linux-kernel - Re: [PATCH v9 5/9] x86/resctrl: Relax checks for mba

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Message-ID: <a87640c0-0cd5-42be-83fc-bd12385bf69f@intel.com>
Date: Tue, 19 Nov 2024 19:54:10 -0800
From: Reinette Chatre <reinette.chatre@...el.com>
To: Tony Luck <tony.luck@...el.com>, Fenghua Yu <fenghua.yu@...el.com>, "Peter
 Newman" <peternewman@...gle.com>, Jonathan Corbet <corbet@....net>,
	<x86@...nel.org>
CC: James Morse <james.morse@....com>, Jamie Iles <quic_jiles@...cinc.com>,
	Babu Moger <babu.moger@....com>, Randy Dunlap <rdunlap@...radead.org>,
	"Shaopeng Tan (Fujitsu)" <tan.shaopeng@...itsu.com>,
	<linux-kernel@...r.kernel.org>, <linux-doc@...r.kernel.org>,
	<patches@...ts.linux.dev>
Subject: Re: [PATCH v9 5/9] x86/resctrl: Relax checks for mba_MBps mount
 option

Hi Tony,

On 11/13/24 4:17 PM, Tony Luck wrote:
> This option may be used with any memory bandwidth monitoring event.

Needs a changelog.

> 
> Signed-off-by: Tony Luck <tony.luck@...el.com>
> ---
>  arch/x86/kernel/cpu/resctrl/rdtgroup.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> index a8022bddf9f7..3a89516e6f56 100644
> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> @@ -2334,7 +2334,7 @@ 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() &&
> +	return (is_mbm_enabled() &&
>  		r->alloc_capable && is_mba_linear() &&
>  		r->ctrl_scope == rmbm->mon_scope);
>  }

I *thought* I had a handle on things with the understanding that rdtgroup.mba_mbps_event
is only valid when mba_sc is enabled. This understanding falls apart with this change since
at this point in series if a system only supports total MBM then mba_sc may be true
but rdtgroup.mba_mbps_event will be zero.

The expectation is that patches build on each other to create a solution but this series
does not respect this making it difficult to reason about this work.  I think this series
will be easier to understand if "x86/resctrl: Make mba_sc use total bandwidth if local
is not supported" is moved before this change.

> @@ -2759,7 +2759,7 @@ static int rdt_parse_param(struct fs_context *fc, struct fs_parameter *param)
>  		ctx->enable_cdpl2 = true;
>  		return 0;
>  	case Opt_mba_mbps:
> -		msg = "mba_MBps requires local MBM and linear scale MBA at L3 scope";
> +		msg = "mba_MBps requires MBM and linear scale MBA at L3 scope";
>  		if (!supports_mba_mbps())
>  			return invalfc(fc, msg);
>  		ctx->enable_mba_mbps = true;

Reinette