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Message-ID: <aWbLAIjOGj910kIW@google.com>
Date: Tue, 13 Jan 2026 14:45:20 -0800
From: Namhyung Kim <namhyung@...nel.org>
To: Arnaldo Carvalho de Melo <acme@...nel.org>
Cc: Ian Rogers <irogers@...gle.com>, James Clark <james.clark@...aro.org>,
Jiri Olsa <jolsa@...nel.org>,
Adrian Hunter <adrian.hunter@...el.com>,
Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
linux-perf-users@...r.kernel.org
Subject: Re: [PATCH v2 1/2] perf inject: Add --convert-callchain option
On Tue, Jan 13, 2026 at 06:35:25PM -0300, Arnaldo Carvalho de Melo wrote:
> On Fri, Jan 09, 2026 at 05:17:14PM -0800, Namhyung Kim wrote:
> > There are applications not built with frame pointers, so DWARF is needed
> > to get the stack traces. So `perf record --call-graph dwarf` saves the
> > stack and register data for each sample to get the stacktrace offline.
> > But sometimes those data may have sensitive information and we don't
> > want to keep them in the file.
> >
> > This perf inject --convert-callchain option parses the callchains and
> > discard the stack and register after that. This will save storage space
> > and processing time for the new data file. Of course, users should
> > remove the original data file. :)
> >
> > The down side is that it cannot handle inlined callchain entries as they
> > all have the same IPs. Maybe we can add an option to perf report to
> > look up inlined functions using DWARF - IIUC it won't requires stack and
> > register data.
> >
> > This is an example.
> >
> > $ perf record --call-graph dwarf -- perf test -w noploop
> >
> > $ perf report --stdio --no-children --percent-limit=0 > output-prev
> >
> > $ perf inject -i perf.data --convert-callchain -o perf.data.out
> >
> > $ perf report --stdio --no-children --percent-limit=0 -i perf.data.out > output-next
> >
> > $ diff -u output-prev output-next
> > ...
> > 0.23% perf ld-linux-x86-64.so.2 [.] _dl_relocate_object_no_relro
> > |
> > - ---elf_dynamic_do_Rela (inlined)
> > - _dl_relocate_object_no_relro
> > + ---_dl_relocate_object_no_relro
> > _dl_relocate_object
> > dl_main
> > _dl_sysdep_start
> > - _dl_start_final (inlined)
> > _dl_start
> > _start
> >
> > Signed-off-by: Namhyung Kim <namhyung@...nel.org>
> > ---
> > v2 changes)
> > * Use machine__kernel_ip() instead (James)
> > * Check sample types for DWARF callchains (James)
> > * Fix build errors (James)
> > * Add a new test (Ian)
> >
> > tools/perf/Documentation/perf-inject.txt | 5 +
> > tools/perf/builtin-inject.c | 151 +++++++++++++++++++++++
> > 2 files changed, 156 insertions(+)
> >
> > diff --git a/tools/perf/Documentation/perf-inject.txt b/tools/perf/Documentation/perf-inject.txt
> > index c972032f4ca0d248..95dfdf39666efe89 100644
> > --- a/tools/perf/Documentation/perf-inject.txt
> > +++ b/tools/perf/Documentation/perf-inject.txt
> > @@ -109,6 +109,11 @@ include::itrace.txt[]
> > should be used, and also --buildid-all and --switch-events may be
> > useful.
> >
> > +--convert-callchain::
> > + Parse DWARF callchains and convert them to usual callchains. This also
> > + discards stack and register data from the samples. This will lose
> > + inlined callchain entries.
> > +
> > :GMEXAMPLECMD: inject
> > :GMEXAMPLESUBCMD:
> > include::guestmount.txt[]
> > diff --git a/tools/perf/builtin-inject.c b/tools/perf/builtin-inject.c
> > index 6080afec537d2178..02bd388d602fdd75 100644
> > --- a/tools/perf/builtin-inject.c
> > +++ b/tools/perf/builtin-inject.c
> > @@ -122,6 +122,7 @@ struct perf_inject {
> > bool in_place_update;
> > bool in_place_update_dry_run;
> > bool copy_kcore_dir;
> > + bool convert_callchain;
> > const char *input_name;
> > struct perf_data output;
> > u64 bytes_written;
> > @@ -133,6 +134,7 @@ struct perf_inject {
> > struct guest_session guest_session;
> > struct strlist *known_build_ids;
> > const struct evsel *mmap_evsel;
> > + struct ip_callchain *raw_callchain;
> > };
> >
> > struct event_entry {
> > @@ -383,6 +385,89 @@ static int perf_event__repipe_sample(const struct perf_tool *tool,
> > return perf_event__repipe_synth(tool, event);
> > }
> >
> > +static int perf_event__convert_sample_callchain(const struct perf_tool *tool,
> > + union perf_event *event,
> > + struct perf_sample *sample,
> > + struct evsel *evsel,
> > + struct machine *machine)
> > +{
> > + struct perf_inject *inject = container_of(tool, struct perf_inject, tool);
> > + struct callchain_cursor *cursor = get_tls_callchain_cursor();
> > + union perf_event *event_copy = (void *)inject->event_copy;
> > + struct callchain_cursor_node *node;
> > + struct thread *thread;
> > + u64 sample_type = evsel->core.attr.sample_type;
> > + u32 sample_size = event->header.size;
> > + u64 i, k;
> > + int ret;
> > +
> > + if (event_copy == NULL) {
> > + inject->event_copy = malloc(PERF_SAMPLE_MAX_SIZE);
> > + if (!inject->event_copy)
> > + return -ENOMEM;
> > +
> > + event_copy = (void *)inject->event_copy;
> > + }
> > +
> > + if (cursor == NULL)
> > + return perf_event__repipe_synth(tool, event);
>
> So when you don't manage to convert you just repipe the whole event,
> with all the stack that you're supposed to discard? Shouldn't we do this
> adjustment anyway?
>
> + /* adjust sample size for stack and regs */
> + sample_size -= sample->user_stack.size;
> + sample_size -= (hweight64(evsel->core.attr.sample_regs_user) + 1) * sizeof(u64);
> + sample_size += (sample->callchain->nr + 1) * sizeof(u64);
> + event_copy->header.size = sample_size;
>
> I.e. we either manage to convert the stack or we throw it away?
Oh.. good point. I think we should just return failue if cursor is NULL
which is unexpected. But thread can be missing or resolve_callchain may
return failure for some reason. I think then we should adjust samples
with kernel stacks only.
Will send v3.
Thanks,
Namhyung
>
> - Arnaldo
>
> > +
> > + callchain_cursor_reset(cursor);
> > +
> > + thread = machine__find_thread(machine, -1, sample->pid);
> > + if (thread == NULL)
> > + return perf_event__repipe_synth(tool, event);
> > +
> > + /* this will parse DWARF using stack and register data */
> > + ret = thread__resolve_callchain(thread, cursor, evsel, sample,
> > + /*parent=*/NULL, /*root_al=*/NULL,
> > + PERF_MAX_STACK_DEPTH);
> > + thread__put(thread);
> > + if (ret != 0)
> > + return perf_event__repipe_synth(tool, event);
> > +
> > + /* copy kernel callchain and context entries */
> > + for (i = 0; i < sample->callchain->nr; i++) {
> > + inject->raw_callchain->ips[i] = sample->callchain->ips[i];
> > + if (sample->callchain->ips[i] == PERF_CONTEXT_USER) {
> > + i++;
> > + break;
> > + }
> > + }
> > + if (i == 0 || inject->raw_callchain->ips[i - 1] != PERF_CONTEXT_USER)
> > + inject->raw_callchain->ips[i++] = PERF_CONTEXT_USER;
> > +
> > + node = cursor->first;
> > + for (k = 0; k < cursor->nr && i < PERF_MAX_STACK_DEPTH; k++) {
> > + if (machine__kernel_ip(machine, node->ip))
> > + /* kernel IPs were added already */;
> > + else if (node->ms.sym && node->ms.sym->inlined)
> > + /* we can't handle inlined callchains */;
> > + else
> > + inject->raw_callchain->ips[i++] = node->ip;
> > +
> > + node = node->next;
> > + }
> > +
> > + inject->raw_callchain->nr = i;
> > + sample->callchain = inject->raw_callchain;
> > +
> > + memcpy(event_copy, event, sizeof(event->header));
> > +
> > + /* adjust sample size for stack and regs */
> > + sample_size -= sample->user_stack.size;
> > + sample_size -= (hweight64(evsel->core.attr.sample_regs_user) + 1) * sizeof(u64);
> > + sample_size += (sample->callchain->nr + 1) * sizeof(u64);
> > + event_copy->header.size = sample_size;
> > +
> > + /* remove sample_type {STACK,REGS}_USER for synthesize */
> > + sample_type &= ~(PERF_SAMPLE_STACK_USER | PERF_SAMPLE_REGS_USER);
> > +
> > + perf_event__synthesize_sample(event_copy, sample_type,
> > + evsel->core.attr.read_format, sample);
> > + return perf_event__repipe_synth(tool, event_copy);
> > +}
> > +
> > static struct dso *findnew_dso(int pid, int tid, const char *filename,
> > const struct dso_id *id, struct machine *machine)
> > {
> > @@ -2270,6 +2355,15 @@ static int __cmd_inject(struct perf_inject *inject)
> > /* Allow space in the header for guest attributes */
> > output_data_offset += gs->session->header.data_offset;
> > output_data_offset = roundup(output_data_offset, 4096);
> > + } else if (inject->convert_callchain) {
> > + inject->tool.sample = perf_event__convert_sample_callchain;
> > + inject->tool.fork = perf_event__repipe_fork;
> > + inject->tool.comm = perf_event__repipe_comm;
> > + inject->tool.exit = perf_event__repipe_exit;
> > + inject->tool.mmap = perf_event__repipe_mmap;
> > + inject->tool.mmap2 = perf_event__repipe_mmap2;
> > + inject->tool.ordered_events = true;
> > + inject->tool.ordering_requires_timestamps = true;
> > }
> >
> > if (!inject->itrace_synth_opts.set)
> > @@ -2322,6 +2416,23 @@ static int __cmd_inject(struct perf_inject *inject)
> > perf_header__set_feat(&session->header,
> > HEADER_BRANCH_STACK);
> > }
> > +
> > + /*
> > + * The converted data file won't have stack and registers.
> > + * Update the perf_event_attr to remove them before writing.
> > + */
> > + if (inject->convert_callchain) {
> > + struct evsel *evsel;
> > +
> > + evlist__for_each_entry(session->evlist, evsel) {
> > + evsel__reset_sample_bit(evsel, REGS_USER);
> > + evsel__reset_sample_bit(evsel, STACK_USER);
> > + evsel->core.attr.sample_regs_user = 0;
> > + evsel->core.attr.sample_stack_user = 0;
> > + evsel->core.attr.exclude_callchain_user = 0;
> > + }
> > + }
> > +
> > session->header.data_offset = output_data_offset;
> > session->header.data_size = inject->bytes_written;
> > perf_session__inject_header(session, session->evlist, fd, &inj_fc.fc,
> > @@ -2346,6 +2457,18 @@ static int __cmd_inject(struct perf_inject *inject)
> > return ret;
> > }
> >
> > +static bool evsel__has_dwarf_callchain(struct evsel *evsel)
> > +{
> > + struct perf_event_attr *attr = &evsel->core.attr;
> > + const u64 dwarf_callchain_flags =
> > + PERF_SAMPLE_STACK_USER | PERF_SAMPLE_REGS_USER | PERF_SAMPLE_CALLCHAIN;
> > +
> > + if (!attr->exclude_callchain_user)
> > + return false;
> > +
> > + return (attr->sample_type & dwarf_callchain_flags) == dwarf_callchain_flags;
> > +}
> > +
> > int cmd_inject(int argc, const char **argv)
> > {
> > struct perf_inject inject = {
> > @@ -2414,6 +2537,8 @@ int cmd_inject(int argc, const char **argv)
> > OPT_STRING(0, "guestmount", &symbol_conf.guestmount, "directory",
> > "guest mount directory under which every guest os"
> > " instance has a subdir"),
> > + OPT_BOOLEAN(0, "convert-callchain", &inject.convert_callchain,
> > + "Generate callchains using DWARF and drop register/stack data"),
> > OPT_END()
> > };
> > const char * const inject_usage[] = {
> > @@ -2429,6 +2554,9 @@ int cmd_inject(int argc, const char **argv)
> >
> > #ifndef HAVE_JITDUMP
> > set_option_nobuild(options, 'j', "jit", "NO_LIBELF=1", true);
> > +#endif
> > +#ifndef HAVE_LIBDW_SUPPORT
> > + set_option_nobuild(options, 0, "convert-callchain", "NO_LIBDW=1", true);
> > #endif
> > argc = parse_options(argc, argv, options, inject_usage, 0);
> >
> > @@ -2588,6 +2716,28 @@ int cmd_inject(int argc, const char **argv)
> > }
> > }
> >
> > + if (inject.convert_callchain) {
> > + struct evsel *evsel;
> > +
> > + if (inject.output.is_pipe || inject.session->data->is_pipe) {
> > + pr_err("--convert-callchain cannot work with pipe\n");
> > + goto out_delete;
> > + }
> > +
> > + evlist__for_each_entry(inject.session->evlist, evsel) {
> > + if (!evsel__has_dwarf_callchain(evsel)) {
> > + pr_err("--convert-callchain requires DWARF call graph.\n");
> > + goto out_delete;
> > + }
> > + }
> > +
> > + inject.raw_callchain = calloc(PERF_MAX_STACK_DEPTH, sizeof(u64));
> > + if (inject.raw_callchain == NULL) {
> > + pr_err("callchain allocation failed\n");
> > + goto out_delete;
> > + }
> > + }
> > +
> > #ifdef HAVE_JITDUMP
> > if (inject.jit_mode) {
> > inject.tool.mmap2 = perf_event__repipe_mmap2;
> > @@ -2618,5 +2768,6 @@ int cmd_inject(int argc, const char **argv)
> > free(inject.itrace_synth_opts.vm_tm_corr_args);
> > free(inject.event_copy);
> > free(inject.guest_session.ev.event_buf);
> > + free(inject.raw_callchain);
> > return ret;
> > }
> > --
> > 2.52.0.457.g6b5491de43-goog
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