Re: [PATCH v8 8/8] x86/resctrl: Update documentation with Sub-NUMA cluster changes

[Peter Newman]

On Tue, Oct 3, 2023 at 11:30 PM Tony Luck <tony.luck@xxxxxxxxx> 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@xxxxxxxxx>
>
> ---
>
> Changes since v5:
>
> Added addtional details about challenges tracking tasks when SNC
> mode is enabled.
>
> ---
> diff --git a/Documentation/arch/x86/resctrl.rst b/Documentation/arch/x86/resctrl.rst
> index cb05d90111b4..222c507089a5 100644
> --- a/Documentation/arch/x86/resctrl.rst
> +++ b/Documentation/arch/x86/resctrl.rst
> @@ -345,9 +345,9 @@ 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",
>         "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 +452,23 @@ 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 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
>  ==========================================
>

Sounds better, thanks.

Reviewed-by: Peter Newman <peternewman@xxxxxxxxxx>