On 23/11/2023 15:45, James Clark wrote:

On 23/11/2023 05:50, Anshuman Khandual wrote:

quoted

On 11/21/23 03:01, Namhyung Kim wrote:

quoted

On Mon, Nov 13, 2023 at 3:26 AM James Clark [off-list ref] wrote:

quoted

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

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

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/

Can you please explain this a bit more?

I guess the first event counts stall_slot PMU if the event if it's
greater than or equal to 2.  And as threshold_count is not set,
it'd count the stall_slot as is.  E.g. it counts 3 when it sees 3.

Hence without 'threshold_count' being set, the other two config requests
will not have an effect, is that correct ?

Yeah I can mention this. It's implied because 0 is the default value of
config fields, and 0 is a valid value for compare and count field, so
threshold=0 has to be the way to disable it. But I can mention it
explicitly.

To avoid any confusion, I thought you meant threshold here instead of
threshold_count. But I replied in more detail about the same issue on
patch 2.


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