Re: [PATCH v6 3/3] fs/resctrl: Factor MBA parse-time conversion to be per-arch

[Gavin Shan <gshan@redhat.com>]

On 9/12/26 2:36 AM, Ben Horgan wrote:
> From: Dave Martin <dave.martin@arm.com>
> 
> The control value parser for the MB resource currently coerces the memory
> bandwidth percentage value from userspace to be an exact multiple of the
> rdt_resource::resctrl_membw::bw_gran parameter.
> 
> On MPAM systems, this results in somewhat worse-than-worst-case rounding, since
> the bandwidth granularity advertised to resctrl by the MPAM driver is in general
> only an approximation to the actual hardware granularity on these systems, and
> the hardware bandwidth allocation control value is not natively a percentage --
> necessitating a further conversion in the resctrl_arch_update_domains() path,
> regardless of the conversion done at parse time.
> 
> For MPAM and x86 use their custom pre-prepared parse-time conversion,
> resctrl_arch_preconvert_bw(). This will avoid accumulated error from rounding
> the value twice on MPAM systems. For x86 systems there is no functional change.
> 
> Clarify the documentation, but avoid overly exact promises.
> 
> Clamping to bw_min and bw_max still feels generic: leave it in the core code,
> for now.
> 
> [ BH: Split out x86 specific changes ]
> Signed-off-by: Dave Martin <Dave.Martin@arm.com>
> Signed-off-by: Ben Horgan <Ben.Horgan@arm.com>
> Reviewed-by: Ben Horgan <ben.horgan@arm.com>
> Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
> ---
> Changes since v3:
> Parameter order swap (Reinette)
> Reflow commit message to use 80 characters (Reinette)
> Adjust comment line length (Reinette)
> Add Reinette's R-b
> 
> Changes since v4:
> Make Dave the author as he wrote the patch
> 
> Changes since v5:
> Document that the rounding can be up or down
> ---
>   Documentation/filesystems/resctrl.rst | 17 +++++++++--------
>   fs/resctrl/ctrlmondata.c              |  6 +++---
>   2 files changed, 12 insertions(+), 11 deletions(-)
> 

Reviewed-by: Gavin Shan <gshan@redhat.com>