[PATCH v2 8/8] perf record: Make --buildid-mmap the default

Ian Rogers
2025-04-24T19:58:51Z

Support for build IDs in mmap2 perf events has been present since
Linux v5.12:
https://lore.kernel.org/lkml/20210219194619.1780437-1-acme@kernel.org/
Build ID mmap events don't avoid the need to inject build IDs for DSO
touched by samples as the build ID cache is populated by perf
record. They can avoid some cases of symbol mis-resolution caused by
the file system changing from when a sample occurred and when the DSO
is sought. To disable build ID scanning

Unlike the --buildid-mmap option, this doesn't disable the build ID
cache but it does disable the processing of samples looking for DSOs
to inject build IDs for. To disable the build ID cache the -B
(--no-buildid) option should be used.

Making this option the default was raised on the list in:
https://lore.kernel.org/linux-perf-users/CAP-5=fXP7jN_QrGUcd55_QH5J-Y-FCaJ6=NaHVtyx0oyNh8_-Q@mail.gmail.com/

Signed-off-by: Ian Rogers <irogers@google.com>
---
tools/perf/builtin-record.c | 33 +++++++++++++++++++-----------
tools/perf/util/symbol_conf.h | 2 +-
tools/perf/util/synthetic-events.c | 16 +++++++--------
3 files changed, 30 insertions(+), 21 deletions(-)

diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c
index ba20bf7c011d..7b64013ba8c0 100644
--- a/tools/perf/builtin-record.c
+++ b/tools/perf/builtin-record.c
@@ -169,6 +169,7 @@ struct record {
bool no_buildid_cache_set;
bool buildid_all;
bool buildid_mmap;
+ bool buildid_mmap_set;
bool timestamp_filename;
bool timestamp_boundary;
bool off_cpu;
@@ -1795,6 +1796,7 @@ record__finish_output(struct record *rec)
data->dir.files[i].size = lseek(data->dir.files[i].fd, 0, SEEK_CUR);
}

+ /* Buildid scanning disabled or build ID in kernel and synthesized map events. */
if (!rec->no_buildid) {
process_buildids(rec);

@@ -2966,6 +2968,8 @@ static int perf_record_config(const char *var, const char *value, void *cb)
rec->no_buildid = true;
else if (!strcmp(value, "mmap"))
rec->buildid_mmap = true;
+ else if (!strcmp(value, "no-mmap"))
+ rec->buildid_mmap = false;
else
return -1;
return 0;
@@ -3349,6 +3353,7 @@ static struct record record = {
.ctl_fd_ack = -1,
.synth = PERF_SYNTH_ALL,
},
+ .buildid_mmap = true,
};

const char record_callchain_help[] = CALLCHAIN_RECORD_HELP
@@ -3514,8 +3519,8 @@ static struct option __record_options[] = {
"file", "vmlinux pathname"),
OPT_BOOLEAN(0, "buildid-all", &record.buildid_all,
"Record build-id of all DSOs regardless of hits"),
- OPT_BOOLEAN(0, "buildid-mmap", &record.buildid_mmap,
- "Record build-id in map events"),
+ OPT_BOOLEAN_SET(0, "buildid-mmap", &record.buildid_mmap, &record.buildid_mmap_set,
+ "Legacy record build-id in map events option which is now the default. Behaves indentically to --no-buildid. Disable with --no-buildid-mmap"),
OPT_BOOLEAN(0, "timestamp-filename", &record.timestamp_filename,
"append timestamp to output filename"),
OPT_BOOLEAN(0, "timestamp-boundary", &record.timestamp_boundary,
@@ -4042,19 +4047,23 @@ int cmd_record(int argc, const char **argv)
record.opts.record_switch_events = true;
}

+ if (!rec->buildid_mmap) {
+ pr_debug("Disabling build id in synthesized mmap2 events.\n");
+ symbol_conf.no_buildid_mmap2 = true;
+ } else if (rec->buildid_mmap_set) {
+ /*
+ * Explicitly passing --buildid-mmap disables buildid processing
+ * and cache generation.
+ */
+ rec->no_buildid = true;
+ }
+ if (rec->buildid_mmap && !perf_can_record_build_id()) {
+ pr_warning("Missing support for build id in kernel mmap events. Disable this warning with --no-buildid-mmap\n");
+ rec->buildid_mmap = false;
+ }
if (rec->buildid_mmap) {
- if (!perf_can_record_build_id()) {
- pr_err("Failed: no support to record build id in mmap events, update your kernel.\n");
- err = -EINVAL;
- goto out_opts;
- }
- pr_debug("Enabling build id in mmap2 events.\n");
- /* Enable mmap build id synthesizing. */
- symbol_conf.buildid_mmap2 = true;
/* Enable perf_event_attr::build_id bit. */
rec->opts.build_id = true;
- /* Disable build id cache. */
- rec->no_buildid = true;
}

if (rec->opts.record_cgroup && !perf_can_record_cgroup()) {
diff --git a/tools/perf/util/symbol_conf.h b/tools/perf/util/symbol_conf.h
index cd9aa82c7d5a..7a80d2c14d9b 100644
--- a/tools/perf/util/symbol_conf.h
+++ b/tools/perf/util/symbol_conf.h
@@ -43,7 +43,7 @@ struct symbol_conf {
report_individual_block,
inline_name,
disable_add2line_warn,
- buildid_mmap2,
+ no_buildid_mmap2,
guest_code,
lazy_load_kernel_maps,
keep_exited_threads,
diff --git a/tools/perf/util/synthetic-events.c b/tools/perf/util/synthetic-events.c
index 8fb2ea544d3a..f48109c14235 100644
--- a/tools/perf/util/synthetic-events.c
+++ b/tools/perf/util/synthetic-events.c
@@ -532,7 +532,7 @@ int perf_event__synthesize_mmap_events(const struct perf_tool *tool,
event->mmap2.pid = tgid;
event->mmap2.tid = pid;

- if (symbol_conf.buildid_mmap2)
+ if (!symbol_conf.no_buildid_mmap2)
perf_record_mmap2__read_build_id(&event->mmap2, machine, false);

if (perf_tool__process_synth_event(tool, event, machine, process) != 0) {
@@ -690,7 +690,7 @@ static int perf_event__synthesize_modules_maps_cb(struct map *map, void *data)
return 0;

dso = map__dso(map);
- if (symbol_conf.buildid_mmap2) {
+ if (!symbol_conf.no_buildid_mmap2) {
size = PERF_ALIGN(dso__long_name_len(dso) + 1, sizeof(u64));
event->mmap2.header.type = PERF_RECORD_MMAP2;
event->mmap2.header.size = (sizeof(event->mmap2) -
@@ -734,9 +734,9 @@ int perf_event__synthesize_modules(const struct perf_tool *tool, perf_event__han
.process = process,
.machine = machine,
};
- size_t size = symbol_conf.buildid_mmap2
- ? sizeof(args.event->mmap2)
- : sizeof(args.event->mmap);
+ size_t size = symbol_conf.no_buildid_mmap2
+ ? sizeof(args.event->mmap)
+ : sizeof(args.event->mmap2);

args.event = zalloc(size + machine->id_hdr_size);
if (args.event == NULL) {
@@ -1124,8 +1124,8 @@ static int __perf_event__synthesize_kernel_mmap(const struct perf_tool *tool,
struct machine *machine)
{
union perf_event *event;
- size_t size = symbol_conf.buildid_mmap2 ?
- sizeof(event->mmap2) : sizeof(event->mmap);
+ size_t size = symbol_conf.no_buildid_mmap2 ?
+ sizeof(event->mmap) : sizeof(event->mmap2);
struct map *map = machine__kernel_map(machine);
struct kmap *kmap;
int err;
@@ -1159,7 +1159,7 @@ static int __perf_event__synthesize_kernel_mmap(const struct perf_tool *tool,
event->header.misc = PERF_RECORD_MISC_GUEST_KERNEL;
}

- if (symbol_conf.buildid_mmap2) {
+ if (!symbol_conf.no_buildid_mmap2) {
size = snprintf(event->mmap2.filename, sizeof(event->mmap2.filename),
"%s%s", machine->mmap_name, kmap->ref_reloc_sym->name) + 1;
size = PERF_ALIGN(size, sizeof(u64));
--
2.49.0.850.g28803427d3-goog