Commit da231338 authored by Anand K Mistry's avatar Anand K Mistry Committed by Arnaldo Carvalho de Melo
Browse files

perf record: Use an eventfd to wakeup when done 



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.

Committer notes:

Make this dependent on HAVE_EVENTFD_SUPPORT, so that we continue
building on older systems without the eventfd syscall.

Signed-off-by: default avatarAnand K Mistry <amistry@google.com>
Acked-by: default avatarJiri Olsa <jolsa@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/20200513122012.v3.1.I4d7421c6bbb1f83ea58419082481082e19097841@changeid


Signed-off-by: default avatarArnaldo Carvalho de Melo <acme@redhat.com>
parent ba35fe93

tools/perf/builtin-record.c

+39 −0
Original line number Diff line number Diff line
@@ -56,6 +56,9 @@ 
#include <unistd.h>

#include <sched.h>

#include <signal.h>

#ifdef HAVE_EVENTFD_SUPPORT

#include <sys/eventfd.h>

#endif

#include <sys/mman.h>

#include <sys/wait.h>

#include <sys/types.h>
@@ -538,6 +541,9 @@ static int record__pushfn(struct mmap *map, void *to, void *bf, size_t size) 


static volatile int signr = -1;

static volatile int child_finished;

#ifdef HAVE_EVENTFD_SUPPORT

static int done_fd = -1;

#endif



static void sig_handler(int sig)

{
@@ -547,6 +553,21 @@ static void sig_handler(int sig) 
		signr = sig;



	done = 1;

#ifdef HAVE_EVENTFD_SUPPORT

{

	u64 tmp = 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");

}

#endif // HAVE_EVENTFD_SUPPORT

}



static void sigsegv_handler(int sig)
@@ -1547,6 +1568,20 @@ static int __cmd_record(struct record *rec, int argc, const char **argv) 
		pr_err("Compression initialization failed.\n");

		return -1;

	}

#ifdef HAVE_EVENTFD_SUPPORT

	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;

	}

#endif // HAVE_EVENTFD_SUPPORT



	session->header.env.comp_type  = PERF_COMP_ZSTD;

	session->header.env.comp_level = rec->opts.comp_level;
@@ -1905,6 +1940,10 @@ out_child: 
	}



out_delete_session:

#ifdef HAVE_EVENTFD_SUPPORT

	if (done_fd >= 0)

		close(done_fd);

#endif

	zstd_fini(&session->zstd_data);

	perf_session__delete(session);

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