From 507f3046d104db2be2c33d5e8fc0d5322818a62c Mon Sep 17 00:00:00 2001
From: Ben Noordhuis <info@bnoordhuis.nl>
Date: Wed, 14 Feb 2024 11:20:44 +0100
Subject: [PATCH] linux: create io_uring sqpoll ring lazily (#4315)

It's been reported that creating many event loops introduces measurable
overhead now that libuv creates an sqpoll-enabled ring.

I don't really see any change in CPU time with or without this change
but deferring ring creation until it's actually used seems like a good
idea, and comes with no downsides that I can think of, so let's do it.

Fixes: https://github.com/libuv/libuv/issues/4308
---
 src/unix/linux.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/src/unix/linux.c b/src/unix/linux.c
index 4164e90d..0dd11438 100644
--- a/src/unix/linux.c
+++ b/src/unix/linux.c
@@ -622,7 +622,7 @@ fail:
 
 
 static void uv__iou_delete(struct uv__iou* iou) {
-  if (iou->ringfd != -1) {
+  if (iou->ringfd > -1) {
     munmap(iou->sq, iou->maxlen);
     munmap(iou->sqe, iou->sqelen);
     uv__close(iou->ringfd);
@@ -636,7 +636,7 @@ int uv__platform_loop_init(uv_loop_t* loop) {
 
   lfields = uv__get_internal_fields(loop);
   lfields->ctl.ringfd = -1;
-  lfields->iou.ringfd = -1;
+  lfields->iou.ringfd = -2;  /* "uninitialized" */
 
   loop->inotify_watchers = NULL;
   loop->inotify_fd = -1;
@@ -645,7 +645,6 @@ int uv__platform_loop_init(uv_loop_t* loop) {
   if (loop->backend_fd == -1)
     return UV__ERR(errno);
 
-  uv__iou_init(loop->backend_fd, &lfields->iou, 64, UV__IORING_SETUP_SQPOLL);
   uv__iou_init(loop->backend_fd, &lfields->ctl, 256, 0);
 
   return 0;
@@ -764,6 +763,15 @@ static struct uv__io_uring_sqe* uv__iou_get_sqe(struct uv__iou* iou,
   uint32_t mask;
   uint32_t slot;
 
+  /* Lazily create the ring. State machine: -2 means uninitialized, -1 means
+   * initialization failed. Anything else is a valid ring file descriptor.
+   */
+  if (iou->ringfd == -2) {
+    uv__iou_init(loop->backend_fd, iou, 64, UV__IORING_SETUP_SQPOLL);
+    if (iou->ringfd == -2)
+      iou->ringfd = -1;  /* "failed" */
+  }
+
   if (iou->ringfd == -1)
     return NULL;