linux-kernel - Re: [PATCH v5 7/8] x86/resctrl: Update documentation with Sub-NUMA cluster changes

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Message-ID: <2da0c9a0-ebb4-f7fa-89f6-d32ce558fa00@intel.com>
Date:   Mon, 25 Sep 2023 16:30:22 -0700
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>,
        "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>,
        Randy Dunlap <rdunlap@...radead.org>,
        <linux-kernel@...r.kernel.org>, <linux-doc@...r.kernel.org>,
        <patches@...ts.linux.dev>
Subject: Re: [PATCH v5 7/8] x86/resctrl: Update documentation with Sub-NUMA
 cluster changes

Hi Tony,

On 8/29/2023 4:44 PM, 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.
> 
> Signed-off-by: Tony Luck <tony.luck@...el.com>
> ---
>  Documentation/arch/x86/resctrl.rst | 25 ++++++++++++++++++++++---
>  1 file changed, 22 insertions(+), 3 deletions(-)
> 
> diff --git a/Documentation/arch/x86/resctrl.rst b/Documentation/arch/x86/resctrl.rst
> index cb05d90111b4..407764f43f25 100644
> --- a/Documentation/arch/x86/resctrl.rst
> +++ b/Documentation/arch/x86/resctrl.rst
> @@ -345,9 +345,15 @@ 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. E.g. on a system with
> +	SNC mode disabled with two L3 domains there will be subdirectories
> +	"mon_L3_00" and "mon_L3_01". The numerical suffix refers to the
> +	L3 cache id.  With SNC enabled the directory names are the same,
> +	but the numerical suffix refers to the node id.
> +        Mappings from node ids to CPUs are available in the
> +        /sys/devices/system/node/node*/cpulist files. Each of these

Please be consistent with tabs vs spaces.

>  	directories have one file per event (e.g. "llc_occupancy",
>  	"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
> @@ -452,6 +458,19 @@ and 0xA are not.  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 the "llc_occupancy", "mbm_total_bytes", and
> +"mbm_local_bytes" will only give accurate results for well behaved NUMA
> +applications. I.e. those that perform the majority of memory accesses
> +to memory on the local NUMA node to the CPU where the task is executing.
> +

Following our discussion of the cover letter I believe this will change to
be specific about what user space can expect and what is actually "accurate".

> +The cache allocation feature still provides the same number of
> +bits in a mask to control allocation into the L3 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
>  ==========================================
>  

Reinette