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

Reinette Chatre
2024-06-20T21:21:18Z

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@intel.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