Re: [PATCH v10 08/17] x86/resctrl: Enforce system RMID limit on AET event groups

[Reinette Chatre]

Hi Tony,

On 7/29/26 10:27 AM, Tony Luck wrote:
> AET (Application Energy Telemetry) event groups each support a specific
> number of RMIDs. But that number may be lower than the number supported
> by the system. Especially true on systems with SNC (Sub-NUMA Cluster)
> enabled as that reduces the number of supported RMIDs.
> 
> Fix get_rdt_mon_resources() to return true when any monitor resource is

hmmm ... "Fix" makes one look for the accompanying "Fixes:" tag. What is
the fix here? What is wrong with existing implementation that needs fixing?
To me this does not look like a fix though (more below).

> possibly enabled. Call intel_aet_init() to adjust the event_group::num_rmid
> values to not exceed the system supported maximum.

Last sentence just documents the code. Please describe why this is needed.
Is this a separate logical change?

> 
> Signed-off-by: Tony Luck <tony.luck@xxxxxxxxx>
> ---

...

> diff --git a/arch/x86/kernel/cpu/resctrl/core.c b/arch/x86/kernel/cpu/resctrl/core.c
> index 092764cf693f..2c938b97b147 100644
> --- a/arch/x86/kernel/cpu/resctrl/core.c
> +++ b/arch/x86/kernel/cpu/resctrl/core.c
> @@ -1019,10 +1019,10 @@ static __init bool get_rdt_mon_resources(void)
>  	if (rdt_cpu_has(X86_FEATURE_ABMC))
>  		ret = true;
>  
> -	if (!ret)
> -		return false;
> +	if (ret)
> +		rdt_get_l3_mon_config(r);
>  
> -	return !rdt_get_l3_mon_config(r);
> +	return boot_cpu_data.x86_cache_max_rmid > 0;
>  }

>From what I can tell this will return true when the system supports monitoring,
but no resource may actually have monitoring enabled at this point. Specifically,
no resource has rdt_resource::mon_capable set. 

The resctrl initialization now proceeds where it used to stop. resctrl_arch_late_init()
will proceed and initialize the resctrl filesystem, which in turn would allow user space
to mount it.

rdt_get_tree() handling the user mount request could thus be run on a system that does
not have a monitoring or allocation capable resource and then we see in rdt_get_tree():
	if (resctrl_arch_alloc_capable() || resctrl_arch_mon_capable())
		resctrl_mounted = true;

The above flow change would cause resctrl to think the system supports monitoring
but resctrl_arch_mon_capable() returns false. If there are no allocation features
then this will result in resctrl fs mounted ... but resctrl_mounted is not set to
true and thus allow a remount that is not supported.

Reinette