[PATCH v3] perf record: Use an eventfd to wakeup when done

Anand K Mistry
2020-05-13T02:20:37Z

The setting and checking of 'done' contains a rare race where the signal
handler setting 'done' is run after checking to break the loop, but
before waiting in evlist__poll(). In this case, the main loop won't wake
up until either another signal is sent, or the perf data fd causes a
wake up.

The following simple script can trigger this condition (but you might
need to run it for several hours):
for ((i = 0; i >= 0; i++)) ; do
echo "Loop $i"
delay=$(echo "scale=4; 0.1 * $RANDOM/32768" | bc)
./perf record -- sleep 30000000 >/dev/null&
pid=$!
sleep $delay
kill -TERM $pid
echo "PID $pid"
wait $pid
done

At some point, the loop will stall. Adding logging, even though perf has
received the SIGTERM and set 'done = 1', perf will remain sleeping until
a second signal is sent.

Signed-off-by: Anand K Mistry <amistry@google.com>

---

Changes in v3:
- Move done_fd creation to below session initialisation
- Close done_fd on exit
- Log errno when write(done_fd) fails

Changes in v2:
- Added comment to signal handler explaining why the eventfd is added
- Added error handling when creating done_fd

tools/perf/builtin-record.c | 29 +++++++++++++++++++++++++++++
1 file changed, 29 insertions(+)

diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c
index 1ab349abe90469..a1af6857f24748 100644
--- a/tools/perf/builtin-record.c
+++ b/tools/perf/builtin-record.c
@@ -53,6 +53,7 @@
#include <unistd.h>
#include <sched.h>
#include <signal.h>
+#include <sys/eventfd.h>
#include <sys/mman.h>
#include <sys/wait.h>
#include <sys/types.h>
@@ -518,15 +519,28 @@ static int record__pushfn(struct mmap *map, void *to, void *bf, size_t size)

static volatile int signr = -1;
static volatile int child_finished;
+static int done_fd = -1;

static void sig_handler(int sig)
{
+ u64 tmp = 1;
if (sig == SIGCHLD)
child_finished = 1;
else
signr = sig;

done = 1;
+
+ /*
+ * It is possible for this signal handler to run after done is checked
+ * in the main loop, but before the perf counter fds are polled. If this
+ * happens, the poll() will continue to wait even though done is set,
+ * and will only break out if either another signal is received, or the
+ * counters are ready for read. To ensure the poll() doesn't sleep when
+ * done is set, use an eventfd (done_fd) to wake up the poll().
+ */
+ if (write(done_fd, &tmp, sizeof(tmp)) < 0)
+ pr_err("failed to signal wakeup fd, error: %m\n");
}

static void sigsegv_handler(int sig)
@@ -1466,6 +1480,19 @@ static int __cmd_record(struct record *rec, int argc, const char **argv)
return -1;
}

+ done_fd = eventfd(0, EFD_NONBLOCK);
+ if (done_fd < 0) {
+ pr_err("Failed to create wakeup eventfd, error: %m\n");
+ status = -1;
+ goto out_delete_session;
+ }
+ err = evlist__add_pollfd(rec->evlist, done_fd);
+ if (err < 0) {
+ pr_err("Failed to add wakeup eventfd to poll list\n");
+ status = err;
+ goto out_delete_session;
+ }
+
session->header.env.comp_type = PERF_COMP_ZSTD;
session->header.env.comp_level = rec->opts.comp_level;

@@ -1827,6 +1854,8 @@ static int __cmd_record(struct record *rec, int argc, const char **argv)
}

out_delete_session:
+ if (done_fd >= 0)
+ close(done_fd);
zstd_fini(&session->zstd_data);
perf_session__delete(session);

--
2.26.2.645.ge9eca65c58-goog