Document the flag along with PMU events to hint what it's used for and
give an example with other useful options to get minimal output.
Reviewed-by: Yeoreum Yun <yeoreum.yun@xxxxxxx>
Signed-off-by: James Clark <james.clark@xxxxxxxxxx>
---
tools/perf/Documentation/perf-arm-spe.txt | 26 +++++++++++++++++++++++
1 file changed, 26 insertions(+)
diff --git a/tools/perf/Documentation/perf-arm-spe.txt b/tools/perf/Documentation/perf-arm-spe.txt
index de2b0b479249..37afade4f1b2 100644
--- a/tools/perf/Documentation/perf-arm-spe.txt
+++ b/tools/perf/Documentation/perf-arm-spe.txt
@@ -150,6 +150,7 @@ arm_spe/load_filter=1,min_latency=10/'
pct_enable=1 - collect physical timestamp instead of virtual timestamp (PMSCR.PCT) - requires privilege
store_filter=1 - collect stores only (PMSFCR.ST)
ts_enable=1 - enable timestamping with value of generic timer (PMSCR.TS)
+ discard=1 - enable SPE PMU events but don't collect sample data - see 'Discard mode' (PMBLIMITR.FM = DISCARD)
+++*+++ Latency is the total latency from the point at which sampling started on that instruction, rather
than only the execution latency.
@@ -220,6 +221,31 @@ Common errors
Increase sampling interval (see above)
+PMU events
+~~~~~~~~~~
+
+SPE has events that can be counted on core PMUs. These are prefixed with
+SAMPLE_, for example SAMPLE_POP, SAMPLE_FEED, SAMPLE_COLLISION and
+SAMPLE_FEED_BR.
+
+These events will only count when an SPE event is running on the same core that
+the PMU event is opened on, otherwise they read as 0. There are various ways to
+ensure that the PMU event and SPE event are scheduled together depending on the
+way the event is opened. For example opening both events as per-process events
+on the same process, although it's not guaranteed that the PMU event is enabled
+first when context switching. For that reason it may be better to open the PMU
+event as a systemwide event and then open SPE on the process of interest.
+
+Discard mode
+~~~~~~~~~~~~
+
+SPE related (SAMPLE_* etc) core PMU events can be used without the overhead of
+collecting sample data if discard mode is supported (optional from Armv8.6).
+First run a system wide SPE session (or on the core of interest) using options
+to minimize output. Then run perf stat:
+
+ perf record -e arm_spe/discard/ -a -N -B --no-bpf-event -o - > /dev/null &
+ perf stat -e SAMPLE_FEED_LD
SEE ALSO
--------
--
2.34.1
