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

[Zeng Heng]

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@xxxxxxxxxx>
> > > ---
> > >   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@xxxxxxxxxx/


Thanks for your review in advance,
Zeng Heng