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

[Ben Horgan <ben.horgan@arm.com>]

Hi Reinette,

On 5/4/26 17:01, Reinette Chatre wrote:
> 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.

Yes, that makes sense to me. I'll use that.

Thanks,

Ben

>   */
>  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