[PATCH v4 37/39] perf record: Enable defer_callchain for user callchains

Josh Poimboeuf
2025-01-22T02:32:32Z

From: Namhyung Kim <namhyung@kernel.org>

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
------------------------------------------------------------
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@kernel.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.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 09b9735f2fb1..e3be3cc7632d 100644
--- a/tools/perf/util/evsel.c
+++ b/tools/perf/util/evsel.c
@@ -998,6 +998,14 @@ 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 (function) {
pr_info("Disabling user space callchains for function trace event.\n");
attr->exclude_callchain_user = 1;
@@ -2038,6 +2046,8 @@ 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 = 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;
@@ -2244,6 +2254,15 @@ static bool evsel__detect_missing_features(struct evsel *evsel)

/* Please add new feature detection here. */

+ attr.defer_callchain = true;
+ attr.sample_type = PERF_SAMPLE_CALLCHAIN;
+ 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.sample_type = 0;
+
attr.inherit = true;
attr.sample_type = PERF_SAMPLE_READ;
if (has_attr_feature(&attr, /*flags=*/0))
@@ -2355,6 +2374,11 @@ static bool evsel__detect_missing_features(struct evsel *evsel)
errno = old_errno;

check:
+ if (evsel->core.attr.defer_callchain &&
+ evsel->core.attr.sample_type & PERF_SAMPLE_CALLCHAIN &&
+ 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 04934a7af174..b90a970f9a30 100644
--- a/tools/perf/util/evsel.h
+++ b/tools/perf/util/evsel.h
@@ -206,6 +206,7 @@ struct perf_missing_features {
bool read_lost;
bool branch_counters;
bool inherit_sample_read;
+ bool defer_callchain;
};

extern struct perf_missing_features perf_missing_features;
--
2.48.1