[PATCH 5/5] io_uring: use TWA_SIGNAL_NO_IPI if IORING_SETUP_NO_RESCHED is used

[Jens Axboe <axboe@kernel.dk>]

If this is set, io_uring will never use an IPI to deliver a task_work
notification. This can be used in the common case where a single task or
thread communicates with the ring, and doesn't rely on
io_uring_cqe_peek().

This provides a noticeable win in performance, both from eliminating
the IPI itself, but also from avoiding interrupting the submitting
task unnecessarily.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 fs/io_uring.c                 | 8 +++++---
 include/uapi/linux/io_uring.h | 1 +
 2 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 45992ada2f06..7824a615f06d 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -11324,9 +11324,10 @@ static __cold int io_uring_create(unsigned entries, struct io_uring_params *p,
 		ctx->user = get_uid(current_user());
 
 	/*
-	 * For SQPOLL, we just need a wakeup, always.
+	 * For SQPOLL, we just need a wakeup, always. For !SQPOLL, if
+	 * NO_RESCHED is set, then IPIs are never needed by the app.
 	 */
-	if (ctx->flags & IORING_SETUP_SQPOLL)
+	if (ctx->flags & (IORING_SETUP_SQPOLL|IORING_SETUP_NO_RESCHED))
 		ctx->notify_method = TWA_SIGNAL_NO_IPI;
 	else
 		ctx->notify_method = TWA_SIGNAL;
@@ -11428,7 +11429,8 @@ 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_ATTACH_WQ |
-			IORING_SETUP_R_DISABLED | IORING_SETUP_SUBMIT_ALL))
+			IORING_SETUP_R_DISABLED | IORING_SETUP_SUBMIT_ALL |
+			IORING_SETUP_NO_RESCHED))
 		return -EINVAL;
 
 	return  io_uring_create(entries, &p, params);
diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
index 980d82eb196e..8a32230aa6f4 100644
--- a/include/uapi/linux/io_uring.h
+++ b/include/uapi/linux/io_uring.h
@@ -102,6 +102,7 @@ enum {
 #define IORING_SETUP_ATTACH_WQ	(1U << 5)	/* attach to existing wq */
 #define IORING_SETUP_R_DISABLED	(1U << 6)	/* start with ring disabled */
 #define IORING_SETUP_SUBMIT_ALL	(1U << 7)	/* continue submit on error */
+#define IORING_SETUP_NO_RESCHED	(1U << 8)	/* work doesn't need resched */
 
 enum {
 	IORING_OP_NOP,
-- 
2.35.1