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Message-ID: <20231020213100.123598-9-tony.luck@intel.com>
Date: Fri, 20 Oct 2023 14:31:00 -0700
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>,
linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
patches@...ts.linux.dev, Tony Luck <tony.luck@...el.com>
Subject: [PATCH v9 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.
Reviewed-by: Peter Newman <peternewman@...gle.com>
Signed-off-by: Tony Luck <tony.luck@...el.com>
---
Changes since v6:
Added Peter's review tag
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 a6279df64a9d..d1db200db5f9 100644
--- a/Documentation/arch/x86/resctrl.rst
+++ b/Documentation/arch/x86/resctrl.rst
@@ -366,9 +366,9 @@ 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
@@ -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
==========================================
--
2.41.0
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