linux: disable SQPOLL io_uring by default (#4492)

The SQPOLL io_uring instance wasn't providing consistent behaviour to
users depending on kernel versions, load shape, ... creating issues
difficult to track and fix. Don't use this ring by default but allow
enabling it by calling `uv_loop_configure()` with
`UV_LOOP_ENABLE_IO_URING_SQPOLL`.

3 files changed, 18 insertions, 1 deletions

diff --git a/src/unix/internal.h b/src/unix/internal.h
index a8ff3909..8055730a 100644
--- a/src/unix/internal.h
+++ b/src/unix/internal.h
@@ -157,7 +157,8 @@ typedef struct uv__stream_queued_fds_s uv__stream_queued_fds_t;
 /* loop flags */
 enum {
   UV_LOOP_BLOCK_SIGPROF = 0x1,
-  UV_LOOP_REAP_CHILDREN = 0x2
+  UV_LOOP_REAP_CHILDREN = 0x2,
+  UV_LOOP_ENABLE_IO_URING_SQPOLL = 0x4
 };
 
 /* flags of excluding ifaddr */
diff --git a/src/unix/linux.c b/src/unix/linux.c
index ad07430e..a5f74e89 100644
--- a/src/unix/linux.c
+++ b/src/unix/linux.c
@@ -761,6 +761,14 @@ static struct uv__io_uring_sqe* uv__iou_get_sqe(struct uv__iou* iou,
    * initialization failed. Anything else is a valid ring file descriptor.
    */
   if (iou->ringfd == -2) {
+    /* By default, the SQPOLL is not created. Enable only if the loop is
+     * configured with UV_LOOP_USE_IO_URING_SQPOLL.
+     */
+    if ((loop->flags & UV_LOOP_ENABLE_IO_URING_SQPOLL) == 0) {
+      iou->ringfd = -1;
+      return NULL;
+    }
+
     uv__iou_init(loop->backend_fd, iou, 64, UV__IORING_SETUP_SQPOLL);
     if (iou->ringfd == -2)
       iou->ringfd = -1;  /* "failed" */
diff --git a/src/unix/loop.c b/src/unix/loop.c
index a9468e8e..179ee999 100644
--- a/src/unix/loop.c
+++ b/src/unix/loop.c
@@ -217,6 +217,14 @@ int uv__loop_configure(uv_loop_t* loop, uv_loop_option option, va_list ap) {
     return 0;
   }
 
+#if defined(__linux__)
+  if (option == UV_LOOP_USE_IO_URING_SQPOLL) {
+    loop->flags |= UV_LOOP_ENABLE_IO_URING_SQPOLL;
+    return 0;
+  }
+#endif
+
+
   if (option != UV_LOOP_BLOCK_SIGNAL)
     return UV_ENOSYS;