HACK: Allow io_uring_submit batching just ahead of polllaforge/io_uring_batch

Let's add a mode (enabled via the LIBOSMO_IO_URING_BATCH environment
variable), where we don't call io_uring_submit() after every operation
we add to the submission queue.  Rather, do that once before we go into
poll.

This should massively reduce the amount of io_uring_enter() syscalls
we're seeing.

Change-Id: Ia27ada909df246f21d110d8ae0b40229aacd579d

2 files changed, 18 insertions, 2 deletions

diff --git a/src/core/osmo_io_uring.c b/src/core/osmo_io_uring.c
index 569f1505..6801eb4f 100644
--- a/src/core/osmo_io_uring.c
+++ b/src/core/osmo_io_uring.c
@@ -60,6 +60,7 @@ struct osmo_io_uring {
 };
 
 static __thread struct osmo_io_uring g_ring;
+static bool g_batch = false;
 
 static void iofd_uring_cqe(struct io_uring *ring);
 
@@ -90,6 +91,9 @@ void osmo_iofd_uring_init(void)
 {
 	int rc, evfd;
 
+	if (getenv("LIBOSMO_IO_URING_BATCH"))
+		g_batch = true;
+
 	rc = io_uring_queue_init(IOFD_URING_ENTRIES, &g_ring.ring, 0);
 	if (rc < 0)
 		osmo_panic("failure during io_uring_queue_init(): %s\n", strerror(-rc));
@@ -175,7 +179,8 @@ static void iofd_uring_submit_recv(struct osmo_io_fd *iofd, enum iofd_msg_action
 	}
 	io_uring_sqe_set_data(sqe, msghdr);
 
-	io_uring_submit(&g_ring.ring);
+	if (!g_batch)
+		io_uring_submit(&g_ring.ring);
 	/* NOTE: This only works if we have one read per fd */
 	iofd->u.uring.read_msghdr = msghdr;
 }
@@ -315,7 +320,8 @@ static int iofd_uring_submit_tx(struct osmo_io_fd *iofd)
 		OSMO_ASSERT(0);
 	}
 
-	io_uring_submit(&g_ring.ring);
+	if (!g_batch)
+		io_uring_submit(&g_ring.ring);
 	iofd->u.uring.write_msghdr = msghdr;
 
 	return 0;
@@ -529,4 +535,10 @@ const struct iofd_backend_ops iofd_uring_ops = {
 	.notify_connected = iofd_uring_notify_connected,
 };
 
+void osmo_io_uring_submit(void)
+{
+	if (g_batch)
+		io_uring_submit(&g_ring.ring);
+}
+
 #endif /* defined(__linux__) */
diff --git a/src/core/select.c b/src/core/select.c
index 70047f09..0a4302f2 100644
--- a/src/core/select.c
+++ b/src/core/select.c
@@ -426,12 +426,16 @@ static int poll_disp_fds(unsigned int n_fd)
 	return work;
 }
 
+void osmo_io_uring_submit(void);
+
 static int _osmo_select_main(int polling)
 {
 	unsigned int n_poll;
 	int rc;
 	int timeout = 0;
 
+	osmo_io_uring_submit();
+
 	/* prepare read and write fdsets */
 	n_poll = poll_fill_fds();