resctrl.
>>> + *
>>> + * This function performs a one-time check to determine if
>>> +Narrow-PARTID
>>> + * can be used. It must be called after mpam_resctrl_pick_mba() has
>>> + * initialized the MBA resource, as the MBA class properties are used
>>> + * to detect Narrow-PARTID support.
>>> + *
>>> + * The first call occurs in mpam_resctrl_pick_counters(), ensuring the
>>> + * prerequisite initialization is complete.
>>> + */
>>> +static u32 get_num_reqpartid(void)
>>> +{
>>> +    struct mpam_resctrl_res *res;
>>> +    struct rdt_resource *r_mba;
>>> +    struct mpam_props *cprops;
>>> +    static bool first = true;
>>> +
>>> +    if (first) {
>>> +        r_mba = resctrl_arch_get_resource(RDT_RESOURCE_MBA);
>>> +        res = container_of(r_mba, struct mpam_resctrl_res,
>>> resctrl_res);
>>> +        if (!res->class)
>>> +            goto out;
>>> +
>>> +        /* If MBA MSCs lack Narrow-PARTID support, roll back. */
>>> +        cprops = &res->class->props;
>>> +        if (!mpam_has_feature(mpam_feat_partid_nrw, cprops))
>>> +            mpam_partid_max = mpam_intpartid_max;
>>
>> Isn't max reqpartid stored in mpam_partid_max? Why is a rollback 
>> necessary?
>>
> 
> "In order to be able to assign multiple reqPARTIDs freely to resource
> control groups, it is necessary that all MSCs that are used by resctrl
> either implement Narrow PARTID (so that PARTIDs can be remapped
> explicitly) or otherwise only have controls whose resource regulation
> is stateless and so whose behaviour would not be affected by splitting
> a control group across multiple PARTIDs."
> 
> 

I've rewritten the commit message in an attempt to explain this more
clearly:
https://lore.kernel.org/all/20260413085405.1166412-4-zengheng4@huawei.com/


Thanks for your review in advance,
Zeng Heng