[PATCH v7 8/8] x86/resctrl: Update documentation with Sub-NUMA cluster changes

From: Tony Luck
Date: Tue Oct 03 2023 - 12:08:48 EST


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.

Documentation/arch/x86/resctrl.rst | 23 ++++++++++++++++++++---
1 file changed, 20 insertions(+), 3 deletions(-)

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
==========================================

--
2.41.0