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Message-ID: <CAP-5=fXX_h8q=0uTGkU89Z_r6Rw30oMfxyJw5p-O45t0OSppJw@mail.gmail.com>
Date: Wed, 19 Nov 2025 08:50:08 -0800
From: Ian Rogers <irogers@...gle.com>
To: Namhyung Kim <namhyung@...nel.org>
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 3/5] perf record: Add --call-graph fp,defer option for
deferred callchains
On Sat, Nov 15, 2025 at 3:41 PM Namhyung Kim <namhyung@...nel.org> wrote:
>
> Add a new callchain record mode option for deferred callchains. For now
> it only works with FP (frame-pointer) mode.
>
> And add the missing feature detection logic to clear the flag on old
> kernels.
>
> $ perf record --call-graph fp,defer -vv true
> ...
> ------------------------------------------------------------
> perf_event_attr:
> type 0 (PERF_TYPE_HARDWARE)
> size 136
> config 0 (PERF_COUNT_HW_CPU_CYCLES)
> { sample_period, sample_freq } 4000
> sample_type IP|TID|TIME|CALLCHAIN|PERIOD
> read_format ID|LOST
> disabled 1
> inherit 1
> mmap 1
> comm 1
> freq 1
> enable_on_exec 1
> task 1
> sample_id_all 1
> mmap2 1
> comm_exec 1
> ksymbol 1
> bpf_event 1
> defer_callchain 1
> defer_output 1
> ------------------------------------------------------------
> sys_perf_event_open: pid 162755 cpu 0 group_fd -1 flags 0x8
> sys_perf_event_open failed, error -22
> switching off deferred callchain support
>
> Signed-off-by: Namhyung Kim <namhyung@...nel.org>
Reviewed-by: Ian Rogers <irogers@...gle.com>
Thanks,
Ian
> ---
> tools/perf/Documentation/perf-config.txt | 3 +++
> tools/perf/Documentation/perf-record.txt | 4 ++++
> tools/perf/util/callchain.c | 16 +++++++++++++---
> tools/perf/util/callchain.h | 1 +
> tools/perf/util/evsel.c | 19 +++++++++++++++++++
> tools/perf/util/evsel.h | 1 +
> 6 files changed, 41 insertions(+), 3 deletions(-)
>
> diff --git a/tools/perf/Documentation/perf-config.txt b/tools/perf/Documentation/perf-config.txt
> index c6f33565966735fe..642d1c490d9e3bcd 100644
> --- a/tools/perf/Documentation/perf-config.txt
> +++ b/tools/perf/Documentation/perf-config.txt
> @@ -452,6 +452,9 @@ Variables
> kernel space is controlled not by this option but by the
> kernel config (CONFIG_UNWINDER_*).
>
> + The 'defer' mode can be used with 'fp' mode to enable deferred
> + user callchains (like 'fp,defer').
> +
> call-graph.dump-size::
> The size of stack to dump in order to do post-unwinding. Default is 8192 (byte).
> When using dwarf into record-mode, the default size will be used if omitted.
> diff --git a/tools/perf/Documentation/perf-record.txt b/tools/perf/Documentation/perf-record.txt
> index 067891bd7da6edc8..e8b9aadbbfa50574 100644
> --- a/tools/perf/Documentation/perf-record.txt
> +++ b/tools/perf/Documentation/perf-record.txt
> @@ -325,6 +325,10 @@ OPTIONS
> by default. User can change the number by passing it after comma
> like "--call-graph fp,32".
>
> + Also "defer" can be used with "fp" (like "--call-graph fp,defer") to
> + enable deferred user callchain which will collect user-space callchains
> + when the thread returns to the user space.
> +
> -q::
> --quiet::
> Don't print any warnings or messages, useful for scripting.
> diff --git a/tools/perf/util/callchain.c b/tools/perf/util/callchain.c
> index d7b7eef740b9d6ed..2884187ccbbecfdc 100644
> --- a/tools/perf/util/callchain.c
> +++ b/tools/perf/util/callchain.c
> @@ -275,9 +275,13 @@ int parse_callchain_record(const char *arg, struct callchain_param *param)
> if (tok) {
> unsigned long size;
>
> - size = strtoul(tok, &name, 0);
> - if (size < (unsigned) sysctl__max_stack())
> - param->max_stack = size;
> + if (!strncmp(tok, "defer", sizeof("defer"))) {
> + param->defer = true;
> + } else {
> + size = strtoul(tok, &name, 0);
> + if (size < (unsigned) sysctl__max_stack())
> + param->max_stack = size;
> + }
> }
> break;
>
> @@ -314,6 +318,12 @@ int parse_callchain_record(const char *arg, struct callchain_param *param)
> } while (0);
>
> free(buf);
> +
> + if (param->defer && param->record_mode != CALLCHAIN_FP) {
> + pr_err("callchain: deferred callchain only works with FP\n");
> + return -EINVAL;
> + }
> +
> return ret;
> }
>
> diff --git a/tools/perf/util/callchain.h b/tools/perf/util/callchain.h
> index 86ed9e4d04f9ee7b..d5ae4fbb7ce5fa44 100644
> --- a/tools/perf/util/callchain.h
> +++ b/tools/perf/util/callchain.h
> @@ -98,6 +98,7 @@ extern bool dwarf_callchain_users;
>
> struct callchain_param {
> bool enabled;
> + bool defer;
> enum perf_call_graph_mode record_mode;
> u32 dump_size;
> enum chain_mode mode;
> diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
> index 5ee3e7dee93fbbcb..7772ee9cfe3ac1c7 100644
> --- a/tools/perf/util/evsel.c
> +++ b/tools/perf/util/evsel.c
> @@ -1065,6 +1065,9 @@ static void __evsel__config_callchain(struct evsel *evsel, struct record_opts *o
> pr_info("Disabling user space callchains for function trace event.\n");
> attr->exclude_callchain_user = 1;
> }
> +
> + if (param->defer && !attr->exclude_callchain_user)
> + attr->defer_callchain = 1;
> }
>
> void evsel__config_callchain(struct evsel *evsel, struct record_opts *opts,
> @@ -1511,6 +1514,7 @@ void evsel__config(struct evsel *evsel, struct record_opts *opts,
> attr->mmap2 = track && !perf_missing_features.mmap2;
> attr->comm = track;
> attr->build_id = track && opts->build_id;
> + attr->defer_output = track && callchain->defer;
>
> /*
> * ksymbol is tracked separately with text poke because it needs to be
> @@ -2199,6 +2203,10 @@ static int __evsel__prepare_open(struct evsel *evsel, struct perf_cpu_map *cpus,
>
> static void evsel__disable_missing_features(struct evsel *evsel)
> {
> + if (perf_missing_features.defer_callchain && evsel->core.attr.defer_callchain)
> + evsel->core.attr.defer_callchain = 0;
> + if (perf_missing_features.defer_callchain && evsel->core.attr.defer_output)
> + evsel->core.attr.defer_output = 0;
> if (perf_missing_features.inherit_sample_read && evsel->core.attr.inherit &&
> (evsel->core.attr.sample_type & PERF_SAMPLE_READ))
> evsel->core.attr.inherit = 0;
> @@ -2473,6 +2481,13 @@ static bool evsel__detect_missing_features(struct evsel *evsel, struct perf_cpu
>
> /* Please add new feature detection here. */
>
> + attr.defer_callchain = true;
> + if (has_attr_feature(&attr, /*flags=*/0))
> + goto found;
> + perf_missing_features.defer_callchain = true;
> + pr_debug2("switching off deferred callchain support\n");
> + attr.defer_callchain = false;
> +
> attr.inherit = true;
> attr.sample_type = PERF_SAMPLE_READ | PERF_SAMPLE_TID;
> if (has_attr_feature(&attr, /*flags=*/0))
> @@ -2584,6 +2599,10 @@ static bool evsel__detect_missing_features(struct evsel *evsel, struct perf_cpu
> errno = old_errno;
>
> check:
> + if ((evsel->core.attr.defer_callchain || evsel->core.attr.defer_output) &&
> + perf_missing_features.defer_callchain)
> + return true;
> +
> if (evsel->core.attr.inherit &&
> (evsel->core.attr.sample_type & PERF_SAMPLE_READ) &&
> perf_missing_features.inherit_sample_read)
> diff --git a/tools/perf/util/evsel.h b/tools/perf/util/evsel.h
> index 3ae4ac8f9a37e009..a08130ff2e47a887 100644
> --- a/tools/perf/util/evsel.h
> +++ b/tools/perf/util/evsel.h
> @@ -221,6 +221,7 @@ struct perf_missing_features {
> bool branch_counters;
> bool aux_action;
> bool inherit_sample_read;
> + bool defer_callchain;
> };
>
> extern struct perf_missing_features perf_missing_features;
> --
> 2.52.0.rc1.455.g30608eb744-goog
>
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