Why is there a --off-cpu-thresh 2000000?

We collect an off-cpu period __ONLY ONCE__, either in direct sample form,
or in accumulated form (in BPF stack trace map).

If I don't add --off-cpu-thresh 200000, the sample in the original test
goes into the ring buffer instead of the BPF stack trace map.

Additionally, when using -e dummy, the ring buffer is not open, causing
us to lose a sample.

Signed-off-by: Howard Chu <howardchu95@gmail.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: James Clark <james.clark@linaro.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20241108204137.2444151-11-howardchu95@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
---
 tools/perf/tests/builtin-test.c         |  1 +
 tools/perf/tests/shell/record_offcpu.sh | 35 ++++++++++++++++++++++++-
 tools/perf/tests/tests.h                |  1 +
 tools/perf/tests/workloads/Build        |  1 +
 tools/perf/tests/workloads/offcpu.c     | 16 +++++++++++
 5 files changed, 53 insertions(+), 1 deletion(-)
 create mode 100644 tools/perf/tests/workloads/offcpu.c

diff --git a/tools/perf/tests/builtin-test.c b/tools/perf/tests/builtin-test.c
index 8dcf74d3c0a3..43dc04075ecb 100644
--- a/tools/perf/tests/builtin-test.c
+++ b/tools/perf/tests/builtin-test.c
@@ -146,6 +146,7 @@ static struct test_workload *workloads[] = {
 	&workload__brstack,
 	&workload__datasym,
 	&workload__landlock,
+	&workload__offcpu,
 };
 
 #define workloads__for_each(workload) \
diff --git a/tools/perf/tests/shell/record_offcpu.sh b/tools/perf/tests/shell/record_offcpu.sh
index 678947fe69ee..8719130fbf70 100755
--- a/tools/perf/tests/shell/record_offcpu.sh
+++ b/tools/perf/tests/shell/record_offcpu.sh
@@ -6,6 +6,10 @@ set -e
 
 err=0
 perfdata=$(mktemp /tmp/__perf_test.perf.data.XXXXX)
+TEST_PROGRAM="perf test -w offcpu"
+
+ts=$(printf "%u" $((~0 << 32))) # OFF_CPU_TIMESTAMP
+dummy_timestamp=${ts%???} # remove the last 3 digits to match perf script
 
 cleanup() {
   rm -f ${perfdata}
@@ -39,7 +43,11 @@ test_offcpu_priv() {
 test_offcpu_basic() {
   echo "Basic off-cpu test"
 
-  if ! perf record --off-cpu -e dummy -o ${perfdata} sleep 1 2> /dev/null
+  # We collect an off-cpu period __ONLY ONCE__, either in direct sample form, or in accumulated form
+  # (in BPF stack trace map). Without the --off-cpu-thresh 200000 below, the sample will go into the
+  # ring buffer instead of the BPF stack trace map. Additionally, when using -e dummy, the ring
+  # buffer is not enabled, resulting in a lost sample.
+  if ! perf record --off-cpu --off-cpu-thresh 2000000 -e dummy -o ${perfdata} sleep 1 2> /dev/null
   then
     echo "Basic off-cpu test [Failed record]"
     err=1
@@ -88,6 +96,27 @@ test_offcpu_child() {
   echo "Child task off-cpu test [Success]"
 }
 
+test_offcpu_direct() {
+  echo "Direct off-cpu test"
+
+  # dump off-cpu samples for task blocked for more than 1.999999s
+  # -D for initial delay, to enable evlist
+  if ! perf record -e dummy -D 500 --off-cpu --off-cpu-thresh 1999999 -o ${perfdata} ${TEST_PROGRAM} 2> /dev/null
+  then
+    echo "Direct off-cpu test [Failed record]"
+    err=1
+    return
+  fi
+  # Direct sample's timestamp should be lower than the dummy_timestamp of the at-the-end sample.
+  if ! perf script -i ${perfdata} -F time,period | sed "s/[\.:]//g" | \
+       awk "{ if (\$1 < ${dummy_timestamp} && \$2 > 1999999999) exit 0; else exit 1; }"
+  then
+    echo "Direct off-cpu test [Failed missing direct sample]"
+    err=1
+    return
+  fi
+  echo "Direct off-cpu test [Success]"
+}
 
 test_offcpu_priv
 
@@ -99,5 +128,9 @@ if [ $err = 0 ]; then
   test_offcpu_child
 fi
 
+if [ $err = 0 ]; then
+  test_offcpu_direct
+fi
+
 cleanup
 exit $err
diff --git a/tools/perf/tests/tests.h b/tools/perf/tests/tests.h
index cb58b43aa063..2e655a617b30 100644
--- a/tools/perf/tests/tests.h
+++ b/tools/perf/tests/tests.h
@@ -217,6 +217,7 @@ DECLARE_WORKLOAD(sqrtloop);
 DECLARE_WORKLOAD(brstack);
 DECLARE_WORKLOAD(datasym);
 DECLARE_WORKLOAD(landlock);
+DECLARE_WORKLOAD(offcpu);
 
 extern const char *dso_to_test;
 extern const char *test_objdump_path;
diff --git a/tools/perf/tests/workloads/Build b/tools/perf/tests/workloads/Build
index 5af17206f04d..0e78fd01eaf1 100644
--- a/tools/perf/tests/workloads/Build
+++ b/tools/perf/tests/workloads/Build
@@ -7,6 +7,7 @@ perf-test-y += sqrtloop.o
 perf-test-y += brstack.o
 perf-test-y += datasym.o
 perf-test-y += landlock.o
+perf-test-y += offcpu.o
 
 CFLAGS_sqrtloop.o         = -g -O0 -fno-inline -U_FORTIFY_SOURCE
 CFLAGS_leafloop.o         = -g -O0 -fno-inline -fno-omit-frame-pointer -U_FORTIFY_SOURCE
diff --git a/tools/perf/tests/workloads/offcpu.c b/tools/perf/tests/workloads/offcpu.c
new file mode 100644
index 000000000000..57cee201a4c3
--- /dev/null
+++ b/tools/perf/tests/workloads/offcpu.c
@@ -0,0 +1,16 @@
+#include <linux/compiler.h>
+#include <unistd.h>
+#include "../tests.h"
+
+static int offcpu(int argc __maybe_unused, const char **argv __maybe_unused)
+{
+	/* get past the initial delay */
+	sleep(1);
+
+	/* what we want to collect as a direct sample */
+	sleep(2);
+
+	return 0;
+}
+
+DEFINE_WORKLOAD(offcpu);
-- 
2.43.0