linux-kernel - Re: [PATCH v13 8/8] x86/resctrl: Update documentation with Sub-NUMA cluster changes

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Message-ID: <7b66283e-5801-ee37-aa76-af0c04240a20@amd.com>
Date:   Mon, 4 Dec 2023 17:04:30 -0600
From:   "Moger, Babu" <bmoger@....com>
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>,
        Randy Dunlap <rdunlap@...radead.org>,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        patches@...ts.linux.dev, Shaopeng Tan <tan.shaopeng@...fujitsu.com>
Subject: Re: [PATCH v13 8/8] x86/resctrl: Update documentation with Sub-NUMA
 cluster changes


On 12/4/2023 12:53 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>
> Tested-by: Shaopeng Tan<tan.shaopeng@...fujitsu.com>
> Reviewed-by: Peter Newman<peternewman@...gle.com>
> Reviewed-by: Reinette Chatre<reinette.chatre@...el.com>
> Reviewed-by: Shaopeng Tan<tan.shaopeng@...fujitsu.com>

Reviewed-by: Babu Moger <babu.moger@...il.com>


> ---
>   Documentation/arch/x86/resctrl.rst | 25 +++++++++++++++++++++----
>   1 file changed, 21 insertions(+), 4 deletions(-)
>
> diff --git a/Documentation/arch/x86/resctrl.rst b/Documentation/arch/x86/resctrl.rst
> index a6279df64a9d..15f1cff6ee76 100644
> --- a/Documentation/arch/x86/resctrl.rst
> +++ b/Documentation/arch/x86/resctrl.rst
> @@ -366,10 +366,10 @@ 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
> -	directories have one file per event (e.g. "llc_occupancy",
> +	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 has 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
>   	all tasks in the group. In CTRL_MON groups these files provide
> @@ -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
> +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
>   ==========================================
>