io_uring: use TWA_SIGNAL_NO_IPI if IORING_SETUP_COOP_TASKRUN is used

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.

Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/20220426014904.60384-6-axboe@kernel.dk
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -11327,12 +11327,20 @@ 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
+	 * COOP_TASKRUN is set, then IPIs are never needed by the app.
 	 */
-	if (ctx->flags & IORING_SETUP_SQPOLL)
+	ret = -EINVAL;
+	if (ctx->flags & IORING_SETUP_SQPOLL) {
+		/* IPI related flags don't make sense with SQPOLL */
+		if (ctx->flags & IORING_SETUP_COOP_TASKRUN)
+			goto err;
 		ctx->notify_method = TWA_SIGNAL_NO_IPI;
-	else
+	} else if (ctx->flags & IORING_SETUP_COOP_TASKRUN) {
+		ctx->notify_method = TWA_SIGNAL_NO_IPI;
+	} else {
 		ctx->notify_method = TWA_SIGNAL;
+	}
 
 	/*
 	 * This is just grabbed for accounting purposes. When a process exits,
@@ -11431,7 +11439,8 @@ 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_COOP_TASKRUN))
 		return -EINVAL;
 
 	return  io_uring_create(entries, &p, params);

include/uapi/linux/io_uring.h

@@ -102,6 +102,14 @@ 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 */
+/*
+ * Cooperative task running. When requests complete, they often require
+ * forcing the submitter to transition to the kernel to complete. If this
+ * flag is set, work will be done when the task transitions anyway, rather
+ * than force an inter-processor interrupt reschedule. This avoids interrupting
+ * a task running in userspace, and saves an IPI.
+ */
+#define IORING_SETUP_COOP_TASKRUN	(1U << 8)
 
 enum {
 	IORING_OP_NOP,