Re: [PATCH v2 1/5] x86,fs/resctrl: Remove unused non-linear bandwidth delay mapping

[Jinjie Ruan]

在 2026/10/8 5:06, Reinette Chatre 写道:
> Intel MBA throttles memory bandwidth with delay values, while resctrl
> exposes memory bandwidth to user space as a percentage. When delay values
> are linear there is a direct mapping between the two. Support for
> non-linear delay values would need a pre-calibrated table mapping
> percentages to delay values.
> 
> Commit
> 
>   05b93417ce5b ("x86/intel_rdt/mba: Add primary support for Memory Bandwidth Allocation (MBA)")
> 
> added a placeholder for such a table to the resctrl_membw structure, but
> the table was never populated and never read. rdt_get_mb_table(), which
> would provide it, always fails. As a result, an Intel MBA resource with
> non-linear delay values is never marked as capable of allocation, and its
> control values are never written to hardware.
> 
> Remove the unused mapping table and fold the linear percentage-to-delay
> conversion into the function that writes the control values to hardware.
> 
> Keep the check that delay values are linear as a defensive measure in
> preparation for supporting multiple controls, which turns the memory
> bandwidth control into a generic scalar control that indicates whether it
> requires linear values.
> 
> Signed-off-by: Reinette Chatre <reinette.chatre@xxxxxxxxx>
> Reviewed-by: Chen Yu <yu.c.chen@xxxxxxxxx>
> ---
> Changes since RFC v1:
> - Also drop separate delay_bw_map() and instead integrate the only
>   mapping to when control value is written.
> 
> Changes since RFC v2:
> - Rework changelog.
> 
> Changes since v1:
> - Rewrite changelog, fix subject prefix.
> - Match error message about linear support with similar message during
>   enumeration added in next patch.
> - Add Chenyu's Reviewed-by tag
> ---
>  arch/x86/kernel/cpu/resctrl/core.c | 23 +++++++----------------
>  include/linux/resctrl.h            |  2 --
>  2 files changed, 7 insertions(+), 18 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/resctrl/core.c b/arch/x86/kernel/cpu/resctrl/core.c
> index 55214d6fdc49..f9677e52790e 100644
> --- a/arch/x86/kernel/cpu/resctrl/core.c
> +++ b/arch/x86/kernel/cpu/resctrl/core.c
> @@ -323,29 +323,20 @@ static void mba_wrmsr_amd(struct msr_param *m)
>  		wrmsrq(hw_res->msr_base + i, hw_dom->ctrl_val[i]);
>  }
>  
> -/*
> - * Map the memory b/w percentage value to delay values
> - * that can be written to QOS_MSRs.
> - * There are currently no SKUs which support non linear delay values.
> - */
> -static u32 delay_bw_map(unsigned long bw, struct rdt_resource *r)
> -{
> -	if (r->membw.delay_linear)
> -		return MAX_MBA_BW - bw;
> -
> -	pr_warn_once("Non Linear delay-bw map not supported but queried\n");
> -	return MAX_MBA_BW;
> -}
> -
>  static void mba_wrmsr_intel(struct msr_param *m)
>  {
>  	struct rdt_hw_ctrl_domain *hw_dom = resctrl_to_arch_ctrl_dom(m->dom);
>  	struct rdt_hw_resource *hw_res = resctrl_to_arch_res(m->res);
>  	unsigned int i;
>  
> -	/*  Write the delay values for mba. */
> +	if (!m->res->membw.delay_linear) {
> +		pr_warn_once("Non-linear MBA delay values not supported\n");
> +		return;
> +	}
> +
> +	/* Program bandwidth percentage mapped to linear delay value. */
>  	for (i = m->low; i < m->high; i++)
> -		wrmsrq(hw_res->msr_base + i, delay_bw_map(hw_dom->ctrl_val[i], m->res));
> +		wrmsrq(hw_res->msr_base + i, MAX_MBA_BW - hw_dom->ctrl_val[i]);

LGTM
Reviewed-by: Jinjie Ruan <ruanjinjie@xxxxxxxxxx>

>  }
>  
>  static void cat_wrmsr(struct msr_param *m)
> diff --git a/include/linux/resctrl.h b/include/linux/resctrl.h
> index 10dfdca7f4bf..b63dc62690ca 100644
> --- a/include/linux/resctrl.h
> +++ b/include/linux/resctrl.h
> @@ -253,7 +253,6 @@ enum membw_throttle_mode {
>   * @throttle_mode:	Bandwidth throttling mode when threads request
>   *			different memory bandwidths
>   * @mba_sc:		True if MBA software controller(mba_sc) is enabled
> - * @mb_map:		Mapping of memory B/W percentage to memory B/W delay
>   */
>  struct resctrl_membw {
>  	u32				min_bw;
> @@ -263,7 +262,6 @@ struct resctrl_membw {
>  	bool				arch_needs_linear;
>  	enum membw_throttle_mode	throttle_mode;
>  	bool				mba_sc;
> -	u32				*mb_map;
>  };
>  
>  struct resctrl_schema;

-- 
Best regards,
Jinjie