linux-kernel - [PATCH 5/5] perf docs: arm_spe: Document new discard mode

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Message-Id: <20241217115610.371755-6-james.clark@linaro.org>
Date: Tue, 17 Dec 2024 11:56:08 +0000
From: James Clark <james.clark@...aro.org>
To: linux-arm-kernel@...ts.infradead.org,
	linux-perf-users@...r.kernel.org
Cc: James Clark <james.clark@...aro.org>,
	Will Deacon <will@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Namhyung Kim <namhyung@...nel.org>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Jiri Olsa <jolsa@...nel.org>,
	Ian Rogers <irogers@...gle.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	"Liang, Kan" <kan.liang@...ux.intel.com>,
	John Garry <john.g.garry@...cle.com>,
	Mike Leach <mike.leach@...aro.org>,
	Leo Yan <leo.yan@...ux.dev>,
	Graham Woodward <graham.woodward@....com>,
	linux-kernel@...r.kernel.org,
	bpf@...r.kernel.org
Subject: [PATCH 5/5] perf docs: arm_spe: Document new discard mode

Document the flag, hint what it's used for and give an example with
other useful options to get minimal output.

Signed-off-by: James Clark <james.clark@...aro.org>
---
 tools/perf/Documentation/perf-arm-spe.txt | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/tools/perf/Documentation/perf-arm-spe.txt b/tools/perf/Documentation/perf-arm-spe.txt
index de2b0b479249..588eead438bc 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 +221,16 @@ Common errors
 
    Increase sampling interval (see above)
 
+Discard mode
+~~~~~~~~~~~~
+
+SPE 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