[PATCH v2] perf record: add dummy event during system wide synthesis

From: Ian Rogers
Date: Tue Apr 21 2020 - 02:11:14 EST


During the processing of /proc during event synthesis new processes may
start. Add a dummy event if /proc is to be processed, to capture mmaps
for starting processes. This reuses the existing logic for
initial-delay.

v2 fixes the dummy event configuration and a branch stack issue.

Suggested-by: Stephane Eranian <eranian@xxxxxxxxxx>
Signed-off-by: Ian Rogers <irogers@xxxxxxxxxx>
---
tools/perf/builtin-record.c | 19 ++++++++++++++-----
tools/perf/util/evsel.c | 5 ++++-
2 files changed, 18 insertions(+), 6 deletions(-)

diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c
index 1ab349abe904..8d1e93351298 100644
--- a/tools/perf/builtin-record.c
+++ b/tools/perf/builtin-record.c
@@ -805,19 +805,28 @@ static int record__open(struct record *rec)
int rc = 0;

/*
- * For initial_delay we need to add a dummy event so that we can track
- * PERF_RECORD_MMAP while we wait for the initial delay to enable the
- * real events, the ones asked by the user.
+ * For initial_delay or system wide, we need to add a dummy event so
+ * that we can track PERF_RECORD_MMAP to cover the delay of waiting or
+ * event synthesis.
*/
- if (opts->initial_delay) {
+ if (opts->initial_delay || target__has_cpu(&opts->target)) {
if (perf_evlist__add_dummy(evlist))
return -ENOMEM;

+ /* Disable tracking of mmaps on lead event. */
pos = evlist__first(evlist);
pos->tracking = 0;
+ /* Set up dummy event. */
pos = evlist__last(evlist);
pos->tracking = 1;
- pos->core.attr.enable_on_exec = 1;
+ /*
+ * Enable the dummy event when the process is forked for
+ * initial_delay, immediately for system wide.
+ */
+ if (opts->initial_delay)
+ pos->core.attr.enable_on_exec = 1;
+ else
+ pos->immediate = 1;
}

perf_evlist__config(evlist, opts, &callchain_param);
diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
index 6a571d322bb2..ca8f9533d8f9 100644
--- a/tools/perf/util/evsel.c
+++ b/tools/perf/util/evsel.c
@@ -1163,11 +1163,14 @@ void perf_evsel__config(struct evsel *evsel, struct record_opts *opts,
}

/*
+ * A dummy event never triggers any actual counter and therefore
+ * cannot be used with branch_stack.
+ *
* For initial_delay, a dummy event is added implicitly.
* The software event will trigger -EOPNOTSUPP error out,
* if BRANCH_STACK bit is set.
*/
- if (opts->initial_delay && is_dummy_event(evsel))
+ if (is_dummy_event(evsel))
perf_evsel__reset_sample_bit(evsel, BRANCH_STACK);
}

--
2.26.1.301.g55bc3eb7cb9-goog