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Subject[tip: perf/core] perf record: Add a function to test for kernel support for AUX area sampling
The following commit has been merged into the perf/core branch of tip:

Commit-ID: 9bca1a4ef5034f0a82861ac0375eb0272c5ce04e
Gitweb: https://git.kernel.org/tip/9bca1a4ef5034f0a82861ac0375eb0272c5ce04e
Author: Adrian Hunter <adrian.hunter@intel.com>
AuthorDate: Fri, 15 Nov 2019 14:42:13 +02:00
Committer: Arnaldo Carvalho de Melo <acme@redhat.com>
CommitterDate: Fri, 22 Nov 2019 10:43:24 -03:00

perf record: Add a function to test for kernel support for AUX area sampling

Architectures are expected to know if AUX area sampling is supported by
the hardware. Add a function perf_can_aux_sample() which will determine
whether the kernel supports it.

Committer notes:

I reported that this message was taking place on a kernel without the
required bits:

# perf record --aux-sample -e '{intel_pt//u,branch-misses:u}'
Error:
The sys_perf_event_open() syscall returned with 7 (Argument list too long) for event (branch-misses:u).
/bin/dmesg | grep -i perf may provide additional information.

Adrian sent a patch addressing it, with this explanation:

----
perf_can_aux_sample_size() always returned true because it did not pass
the attribute size to sys_perf_event_open, nor correctly check the
return value and errno.
----

After applying it I get, later in the series, when --aux-sample is
added:

# perf record --aux-sample -e '{intel_pt//u,branch-misses:u}'
AUX area sampling is not supported by kernel

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lore.kernel.org/lkml/20191115124225.5247-4-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
---
tools/perf/util/evlist.h | 1 +
tools/perf/util/record.c | 31 +++++++++++++++++++++++++++++++
2 files changed, 32 insertions(+)

diff --git a/tools/perf/util/evlist.h b/tools/perf/util/evlist.h
index 1305140..3655b9e 100644
--- a/tools/perf/util/evlist.h
+++ b/tools/perf/util/evlist.h
@@ -176,6 +176,7 @@ void perf_evlist__set_id_pos(struct evlist *evlist);
bool perf_can_sample_identifier(void);
bool perf_can_record_switch_events(void);
bool perf_can_record_cpu_wide(void);
+bool perf_can_aux_sample(void);
void perf_evlist__config(struct evlist *evlist, struct record_opts *opts,
struct callchain_param *callchain);
int record_opts__config(struct record_opts *opts);
diff --git a/tools/perf/util/record.c b/tools/perf/util/record.c
index 8579505..7def661 100644
--- a/tools/perf/util/record.c
+++ b/tools/perf/util/record.c
@@ -136,6 +136,37 @@ bool perf_can_record_cpu_wide(void)
return true;
}

+/*
+ * Architectures are expected to know if AUX area sampling is supported by the
+ * hardware. Here we check for kernel support.
+ */
+bool perf_can_aux_sample(void)
+{
+ struct perf_event_attr attr = {
+ .size = sizeof(struct perf_event_attr),
+ .exclude_kernel = 1,
+ /*
+ * Non-zero value causes the kernel to calculate the effective
+ * attribute size up to that byte.
+ */
+ .aux_sample_size = 1,
+ };
+ int fd;
+
+ fd = sys_perf_event_open(&attr, -1, 0, -1, 0);
+ /*
+ * If the kernel attribute is big enough to contain aux_sample_size
+ * then we assume that it is supported. We are relying on the kernel to
+ * validate the attribute size before anything else that could be wrong.
+ */
+ if (fd < 0 && errno == E2BIG)
+ return false;
+ if (fd >= 0)
+ close(fd);
+
+ return true;
+}
+
void perf_evlist__config(struct evlist *evlist, struct record_opts *opts,
struct callchain_param *callchain)
{
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