Re: [PATCH v6 8/8] x86/resctrl: Update documentation with Sub-NUMA cluster changes

[Peter Newman]

Hi Tony,

On Thu, Sep 28, 2023 at 9:14 PM Tony Luck <tony.luck@xxxxxxxxx> wrote:
> diff --git a/Documentation/arch/x86/resctrl.rst b/Documentation/arch/x86/resctrl.rst
> index cb05d90111b4..d6b6a4cfd967 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

The explanation of mon_data seems overwhelmingly SNC-centric now.
Maybe the SNC section should be responsible for explaining its impact
on the mon_data directory. Mainly by reminding the reader that domain
ids in the mon_data directory are node ids in SNC mode.


>         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 +458,28 @@ 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 meaningful 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.

Not being specific about why the results aren't meaningful, this
sounds vague and alarming.

> +Note that Linux may load balance tasks between Sub-NUMA nodes much
> +more readily than between regular NUMA nodes since the CPUs on SNC
> +share the same L3 cache and the system may report the NUMA distance
> +between SNC nodes with a lower value than used for regular NUMA nodes.
> +Tasks that migrate between nodes will have their traffic recorded by the
> +counters in different SNC nodes so a user will need to read mon_data
> +files from each node on which the task executed to get the full
> +view of traffic for which the task was 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
>  ==========================================
>
> --
> 2.41.0
>

Reviewed-by: Peter Newman <peternewman@xxxxxxxxxx>