On 24/11/2023 10:28, James Clark wrote:
Add documentation for the new Perf event open parameters and
the threshold_max capability file.
Signed-off-by: James Clark <james.clark@xxxxxxx>
Reviewed-by: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
---
Documentation/arch/arm64/perf.rst | 72 +++++++++++++++++++++++++++++++
1 file changed, 72 insertions(+)
diff --git a/Documentation/arch/arm64/perf.rst b/Documentation/arch/arm64/perf.rst
index 1f87b57c2332..41eee68951ff 100644
--- a/Documentation/arch/arm64/perf.rst
+++ b/Documentation/arch/arm64/perf.rst
@@ -164,3 +164,75 @@ and should be used to mask the upper bits as needed.
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/perf/arch/arm64/tests/user-events.c
.. _tools/lib/perf/tests/test-evsel.c:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/lib/perf/tests/test-evsel.c
+
+Event Counting Threshold
+==========================================
+
+Overview
+--------
+
+FEAT_PMUv3_TH (Armv8.8) permits a PMU counter to increment only on
+events whose count meets a specified threshold condition. For example if
+threshold_compare is set to 2 ('Greater than or equal'), and the
+threshold is set to 2, then the PMU counter will now only increment by
+when an event would have previously incremented the PMU counter by 2 or
+more on a single processor cycle.
+
+To increment by 1 after passing the threshold condition instead of the
+number of events on that cycle, add the 'threshold_count' option to the
+commandline.
+
+How-to
+------
+
+These are the parameters for controlling the feature:
+
+.. list-table::
+ :header-rows: 1
+
+ * - Parameter
+ - Description
+ * - threshold
+ - Value to threshold the event by. A value of 0 means that
+ thresholding is disabled and the other parameters have no effect.
+ * - threshold_compare
+ - | Comparison function to use, with the following values supported:
+ |
+ | 0: Not-equal
+ | 1: Equals
+ | 2: Greater-than-or-equal
+ | 3: Less-than
+ * - threshold_count
+ - If this is set, count by 1 after passing the threshold condition
+ instead of the value of the event on this cycle.
+
+The threshold, threshold_compare and threshold_count values can be
+provided per event, for example:
+
+.. code-block:: sh
+
+ perf stat -e stall_slot/threshold=2,threshold_compare=2/ \
+ -e dtlb_walk/threshold=10,threshold_compare=3,threshold_count/
+
+In this example the stall_slot event will count by 2 or more on every
+cycle where 2 or more stalls happen. And dtlb_walk will count by 1 on
+every cycle where the number of dtlb walks were less than 10.
+
+The maximum supported threshold value can be read from the caps of each
+PMU, for example:
+
+.. code-block:: sh
+
+ cat /sys/bus/event_source/devices/armv8_pmuv3/caps/threshold_max
+
+ 0x000000ff
+
+If a value higher than this is given, then it will be silently clamped
+to the maximum. The highest possible maximum is 4095, as the config
+field for threshold is limited to 12 bits, and the Perf tool will refuse
+to parse higher values.
+
+If the PMU doesn't support FEAT_PMUv3_TH, then threshold_max will read
+0, and both threshold and threshold_compare will be silently ignored.
+threshold_max will also read as 0 on aarch32 guests, even if the host
+is running on hardware with the feature.
