linux-kernel - [PATCH v15-RFC 8/8] x86/resctrl: Update documentation with Sub-NUMA cluster changes

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Message-ID: <20240130222034.37181-9-tony.luck@intel.com>
Date: Tue, 30 Jan 2024 14:20:34 -0800
From: Tony Luck <tony.luck@...el.com>
To: 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>,
	Drew Fustini <dfustini@...libre.com>,
	linux-kernel@...r.kernel.org,
	linux-doc@...r.kernel.org,
	patches@...ts.linux.dev,
	Tony Luck <tony.luck@...el.com>,
	Shaopeng Tan <tan.shaopeng@...fujitsu.com>,
	Babu Moger <babu.moger@...il.com>
Subject: [PATCH v15-RFC 8/8] x86/resctrl: Update documentation with Sub-NUMA cluster changes

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.

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>
Signed-off-by: Tony Luck <tony.luck@...el.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
 ==========================================
 
-- 
2.43.0