Re: [PATCH v8 next 03/10] arm_mpam: Disable reqPARTID expansion when Narrow-PARTID is unavailable

[James Morse <james.morse@arm.com>]

Hi Zeng,

On 13/04/2026 09:53, Zeng Heng wrote:
> MPAM supports heterogeneous systems where some type of MSCs may implement
> Narrow-PARTID while others do not. However, when an MSC uses
> percentage-based throttling (non-bitmap partition control) and lacks
> Narrow-PARTID support, resctrl cannot correctly apply control group
> configurations across multiple PARTIDs.
> 
> To enable free assignment of multiple reqPARTIDs to resource control
> groups, all MSCs used by resctrl must either: Implement Narrow-PARTID,
> allowing explicit PARTID remapping, or only have stateless resource
> controls (non-percentage-based), such that splitting a control group
> across multiple PARTIDs does not affect behavior.

I prefer Dave's terminology for this: aliasing and non-aliasing. It implies
there are two controls, which stateless does not.


> The detection occurs at initialization time on the first call to
> get_num_reqpartid() from update_rmid_limits(). This call is guaranteed
> to occur after mpam_resctrl_pick_{mba,caches}() have set up the
> resource classes, ensuring the necessary properties are available
> for the Narrow-PARTID capability check.
> 
> When an MSC with percentage-based control lacks Narrow-PARTID support,
> get_num_reqpartid() falls back to returning the number of intPARTIDs,
> effectively disabling the reqPARTID expansion for monitoring groups.

No MSC has percentage based controls - that's an x86ism. The MSCs have
fixed point fractions, bitmaps or a cost/weight.


I think you're thinking about this the wrong way up - we should only enable
this on a small number of platforms that don't have any controls we'd have to discard.
(hopefully yours is such a platform!)

I don't think this should be added to resctrl_arch_system_num_rmid_idx(). Please make
this decision for resctrl at mpam_resctrl_setup() time.


> diff --git a/drivers/resctrl/mpam_resctrl.c b/drivers/resctrl/mpam_resctrl.c
> index 5f4364c8101a..56859f354efa 100644
> --- a/drivers/resctrl/mpam_resctrl.c
> +++ b/drivers/resctrl/mpam_resctrl.c
> @@ -257,9 +257,50 @@ 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,caches}()
> + * have initialized the resource classes, as class properties are used
> + * to detect Narrow-PARTID support.

> + * The first call occurs in update_rmid_limits(), ensuring the
> + * prerequisite initialization is complete.

This is fragile to changes in the order resctrl makes these calls. We need these
properties to be fixed before we call resctrl_init().

(yes - I think CDP is fragile too!)


> + */
> +static u32 get_num_reqpartid(void)
> +{
> +	struct mpam_resctrl_res *res;
> +	struct mpam_props *cprops;
> +	static bool first = true;
> +	int rid;
> +
> +	if (first) {
> +		for_each_mpam_resctrl_control(res, rid) {
> +			if (!res->class)
> +				continue;
> +
> +			cprops = &res->class->props;
> +			if (mpam_has_feature(mpam_feat_partid_nrw, cprops))
> +				continue;


> +			if (mpam_has_feature(mpam_feat_mbw_max, cprops) ||
> +			    mpam_has_feature(mpam_feat_mbw_min, cprops) ||
> +			    mpam_has_feature(mpam_feat_cmax_cmax, cprops) ||
> +			    mpam_has_feature(mpam_feat_cmax_cmin, cprops)) {

Please make this a helper in mpam_internal.h with 'controls' and 'aliasing' in its name.
(maybe has_aliasing_controls()).

What about the priority for PRI and the proportional-stride?

I don't think proportional-stride aliases properly: if I have groups with stride 1 and 2,
I can't add a second '2' without decreasing the first groups stride from 1/3 to 1/5. If I
halve the second groups, they each get half the bandwidth instead of sharing it.

Can you check whether the priority for PRI aliases?


> +				mpam_partid_max = mpam_intpartid_max;
> +				break;
> +			}
> +		}
> +	}
> +
> +	first = false;
> +	return mpam_partid_max + 1;
> +}
> +
>  u32 resctrl_arch_system_num_rmid_idx(void)
>  {
> -	return (mpam_pmg_max + 1) * (mpam_partid_max + 1);
> +	return (mpam_pmg_max + 1) * get_num_reqpartid();
>  }


Thanks,

James