[PATCH 4/4] io_uring: add support for sharing kernel io-wq workqueue

[Jens Axboe <axboe@kernel.dk>]

An id field is added to io_uring_params, which always returns the ID of
the io-wq backend that is associated with an io_uring context. If an 'id'
is provided and IORING_SETUP_SHARED is set in the creation flags, then
we attempt to attach to an existing io-wq instead of setting up a new one.

This allows creation of "sibling" io_urings, where we prefer to keep the
SQ/CQ private, but want to share the async backend to minimize the amount
of overhead associated with having multiple rings that belong to the same
backend.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 fs/io_uring.c                 | 20 +++++++++++++++++---
 include/uapi/linux/io_uring.h |  4 +++-
 2 files changed, 20 insertions(+), 4 deletions(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 9f73586dcfb8..3dad12906db3 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -5673,7 +5673,7 @@ static int io_sq_offload_start(struct io_ring_ctx *ctx,
 {
 	struct io_wq_data data;
 	unsigned concurrency;
-	int ret;
+	int ret, id;
 
 	init_waitqueue_head(&ctx->sqo_wait);
 	mmgrab(current->mm);
@@ -5724,13 +5724,23 @@ static int io_sq_offload_start(struct io_ring_ctx *ctx,
 
 	/* Do QD, or 4 * CPUS, whatever is smallest */
 	concurrency = min(ctx->sq_entries, 4 * num_online_cpus());
-	ctx->io_wq = io_wq_create(concurrency, &data);
+
+	id = 0;
+	if (ctx->flags & IORING_SETUP_ATTACH_WQ) {
+		id = p->id;
+		if (!id) {
+			ret = -EINVAL;
+			goto err;
+		}
+	}
+	ctx->io_wq = io_wq_create_id(concurrency, &data, id);
 	if (IS_ERR(ctx->io_wq)) {
 		ret = PTR_ERR(ctx->io_wq);
 		ctx->io_wq = NULL;
 		goto err;
 	}
 
+	p->id = io_wq_id(ctx->io_wq);
 	return 0;
 err:
 	io_finish_async(ctx);
@@ -6554,7 +6564,11 @@ static long io_uring_setup(u32 entries, struct io_uring_params __user *params)
 
 	if (p.flags & ~(IORING_SETUP_IOPOLL | IORING_SETUP_SQPOLL |
 			IORING_SETUP_SQ_AFF | IORING_SETUP_CQSIZE |
-			IORING_SETUP_CLAMP))
+			IORING_SETUP_CLAMP | IORING_SETUP_ATTACH_WQ))
+		return -EINVAL;
+
+	/* id isn't valid without ATTACH_WQ being set */
+	if (!(p.flags & IORING_SETUP_ATTACH_WQ) && p.id)
 		return -EINVAL;
 
 	ret = io_uring_create(entries, &p);
diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
index 57d05cc5e271..f66e53c74a3d 100644
--- a/include/uapi/linux/io_uring.h
+++ b/include/uapi/linux/io_uring.h
@@ -75,6 +75,7 @@ enum {
 #define IORING_SETUP_SQ_AFF	(1U << 2)	/* sq_thread_cpu is valid */
 #define IORING_SETUP_CQSIZE	(1U << 3)	/* app defines CQ size */
 #define IORING_SETUP_CLAMP	(1U << 4)	/* clamp SQ/CQ ring sizes */
+#define IORING_SETUP_ATTACH_WQ	(1U << 5)	/* attach to existing wq */
 
 enum {
 	IORING_OP_NOP,
@@ -183,7 +184,8 @@ struct io_uring_params {
 	__u32 sq_thread_cpu;
 	__u32 sq_thread_idle;
 	__u32 features;
-	__u32 resv[4];
+	__u32 id;
+	__u32 resv[3];
 	struct io_sqring_offsets sq_off;
 	struct io_cqring_offsets cq_off;
 };
-- 
2.25.0