Re: [PATCH v5 3/7] fs/resctrl: Disallow the software controller when MBM counters are assignable

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

Hi Ben,

On 4/28/26 6:04 AM, Ben Horgan wrote:

> diff --git a/fs/resctrl/rdtgroup.c b/fs/resctrl/rdtgroup.c
> index b382a348dd79..be84bb210e3a 100644
> --- a/fs/resctrl/rdtgroup.c
> +++ b/fs/resctrl/rdtgroup.c
> @@ -2530,7 +2530,8 @@ static bool supports_mba_mbps(void)
>  
>  	return (resctrl_is_mbm_enabled() &&
>  		r->alloc_capable && is_mba_linear() &&
> -		r->ctrl_scope == rmbm->mon_scope);
> +		r->ctrl_scope == rmbm->mon_scope &&
> +		!rmbm->mon.mbm_cntr_assignable);
>  }

For better or worse supports_mba_mbps() has a function description that documents
the above tests. Updating the tests but not the accompanying description creates
an inconsistency. How about something like below?

diff --git a/fs/resctrl/rdtgroup.c b/fs/resctrl/rdtgroup.c
index 36d21652616e..b456a67b2c24 100644
--- a/fs/resctrl/rdtgroup.c
+++ b/fs/resctrl/rdtgroup.c
@@ -2536,10 +2536,12 @@ static void mba_sc_domain_destroy(struct rdt_resource *r,
 }
 
 /*
- * MBA software controller is supported only if
- * MBM is supported and MBA is in linear scale,
- * and the MBM monitor scope is the same as MBA
- * control scope.
+ * MBA software controller is supported only if MBM is supported and MBA is in
+ * linear scale, and the MBM monitor scope is the same as MBA control scope.
+ *
+ * Software controller cannot be supported when the MBM counters are assignable.
+ * There is no guarantee that MBM counters are assigned to the event backing
+ * the software controller in all monitoring domains of all monitoring groups.
  */
 static bool supports_mba_mbps(void)
 {

>  
>  /*
> @@ -2945,7 +2946,7 @@ static int rdt_parse_param(struct fs_context *fc, struct fs_parameter *param)
>  		ctx->enable_cdpl2 = true;
>  		return 0;
>  	case Opt_mba_mbps:
> -		msg = "mba_MBps requires MBM and linear scale MBA at L3 scope";
> +		msg = "mba_MBps requires MBM (mbm_event mode not supported) and linear scale MBA at L3 scope";
>  		if (!supports_mba_mbps())
>  			return invalfc(fc, msg);
>  		ctx->enable_mba_mbps = true;

Reinette