[PATCH v14 32/32] x86,fs/resctrl: Update documentation for telemetry events

Tony Luck
2025-11-24T18:55:15Z

Update resctrl filesystem documentation with the details about the
resctrl files that support telemetry events.

Signed-off-by: Tony Luck <tony.luck@intel.com>
---
Documentation/filesystems/resctrl.rst | 102 +++++++++++++++++++++++---
1 file changed, 90 insertions(+), 12 deletions(-)

diff --git a/Documentation/filesystems/resctrl.rst b/Documentation/filesystems/resctrl.rst
index 8c8ce678148a..138d2663b03d 100644
--- a/Documentation/filesystems/resctrl.rst
+++ b/Documentation/filesystems/resctrl.rst
@@ -252,13 +252,12 @@ with respect to allocation:
bandwidth percentages are directly applied to
the threads running on the core

-If RDT monitoring is available there will be an "L3_MON" directory
+If L3 monitoring is available there will be an "L3_MON" directory
with the following files:

"num_rmids":
- The number of RMIDs available. This is the
- upper bound for how many "CTRL_MON" + "MON"
- groups can be created.
+ The number of RMIDs supported by hardware for
+ L3 monitoring events.

"mon_features":
Lists the monitoring events if
@@ -484,6 +483,25 @@ with the following files:
bytes) at which a previously used LLC_occupancy
counter can be considered for re-use.

+If telemetry monitoring is available there will be a "PERF_PKG_MON" directory
+with the following files:
+
+"num_rmids":
+ The number of RMIDs for telemetry monitoring events. By default,
+ resctrl will not enable telemetry events of a particular type
+ ("perf" or "energy") if the number of RMIDs that can be tracked
+ concurrently for that type is lower than the total number of
+ RMIDs supported by that type. The user can force-enable each
+ type of telemetry events with the "rdt=" boot command line
+ option, but this may reduce the number of monitoring groups that
+ can be created.
+
+"mon_features":
+ Lists the telemetry monitoring events that are enabled on this system.
+
+The upper bound for how many "CTRL_MON" + "MON" can be created
+is the smaller of the L3_MON and PERF_PKG_MON "num_rmids" values.
+
Finally, in the top level of the "info" directory there is a file
named "last_cmd_status". This is reset with every "command" issued
via the file system (making new directories or writing to any of the
@@ -589,15 +607,40 @@ 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",
- "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
- the sum for all tasks in the CTRL_MON group and all tasks in
+ This contains directories for each monitor domain.
+
+ If L3 monitoring is enabled, there will be a "mon_L3_XX" directory for
+ each instance of an L3 cache. Each directory contains files for the enabled
+ L3 events (e.g. "llc_occupancy", "mbm_total_bytes", and "mbm_local_bytes").
+
+ If telemetry monitoring is enabled, there will be a "mon_PERF_PKG_YY"
+ directory for each physical processor package. Each directory contains
+ files for the enabled telemetry events (e.g. "core_energy". "activity",
+ "uops_retired", etc.)
+
+ The info/`*`/mon_features files provide the full list of enabled
+ event/file names.
+
+ "core energy" reports a floating point number for the energy (in Joules)
+ consumed by cores (registers, arithmetic units, TLB and L1/L2 caches)
+ during execution of instructions summed across all logical CPUs on a
+ package for the current monitoring group.
+
+ "activity" also reports a floating point value (in Farads). This provides
+ an estimate of work done independent of the frequency that the CPUs used
+ for execution.
+
+ Note that "core energy" and "activity" only measure energy/activity in the
+ "core" of the CPU (arithmetic units, TLB, L1 and L2 caches, etc.). They
+ do not include L3 cache, memory, I/O devices etc.
+
+ All other events report decimal integer values.
+
+ 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 the sum for all tasks in the CTRL_MON group and all tasks in
MON groups. Please see example section for more details on usage.
+
On systems with Sub-NUMA Cluster (SNC) enabled there are extra
directories for each node (located within the "mon_L3_XX" directory
for the L3 cache they occupy). These are named "mon_sub_L3_YY"
@@ -1590,6 +1633,41 @@ Example with C::
resctrl_release_lock(fd);
}

+Debugfs
+=======
+In addition to the use of debugfs for tracing of pseudo-locking performance,
+architecture code may create debugfs directories associated with monitoring
+features for a specific resource.
+
+The full pathname for these is in the form:
+
+ /sys/kernel/debug/resctrl/info/{resource_name}_MON/{arch}/
+
+The presence, names, and format of these files may vary between architectures
+even if the same resource is present.
+
+PERF_PKG_MON/x86_64
+-------------------
+Three files are present per telemetry aggregator instance that show status.
+The prefix of each file name describes the type ("energy" or "perf"), the
+guid, which processor package it belongs to, and the instance number of the
+aggregator. For example: "energy_0x26696143_pkg1_agg2".
+
+The suffix describes which data is reported in the file and is one of:
+
+data_loss_count:
+ This counts the number of times that this aggregator
+ failed to accumulate a counter value supplied by a CPU.
+
+data_loss_timestamp:
+ This is a "timestamp" from a free running 25MHz uncore
+ timer indicating when the most recent data loss occurred.
+
+last_update_timestamp:
+ Another 25MHz timestamp indicating when the
+ most recent counter update was successfully applied.
+
+
Examples for RDT Monitoring along with allocation usage
=======================================================
Reading monitored data
--
2.51.1