io_uring: pre-calculate syscall iopolling decision

Syscall should only iopoll for events when it's a IOPOLL ring and is not
SQPOLL. Instead of check both flags every time we can save it in ring
flags so it's easier to use. We don't care much about an extra if there,
however it will be inconvenient to copy-paste this chunk with checks in
future patches.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/7fd2f8fc2606305aa06dd8c0ff8f76a66b39c383.1647957378.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -356,6 +356,7 @@ struct io_ring_ctx {
 		unsigned int		drain_active: 1;
 		unsigned int		drain_disabled: 1;
 		unsigned int		has_evfd: 1;
+		unsigned int		syscall_iopoll: 1;
 	} ____cacheline_aligned_in_smp;
 
 	/* submission data */
@@ -10829,14 +10830,7 @@ SYSCALL_DEFINE6(io_uring_enter, unsigned int, fd, u32, to_submit,
 	if (flags & IORING_ENTER_GETEVENTS) {
 		min_complete = min(min_complete, ctx->cq_entries);
 
-		/*
-		 * When SETUP_IOPOLL and SETUP_SQPOLL are both enabled, user
-		 * space applications don't need to do io completion events
-		 * polling again, they can rely on io_sq_thread to do polling
-		 * work, which can reduce cpu usage and uring_lock contention.
-		 */
-		if (ctx->flags & IORING_SETUP_IOPOLL &&
-		    !(ctx->flags & IORING_SETUP_SQPOLL)) {
+		if (ctx->syscall_iopoll) {
 			ret = io_validate_ext_arg(flags, argp, argsz);
 			if (unlikely(ret))
 				goto out;
@@ -11174,6 +11168,17 @@ static __cold int io_uring_create(unsigned entries, struct io_uring_params *p,
 	ctx = io_ring_ctx_alloc(p);
 	if (!ctx)
 		return -ENOMEM;
+
+	/*
+	 * When SETUP_IOPOLL and SETUP_SQPOLL are both enabled, user
+	 * space applications don't need to do io completion events
+	 * polling again, they can rely on io_sq_thread to do polling
+	 * work, which can reduce cpu usage and uring_lock contention.
+	 */
+	if (ctx->flags & IORING_SETUP_IOPOLL &&
+	    !(ctx->flags & IORING_SETUP_SQPOLL))
+		ctx->syscall_iopoll = 1;
+
 	ctx->compat = in_compat_syscall();
 	if (!capable(CAP_IPC_LOCK))
 		ctx->user = get_uid(current_user());