[v4,3/3] Documentation: arm64: Document the PMU event counting threshold feature

Commit Message

James Clark Oct. 25, 2023, 9:57 a.m. UTC

Add documentation for the new Perf event open parameters and
the threshold_max capability file.

Signed-off-by: James Clark <[e-mail]>
---
 Documentation/arch/arm64/perf.rst | 56 +++++++++++++++++++++++++++++++
 1 file changed, 56 insertions(+)

Patch

diff --git a/Documentation/arch/arm64/perf.rst b/Documentation/arch/arm64/perf.rst
index 1f87b57c2332..36b8111a710d 100644
--- a/Documentation/arch/arm64/perf.rst
+++ b/Documentation/arch/arm64/perf.rst
@@ -164,3 +164,59 @@  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
+------
+
+The threshold, threshold_compare and threshold_count values can be
+provided per event:
+
+.. code-block:: sh
+
+  perf stat -e stall_slot/threshold=2,threshold_compare=2/ \
+            -e dtlb_walk/threshold=10,threshold_compare=3,threshold_count/
+
+And the following comparison values are supported:
+
+.. code-block::
+
+  0: Not-equal
+  1: Equals
+  2: Greater-than-or-equal
+  3: Less-than
+
+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.