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>

diff --git a/tools/perf/util/evlist.h b/tools/perf/util/evlist.h
index 1305140..3655b9e 100644 (file)
--- 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 (file)
--- 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)
 {