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SubjectRe: [PATCH v5 07/21] x86/resctrl: Abstract and use supports_mba_mbps()
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Hi James,

On 6/22/2022 9:46 AM, James Morse wrote:

...

> diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> index b32ceff8325a..49d9e2c3c743 100644
> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> @@ -1890,17 +1890,26 @@ void rdt_domain_reconfigure_cdp(struct rdt_resource *r)
> }
>
> /*
> - * Enable or disable the MBA software controller
> - * which helps user specify bandwidth in MBps.
> * MBA software controller is supported only if
> * MBM is supported and MBA is in linear scale.
> */
> +static bool supports_mba_mbps(void)
> +{
> + struct rdt_resource *r = &rdt_resources_all[RDT_RESOURCE_MBA].r_resctrl;
> +
> + return (is_mbm_enabled() &&
> + r->alloc_capable && is_mba_linear());
> +}
> +
> +/*
> + * Enable or disable the MBA software controller
> + * which helps user specify bandwidth in MBps.
> + */
> static int set_mba_sc(bool mba_sc)
> {
> struct rdt_resource *r = &rdt_resources_all[RDT_RESOURCE_MBA].r_resctrl;
>
> - if (!is_mbm_enabled() || !is_mba_linear() ||
> - mba_sc == is_mba_sc(r))
> + if (!supports_mba_mbps() || mba_sc == is_mba_sc(r))
> return -EINVAL;
>
> r->membw.mba_sc = mba_sc;


It seems that this change is propagating an issue that exists
in the current implementation (not introduced by this work).

The supports_mba_mbps() test uses is_mbm_enabled() which is:
(is_mbm_total_enabled() || is_mbm_local_enabled())

That is not accurate since the software controller is only called
when MBM local is enabled:
mbm_update()
{
...
if (is_mbm_local_enabled()) {
...
if (is_mba_sc(NULL))
mbm_bw_count(rmid, &rr);
}
}


Could the "is_mbm_enabled()" in the new supports_mba_mbps() introduced
here be changed to use "is_mbm_local_enabled()" instead? It does not seem
an issue for existing hardware (that supports total and local) but
the kernel parameters do enable users to boot with "rdt=!mbmlocal" to
trigger problematic scenarios.

Reinette

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