lib/ktap: Read /dev/kmsg in blocking mode

We obtain KTAP report from /dev/kmsg.  That file is now opened from
igt_ktest_begin(), a function originally designed for i915 selftests and
now reused with kunit tests.  The original intention of opening that file
was to dump kernel messages to stderr on selftest error.  For that
purpose, the file is now opened in non-blocking mode so we don't end up
waiting for more kernel messages than already available.  Since our ktap
parser code reuses the file descriptor, we now have to loop over
EAGAIN responses, waiting for more KTAP data.  Since we have no sleeps
inside those loops, extremely high CPU usage can be observed.

Simplify reading KTAP reports by first switching the file descriptor back
to blocking mode.

While being at it, fix read errors other than EPIPE likely unintentionally
ignored when reading first line of KTAP data.

v2: Drop EINTR handling as not applicable since SIGINT default signal
    handler kills the whole process anyway,
  - update commit description to also mention read error handling fix.

Signed-off-by: Janusz Krzysztofik <janusz.krzysztofik@linux.intel.com>
Reviewed-by: Mauro Carvalho Chehab <mchehab@kernel.org>

1 files changed, 6 insertions, 1 deletions

diff --git a/lib/igt_kmod.c b/lib/igt_kmod.c
index 739227640..96240543a 100644
--- a/lib/igt_kmod.c
+++ b/lib/igt_kmod.c
@@ -24,6 +24,7 @@
 #include <ctype.h>
 #include <signal.h>
 #include <errno.h>
+#include <fcntl.h>
 #include <sys/utsname.h>
 
 #include "igt_aux.h"
@@ -758,12 +759,16 @@ static void __igt_kunit(struct igt_ktest *tst, const char *opts)
 {
 	struct kmod_module *kunit_kmod;
 	bool is_builtin;
-	int ret;
 	struct ktap_test_results *results;
 	struct ktap_test_results_element *temp;
+	int flags, ret;
 
 	igt_skip_on_f(tst->kmsg < 0, "Could not open /dev/kmsg\n");
 
+	flags = fcntl(tst->kmsg, F_GETFL, 0) & ~O_NONBLOCK;
+	igt_skip_on_f(fcntl(tst->kmsg, F_SETFL, flags) == -1,
+		      "Could not set /dev/kmsg to blocking mode\n");
+
 	igt_skip_on(lseek(tst->kmsg, 0, SEEK_END) < 0);
 
 	igt_skip_on(kmod_module_new_from_name(kmod_ctx(), "kunit", &kunit_kmod));