linux-kernel - Re: [PATCH v11 8/8] x86/resctrl: Update documentation with Sub-NUMA cluster changes

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Message-ID: <932eed52-fd9a-4617-84a4-9376861d1c86@infradead.org>
Date:   Thu, 9 Nov 2023 15:14:44 -0800
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Tony Luck <tony.luck@...el.com>, Fenghua Yu <fenghua.yu@...el.com>,
        Reinette Chatre <reinette.chatre@...el.com>,
        Peter Newman <peternewman@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Shuah Khan <skhan@...uxfoundation.org>, x86@...nel.org
Cc:     Shaopeng Tan <tan.shaopeng@...itsu.com>,
        James Morse <james.morse@....com>,
        Jamie Iles <quic_jiles@...cinc.com>,
        Babu Moger <babu.moger@....com>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, patches@...ts.linux.dev
Subject: Re: [PATCH v11 8/8] x86/resctrl: Update documentation with Sub-NUMA
 cluster changes

Hi Tony,

If you make any updates to this:
(but not critical)

On 11/9/23 15:09, Tony Luck wrote:
> With Sub-NUMA Cluster mode enabled the scope of monitoring resources is
> per-NODE instead of per-L3 cache. Suffixes of directories with "L3" in
> their name refer to Sub-NUMA nodes instead of L3 cache ids.
> 
> Users should be aware that SNC mode also affects the amount of L3 cache
> available for allocation within each SNC node.
> 
> Reviewed-by: Peter Newman <peternewman@...gle.com>
> Signed-off-by: Tony Luck <tony.luck@...el.com>
> ---
>  Documentation/arch/x86/resctrl.rst | 23 ++++++++++++++++++++---
>  1 file changed, 20 insertions(+), 3 deletions(-)
> 
> diff --git a/Documentation/arch/x86/resctrl.rst b/Documentation/arch/x86/resctrl.rst
> index a6279df64a9d..d1db200db5f9 100644
> --- a/Documentation/arch/x86/resctrl.rst
> +++ b/Documentation/arch/x86/resctrl.rst
> @@ -366,9 +366,9 @@ When control is enabled all CTRL_MON groups will also contain:
>  When monitoring is enabled all MON groups will also contain:
>  
>  "mon_data":
> -	This contains a set of files organized by L3 domain and by
> -	RDT event. E.g. on a system with two L3 domains there will
> -	be subdirectories "mon_L3_00" and "mon_L3_01".	Each of these
> +	This contains a set of files organized by L3 domain or by NUMA
> +	node (depending on whether Sub-NUMA Cluster (SNC) mode is disabled
> +	or enabled respectively) and by RDT event.  Each of these
>  	directories have one file per event (e.g. "llc_occupancy",

	            has

>  	"mbm_total_bytes", and "mbm_local_bytes"). In a MON group these
>  	files provide a read out of the current value of the event for
> @@ -478,6 +478,23 @@ if non-contiguous 1s value is supported. On a system with a 20-bit mask
>  each bit represents 5% of the capacity of the cache. You could partition
>  the cache into four equal parts with masks: 0x1f, 0x3e0, 0x7c00, 0xf8000.
>  
> +Notes on Sub-NUMA Cluster mode
> +==============================
> +When SNC mode is enabled Linux may load balance tasks between Sub-NUMA
> +nodes much more readily than between regular NUMA nodes since the CPUs
> +on Sub-NUMA nodes share the same L3 cache and the system may report
> +the NUMA distance between Sub-NUMA nodes with a lower value than used
> +for regular NUMA nodes.  Users who do not bind tasks to the CPUs of a
> +specific Sub-NUMA node must read the "llc_occupancy", "mbm_total_bytes",
> +and "mbm_local_bytes" for all Sub-NUMA nodes where the tasks may execute
> +to get the full view of traffic for which the tasks were the source.
> +
> +The cache allocation feature still provides the same number of
> +bits in a mask to control allocation into the L3 cache. But each

                                                    cache, but each

> +of those ways has its capacity reduced because the cache is divided
> +between the SNC nodes. The values reported in the resctrl
> +"size" files are adjusted accordingly.
> +
>  Memory bandwidth Allocation and monitoring
>  ==========================================
>  

-- 
~Randy