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Message-ID: <CAP-5=fXHay1Qn7c3JUe4nH-cTw8zPhPv6-XWMZdPKpwwT=9n2w@mail.gmail.com>
Date: Thu, 24 Apr 2025 00:20:44 -0700
From: Ian Rogers <irogers@...gle.com>
To: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
Arnaldo Carvalho de Melo <acme@...nel.org>, Namhyung Kim <namhyung@...nel.org>,
Mark Rutland <mark.rutland@....com>,
Alexander Shishkin <alexander.shishkin@...ux.intel.com>, Jiri Olsa <jolsa@...nel.org>,
Ian Rogers <irogers@...gle.com>, Adrian Hunter <adrian.hunter@...el.com>,
Kan Liang <kan.liang@...ux.intel.com>, Athira Rajeev <atrajeev@...ux.ibm.com>,
Kajol Jain <kjain@...ux.ibm.com>, Li Huafei <lihuafei1@...wei.com>,
"Steinar H. Gunderson" <sesse@...gle.com>, Stephen Brennan <stephen.s.brennan@...cle.com>,
James Clark <james.clark@...aro.org>, Andi Kleen <ak@...ux.intel.com>,
Dmitry Vyukov <dvyukov@...gle.com>, Zhongqiu Han <quic_zhonhan@...cinc.com>,
Yicong Yang <yangyicong@...ilicon.com>, Michael Petlan <mpetlan@...hat.com>,
Krzysztof Łopatowski <krzysztof.m.lopatowski@...il.com>,
"Dr. David Alan Gilbert" <linux@...blig.org>, Leo Yan <leo.yan@....com>,
Steve Clevenger <scclevenger@...amperecomputing.com>, Zixian Cai <fzczx123@...il.com>,
Thomas Falcon <thomas.falcon@...el.com>, Martin Liska <martin.liska@....com>,
Martin Liška <m.liska@...link.cz>,
linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 5/5] perf record: Make --buildid-mmap the default
On Wed, Apr 23, 2025 at 11:19 PM Ian Rogers <irogers@...gle.com> wrote:
>
> 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 avoid the need to inject build IDs for DSO
> touched by samples when perf record completes and can avoid some cases
> of symbol mis-resolution.
>
> 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 -N
> (--no-buildid-cache) option is necessary.
>
> 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@...gle.com>
> ---
> tools/perf/builtin-record.c | 35 +++++++++++++++++++-----------
> tools/perf/util/symbol_conf.h | 2 +-
> tools/perf/util/synthetic-events.c | 16 +++++++-------
> 3 files changed, 31 insertions(+), 22 deletions(-)
>
> diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c
> index ba20bf7c011d..0fcb0f469488 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;
> @@ -1795,7 +1796,8 @@ record__finish_output(struct record *rec)
> data->dir.files[i].size = lseek(data->dir.files[i].fd, 0, SEEK_CUR);
> }
>
> - if (!rec->no_buildid) {
> + /* Buildid scanning disabled or build ID in kernel and synthesized map events. */
> + if (!rec->no_buildid && !rec->buildid_mmap) {
So I think this is wrong. It matches current behaviors, but it is
wrong. If we don't process the kernel's mmap events the DSOs won't be
loaded and the build ID cache won't contain the DSOs. There is also
the bug that the sample processing to find maps to find DSOs, doesn't
handle call chains. Given the broken nature of the build ID cache I'm
not sure it makes any sense for perf record to be by default
populating it. I think it probably makes sense to consider the default
population a legacy feature and make -N the default along with
--buildid-mmap.
Thanks,
Ian
> process_buildids(rec);
>
> if (rec->buildid_all)
> @@ -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));
> --
> 2.49.0.805.g082f7c87e0-goog
>
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