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Message-ID: <CAP-5=fVEuYXw+P-+Z7bU7Z-+7dsHPPfABh5pdnPtfvH-23u4Qw@mail.gmail.com>
Date: Fri, 14 Nov 2025 09:59:42 -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 v3 3/5] perf record: Enable defer_callchain for user callchains
On Thu, Nov 13, 2025 at 11:01 PM Namhyung Kim <namhyung@...nel.org> wrote:
>
> And add the missing feature detection logic to clear the flag on old
> kernels.
>
> $ perf record -g -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>
> ---
> tools/perf/util/evsel.c | 24 ++++++++++++++++++++++++
> tools/perf/util/evsel.h | 1 +
> 2 files changed, 25 insertions(+)
>
> diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
> index 244b3e44d090d413..f5652d00b457d096 100644
> --- a/tools/perf/util/evsel.c
> +++ b/tools/perf/util/evsel.c
> @@ -1061,6 +1061,14 @@ static void __evsel__config_callchain(struct evsel *evsel, struct record_opts *o
> }
> }
>
> + if (param->record_mode == CALLCHAIN_FP && !attr->exclude_callchain_user) {
> + /*
> + * Enable deferred callchains optimistically. It'll be switched
> + * off later if the kernel doesn't support it.
> + */
> + attr->defer_callchain = 1;
> + }
If a user has requested frame pointer call chains why would they want
deferred call chains? The point of deferral to my understanding is to
allow the paging in of debug data, but frame pointers don't need that
as the stack should be in the page cache.
Is this being done for code coverage reasons so that deferral is known
to work for later addition of SFrames? In which case this should be an
opt-in not default behavior. When there is a record_mode of
CALLCHAIN_SFRAME then making deferral the default for that mode makes
sense, but not for frame pointers IMO.
Thanks,
Ian
> +
> if (function) {
> pr_info("Disabling user space callchains for function trace event.\n");
> attr->exclude_callchain_user = 1;
> @@ -1511,6 +1519,7 @@ 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;
>
> /*
> * ksymbol is tracked separately with text poke because it needs to be
> @@ -2199,6 +2208,10 @@ 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 +2486,13 @@ 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 +2604,10 @@ 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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