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Message-ID: <aR5nUOtqxSLkatIk@google.com>
Date: Wed, 19 Nov 2025 16:56:48 -0800
From: Namhyung Kim <namhyung@...nel.org>
To: Ian Rogers <irogers@...gle.com>
Cc: Arnaldo Carvalho de Melo <acme@...nel.org>,
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,
Steven Rostedt <rostedt@...dmis.org>,
Josh Poimboeuf <jpoimboe@...nel.org>,
Indu Bhagat <indu.bhagat@...cle.com>,
Jens Remus <jremus@...ux.ibm.com>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
linux-trace-kernel@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [PATCH v4 2/5] perf tools: Minimal DEFERRED_CALLCHAIN support
On Wed, Nov 19, 2025 at 08:48:35AM -0800, Ian Rogers wrote:
> On Sat, Nov 15, 2025 at 3:41 PM Namhyung Kim <namhyung@...nel.org> wrote:
> >
> > Add a new event type for deferred callchains and a new callback for the
> > struct perf_tool. For now it doesn't actually handle the deferred
> > callchains but it just marks the sample if it has the PERF_CONTEXT_
> > USER_DEFFERED in the callchain array.
> >
> > At least, perf report can dump the raw data with this change. Actually
> > this requires the next commit to enable attr.defer_callchain, but if you
> > already have a data file, it'll show the following result.
> >
> > $ perf report -D
> > ...
> > 0x2158@...f.data [0x40]: event: 22
> > .
> > . ... raw event: size 64 bytes
> > . 0000: 16 00 00 00 02 00 40 00 06 00 00 00 0b 00 00 00 ......@.........
> > . 0010: 03 00 00 00 00 00 00 00 a7 7f 33 fe 18 7f 00 00 ..........3.....
> > . 0020: 0f 0e 33 fe 18 7f 00 00 48 14 33 fe 18 7f 00 00 ..3.....H.3.....
> > . 0030: 08 09 00 00 08 09 00 00 e6 7a e7 35 1c 00 00 00 .........z.5....
> >
> > 121163447014 0x2158 [0x40]: PERF_RECORD_CALLCHAIN_DEFERRED(IP, 0x2): 2312/2312: 0xb00000006
> > ... FP chain: nr:3
> > ..... 0: 00007f18fe337fa7
> > ..... 1: 00007f18fe330e0f
> > ..... 2: 00007f18fe331448
> > : unhandled!
> >
> > Signed-off-by: Namhyung Kim <namhyung@...nel.org>
> > ---
> > tools/lib/perf/include/perf/event.h | 13 ++++++++++
> > tools/perf/util/event.c | 1 +
> > tools/perf/util/evsel.c | 31 +++++++++++++++++++++--
> > tools/perf/util/machine.c | 1 +
> > tools/perf/util/perf_event_attr_fprintf.c | 2 ++
> > tools/perf/util/sample.h | 2 ++
> > tools/perf/util/session.c | 20 +++++++++++++++
> > tools/perf/util/tool.c | 1 +
> > tools/perf/util/tool.h | 3 ++-
> > 9 files changed, 71 insertions(+), 3 deletions(-)
> >
> > diff --git a/tools/lib/perf/include/perf/event.h b/tools/lib/perf/include/perf/event.h
> > index aa1e91c97a226e1a..43a8cb04994fa033 100644
> > --- a/tools/lib/perf/include/perf/event.h
> > +++ b/tools/lib/perf/include/perf/event.h
> > @@ -151,6 +151,18 @@ struct perf_record_switch {
> > __u32 next_prev_tid;
> > };
> >
> > +struct perf_record_callchain_deferred {
> > + struct perf_event_header header;
> > + /*
> > + * This is to match kernel and (deferred) user stacks together.
> > + * The kernel part will be in the sample callchain array after
> > + * the PERF_CONTEXT_USER_DEFERRED entry.
> > + */
> > + __u64 cookie;
> > + __u64 nr;
> > + __u64 ips[];
> > +};
> > +
> > struct perf_record_header_attr {
> > struct perf_event_header header;
> > struct perf_event_attr attr;
> > @@ -523,6 +535,7 @@ union perf_event {
> > struct perf_record_read read;
> > struct perf_record_throttle throttle;
> > struct perf_record_sample sample;
> > + struct perf_record_callchain_deferred callchain_deferred;
> > struct perf_record_bpf_event bpf;
> > struct perf_record_ksymbol ksymbol;
> > struct perf_record_text_poke_event text_poke;
> > diff --git a/tools/perf/util/event.c b/tools/perf/util/event.c
> > index fcf44149feb20c35..4c92cc1a952c1d9f 100644
> > --- a/tools/perf/util/event.c
> > +++ b/tools/perf/util/event.c
> > @@ -61,6 +61,7 @@ static const char *perf_event__names[] = {
> > [PERF_RECORD_CGROUP] = "CGROUP",
> > [PERF_RECORD_TEXT_POKE] = "TEXT_POKE",
> > [PERF_RECORD_AUX_OUTPUT_HW_ID] = "AUX_OUTPUT_HW_ID",
> > + [PERF_RECORD_CALLCHAIN_DEFERRED] = "CALLCHAIN_DEFERRED",
> > [PERF_RECORD_HEADER_ATTR] = "ATTR",
> > [PERF_RECORD_HEADER_EVENT_TYPE] = "EVENT_TYPE",
> > [PERF_RECORD_HEADER_TRACING_DATA] = "TRACING_DATA",
> > diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
> > index 989c56d4a23f74f4..5ee3e7dee93fbbcb 100644
> > --- a/tools/perf/util/evsel.c
> > +++ b/tools/perf/util/evsel.c
> > @@ -3089,6 +3089,20 @@ int evsel__parse_sample(struct evsel *evsel, union perf_event *event,
> > data->data_src = PERF_MEM_DATA_SRC_NONE;
> > data->vcpu = -1;
> >
> > + if (event->header.type == PERF_RECORD_CALLCHAIN_DEFERRED) {
> > + const u64 max_callchain_nr = UINT64_MAX / sizeof(u64);
> > +
> > + data->callchain = (struct ip_callchain *)&event->callchain_deferred.nr;
> > + if (data->callchain->nr > max_callchain_nr)
> > + return -EFAULT;
> > +
> > + data->deferred_cookie = event->callchain_deferred.cookie;
> > +
> > + if (evsel->core.attr.sample_id_all)
> > + perf_evsel__parse_id_sample(evsel, event, data);
> > + return 0;
> > + }
> > +
> > if (event->header.type != PERF_RECORD_SAMPLE) {
> > if (!evsel->core.attr.sample_id_all)
> > return 0;
> > @@ -3213,12 +3227,25 @@ int evsel__parse_sample(struct evsel *evsel, union perf_event *event,
> >
> > if (type & PERF_SAMPLE_CALLCHAIN) {
> > const u64 max_callchain_nr = UINT64_MAX / sizeof(u64);
> > + u64 callchain_nr;
> >
> > OVERFLOW_CHECK_u64(array);
> > data->callchain = (struct ip_callchain *)array++;
> > - if (data->callchain->nr > max_callchain_nr)
> > + callchain_nr = data->callchain->nr;
> > + if (callchain_nr > max_callchain_nr)
> > return -EFAULT;
> > - sz = data->callchain->nr * sizeof(u64);
> > + sz = callchain_nr * sizeof(u64);
> > + /*
> > + * Save the cookie for the deferred user callchain. The last 2
> > + * entries in the callchain should be the context marker and the
> > + * cookie. The cookie will be used to match PERF_RECORD_
> > + * CALLCHAIN_DEFERRED later.
> > + */
> > + if (evsel->core.attr.defer_callchain && callchain_nr >= 2 &&
> > + data->callchain->ips[callchain_nr - 2] == PERF_CONTEXT_USER_DEFERRED) {
> > + data->deferred_cookie = data->callchain->ips[callchain_nr - 1];
> > + data->deferred_callchain = true;
> > + }
> > OVERFLOW_CHECK(array, sz, max_size);
> > array = (void *)array + sz;
> > }
> > diff --git a/tools/perf/util/machine.c b/tools/perf/util/machine.c
> > index b5dd42588c916d91..841b711d970e9457 100644
> > --- a/tools/perf/util/machine.c
> > +++ b/tools/perf/util/machine.c
> > @@ -2124,6 +2124,7 @@ static int add_callchain_ip(struct thread *thread,
> > *cpumode = PERF_RECORD_MISC_KERNEL;
> > break;
> > case PERF_CONTEXT_USER:
> > + case PERF_CONTEXT_USER_DEFERRED:
> > *cpumode = PERF_RECORD_MISC_USER;
> > break;
> > default:
> > diff --git a/tools/perf/util/perf_event_attr_fprintf.c b/tools/perf/util/perf_event_attr_fprintf.c
> > index 66b666d9ce649dd7..741c3d657a8b6ae7 100644
> > --- a/tools/perf/util/perf_event_attr_fprintf.c
> > +++ b/tools/perf/util/perf_event_attr_fprintf.c
> > @@ -343,6 +343,8 @@ int perf_event_attr__fprintf(FILE *fp, struct perf_event_attr *attr,
> > PRINT_ATTRf(inherit_thread, p_unsigned);
> > PRINT_ATTRf(remove_on_exec, p_unsigned);
> > PRINT_ATTRf(sigtrap, p_unsigned);
> > + PRINT_ATTRf(defer_callchain, p_unsigned);
> > + PRINT_ATTRf(defer_output, p_unsigned);
> >
> > PRINT_ATTRn("{ wakeup_events, wakeup_watermark }", wakeup_events, p_unsigned, false);
> > PRINT_ATTRf(bp_type, p_unsigned);
> > diff --git a/tools/perf/util/sample.h b/tools/perf/util/sample.h
> > index fae834144ef42105..a8307b20a9ea8066 100644
> > --- a/tools/perf/util/sample.h
> > +++ b/tools/perf/util/sample.h
> > @@ -107,6 +107,8 @@ struct perf_sample {
> > /** @weight3: On x86 holds retire_lat, on powerpc holds p_stage_cyc. */
> > u16 weight3;
> > bool no_hw_idx; /* No hw_idx collected in branch_stack */
> > + bool deferred_callchain; /* Has deferred user callchains */
> > + u64 deferred_cookie;
> > char insn[MAX_INSN];
> > void *raw_data;
> > struct ip_callchain *callchain;
> > diff --git a/tools/perf/util/session.c b/tools/perf/util/session.c
> > index 4b0236b2df2913e1..361e15c1f26a96d0 100644
> > --- a/tools/perf/util/session.c
> > +++ b/tools/perf/util/session.c
> > @@ -720,6 +720,7 @@ static perf_event__swap_op perf_event__swap_ops[] = {
> > [PERF_RECORD_CGROUP] = perf_event__cgroup_swap,
> > [PERF_RECORD_TEXT_POKE] = perf_event__text_poke_swap,
> > [PERF_RECORD_AUX_OUTPUT_HW_ID] = perf_event__all64_swap,
> > + [PERF_RECORD_CALLCHAIN_DEFERRED] = perf_event__all64_swap,
> > [PERF_RECORD_HEADER_ATTR] = perf_event__hdr_attr_swap,
> > [PERF_RECORD_HEADER_EVENT_TYPE] = perf_event__event_type_swap,
> > [PERF_RECORD_HEADER_TRACING_DATA] = perf_event__tracing_data_swap,
> > @@ -854,6 +855,9 @@ static void callchain__printf(struct evsel *evsel,
> > for (i = 0; i < callchain->nr; i++)
> > printf("..... %2d: %016" PRIx64 "\n",
> > i, callchain->ips[i]);
> > +
> > + if (sample->deferred_callchain)
> > + printf("...... (deferred)\n");
> > }
> >
> > static void branch_stack__printf(struct perf_sample *sample,
> > @@ -1123,6 +1127,19 @@ static void dump_sample(struct evsel *evsel, union perf_event *event,
> > sample_read__printf(sample, evsel->core.attr.read_format);
> > }
> >
> > +static void dump_deferred_callchain(struct evsel *evsel, union perf_event *event,
> > + struct perf_sample *sample)
> > +{
> > + if (!dump_trace)
> > + return;
> > +
> > + printf("(IP, 0x%x): %d/%d: %#" PRIx64 "\n",
> > + event->header.misc, sample->pid, sample->tid, sample->deferred_cookie);
> > +
> > + if (evsel__has_callchain(evsel))
> > + callchain__printf(evsel, sample);
> > +}
> > +
> > static void dump_read(struct evsel *evsel, union perf_event *event)
> > {
> > struct perf_record_read *read_event = &event->read;
> > @@ -1353,6 +1370,9 @@ static int machines__deliver_event(struct machines *machines,
> > return tool->text_poke(tool, event, sample, machine);
> > case PERF_RECORD_AUX_OUTPUT_HW_ID:
> > return tool->aux_output_hw_id(tool, event, sample, machine);
> > + case PERF_RECORD_CALLCHAIN_DEFERRED:
> > + dump_deferred_callchain(evsel, event, sample);
> > + return tool->callchain_deferred(tool, event, sample, evsel, machine);
> > default:
> > ++evlist->stats.nr_unknown_events;
> > return -1;
> > diff --git a/tools/perf/util/tool.c b/tools/perf/util/tool.c
> > index 22a8a4ffe05f778e..f732d33e7f895ed4 100644
> > --- a/tools/perf/util/tool.c
> > +++ b/tools/perf/util/tool.c
> > @@ -287,6 +287,7 @@ void perf_tool__init(struct perf_tool *tool, bool ordered_events)
> > tool->read = process_event_sample_stub;
> > tool->throttle = process_event_stub;
> > tool->unthrottle = process_event_stub;
> > + tool->callchain_deferred = process_event_sample_stub;
>
> nit: there's a similar change needed to delegate_tool__init, that code
> is currently unused.
Will update in v2.
>
> Reviewed-by: Ian Rogers <irogers@...gle.com>
Thanks for the review!
Namhyung
>
> > tool->attr = process_event_synth_attr_stub;
> > tool->event_update = process_event_synth_event_update_stub;
> > tool->tracing_data = process_event_synth_tracing_data_stub;
> > diff --git a/tools/perf/util/tool.h b/tools/perf/util/tool.h
> > index 88337cee1e3e2be3..9b9f0a8cbf3de4b5 100644
> > --- a/tools/perf/util/tool.h
> > +++ b/tools/perf/util/tool.h
> > @@ -44,7 +44,8 @@ enum show_feature_header {
> >
> > struct perf_tool {
> > event_sample sample,
> > - read;
> > + read,
> > + callchain_deferred;
> > event_op mmap,
> > mmap2,
> > comm,
> > --
> > 2.52.0.rc1.455.g30608eb744-goog
> >
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