linux-kernel - Re: [PATCH v20 07/18] x86/resctrl: Block use of mba_MBps mount option on Sub-NUMA Cluster (SNC) systems

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Date: Thu, 20 Jun 2024 14:21:12 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: Tony Luck <tony.luck@...el.com>, Fenghua Yu <fenghua.yu@...el.com>, Maciej
 Wieczor-Retman <maciej.wieczor-retman@...el.com>, Peter Newman
	<peternewman@...gle.com>, James Morse <james.morse@....com>, Babu Moger
	<babu.moger@....com>, Drew Fustini <dfustini@...libre.com>, Dave Martin
	<Dave.Martin@....com>
CC: <x86@...nel.org>, <linux-kernel@...r.kernel.org>,
	<patches@...ts.linux.dev>
Subject: Re: [PATCH v20 07/18] x86/resctrl: Block use of mba_MBps mount option
 on Sub-NUMA Cluster (SNC) systems

Hi Tony,

On 6/10/24 11:35 AM, Tony Luck wrote:
> When SNC is enabled there is a mismatch between the MBA control function
> which operates at L3 cache scope and the MBM monitor functions which
> measure memory bandwidth on each SNC node.
> 
> Block use of the mba_MBps when scopes for MBA/MBM do not match.
> 
> Signed-off-by: Tony Luck <tony.luck@...el.com>
> ---
>   arch/x86/kernel/cpu/resctrl/rdtgroup.c | 4 +++-
>   1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> index eb3bbfa96d5a..a0a43dbe011b 100644
> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> @@ -2339,10 +2339,12 @@ static void mba_sc_domain_destroy(struct rdt_resource *r,
>    */
>   static bool supports_mba_mbps(void)
>   {
> +	struct rdt_resource *rmbm = &rdt_resources_all[RDT_RESOURCE_L3].r_resctrl;
>   	struct rdt_resource *r = &rdt_resources_all[RDT_RESOURCE_MBA].r_resctrl;
>   
>   	return (is_mbm_local_enabled() &&
> -		r->alloc_capable && is_mba_linear());
> +		r->alloc_capable && is_mba_linear() &&
> +		r->ctrl_scope == rmbm->mon_scope);
>   }
>   
>   /*

The function comments of supports_mba_mbps() needs an update to accompany
this new requirement.

I also think that the "mba_MBps" mount option is now complicated enough to
warrant a clear error to user space when using it fails. invalfc() is
available for this and enables user space to get detailed log message
from a read() on an fd created by fsopen().

Perhaps something like (please check line length and feel free to improve
since as is it may quite cryptic):
	rdt_parse_param(...)
	{


	...
	case Opt_mba_mbps:
		if (!supports_mba_mbps())
			return invalfc(fc, "mba_MBps requires both MBM and (linear scale) MBA at L3 scope");
	...
	}


Reinette