linux-kernel - Re: [PATCH v20 18/18] x86/resctrl: Update documentation with Sub-NUMA cluster changes

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Message-ID: <694c09c2-c0f8-4583-bf82-873c40a307e4@intel.com>
Date: Thu, 20 Jun 2024 14:35:25 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: Tony Luck <tony.luck@...el.com>, Fenghua Yu <fenghua.yu@...el.com>,
	"Maciej Wieczor-Retman" <maciej.wieczor-retman@...el.com>, Peter Newman
	<peternewman@...gle.com>, James Morse <james.morse@....com>, Babu Moger
	<babu.moger@....com>, Drew Fustini <dfustini@...libre.com>, Dave Martin
	<Dave.Martin@....com>
CC: <x86@...nel.org>, <linux-kernel@...r.kernel.org>,
	<patches@...ts.linux.dev>
Subject: Re: [PATCH v20 18/18] x86/resctrl: Update documentation with Sub-NUMA
 cluster changes

Hi Tony,

On 6/10/24 11:35 AM, Tony Luck wrote:
> With Sub-NUMA Cluster (SNC) mode enabled the scope of monitoring resources
> is per-NODE instead of per-L3 cache. Backwards compatibility is mainatined

mainatined -> maintained

> by providing files in the mon_L3_XX directories that sum event counts
> for all SNC nodes sharing an L3 cache.
> 
> New files provide per-SNC node event counts.
> 
> 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 | 27 +++++++++++++++++++++++++++
>   1 file changed, 27 insertions(+)
> 
> diff --git a/Documentation/arch/x86/resctrl.rst b/Documentation/arch/x86/resctrl.rst
> index 627e23869bca..6695b7bc698d 100644
> --- a/Documentation/arch/x86/resctrl.rst
> +++ b/Documentation/arch/x86/resctrl.rst
> @@ -375,6 +375,10 @@ When monitoring is enabled all MON groups will also contain:
>   	all tasks in the group. In CTRL_MON groups these files provide
>   	the sum for all tasks in the CTRL_MON group and all tasks in
>   	MON groups. Please see example section for more details on usage.
> +	On systems with Sub-NUMA Cluster (SNC) enabled there are extra
> +	directories for each node (located within the "mon_L3_XX" directory
> +	for the L3 cache they occupy). These are named "mon_sub_L3_YY"
> +	where "YY" is the node number.
>   
>   "mon_hw_id":
>   	Available only with debug option. The identifier used by hardware
> @@ -484,6 +488,29 @@ 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.
> +
> +The top-level monitoring files in each "mon_L3_XX" directory provide
> +the sum of data across all SNC nodes sharing an L3 cache instance.
> +Users who bind tasks to the CPUs of a specific Sub-NUMA node can read
> +the "llc_occupancy", "mbm_total_bytes", and "mbm_local_bytes" in the
> +"mon_sub_L3_YY" directories to get node local data.
> +
> +Memory bandwidth allocation is still performed at the L3 cache
> +level. I.e. throttling controls are applied to all SNC nodes.
> +
> +L3 cache allocation bitmaps also apply to all SNC nodes. But note that
> +the amount of L3 cache represented by each bit is divided by the number
> +of SNC nodes per L3 cache. E.g. with a 100MB cache on a system with 10-bit
> +allocation masks each bit normally represents 10MB. With SNC mode enabled
> +with two SNC nodes per L3 cache, each bit would only represent 5MB.

"each bit would only represent 5MB" -> "each bit represents 5MB" or
"each bit only represents 5MB" or "each bit represents only 5MB"?

> +
>   Memory bandwidth Allocation and monitoring
>   ==========================================
>   

| Reviewed-by: Reinette Chatre <reinette.chatre@...el.com>

Reinette