[PATCH 14/16] io_uring: add submission polling

[Jens Axboe <axboe@kernel.dk>]

This enables an application to do IO, without ever entering the kernel.
By using the SQ ring to fill in new sqes and watching for completions
on the CQ ring, we can submit and reap IOs without doing a single system
call. The kernel side thread will poll for new submissions, and in case
of HIPRI/polled IO, it'll also poll for completions.

Proof of concept. If the thread has been idle for 1 second, it will set
sq_ring->flags |= IORING_SQ_NEED_WAKEUP. The application will have to
call io_uring_enter() to start things back up again. If IO is kept busy,
that will never be needed. Basically an application that has this
feature enabled will guard it's io_uring_enter(2) call with:

read_barrier();
if (*sq_ring->flags & IORING_SQ_NEED_WAKEUP)
	io_uring_enter(fd, to_submit, 0, 0);

instead of calling it unconditionally.

Improvements:

1) Maybe have smarter backoff. Busy loop for X time, then go to
   monitor/mwait, finally the schedule we have now after an idle
   second. Might not be worth the complexity.

2) Probably want the application to pass in the appropriate grace
   period, not hard code it at 1 second.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 fs/io_uring.c                 | 215 +++++++++++++++++++++++++++++++++-
 include/uapi/linux/io_uring.h |  10 +-
 2 files changed, 218 insertions(+), 7 deletions(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 00743a5a6fac..6df5da8b5259 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -23,6 +23,7 @@
 #include <linux/percpu.h>
 #include <linux/slab.h>
 #include <linux/workqueue.h>
+#include <linux/kthread.h>
 #include <linux/blkdev.h>
 #include <linux/bvec.h>
 #include <linux/anon_inodes.h>
@@ -91,8 +92,10 @@ struct io_ring_ctx {
 
 	/* IO offload */
 	struct workqueue_struct	*sqo_wq;
+	struct task_struct	*sqo_thread;	/* if using sq thread polling */
 	struct mm_struct	*sqo_mm;
 	struct files_struct	*sqo_files;
+	wait_queue_head_t	sqo_wait;
 
 	/* if used, fixed mapped user buffers */
 	unsigned		nr_user_bufs;
@@ -1112,6 +1115,167 @@ static bool io_peek_sqring(struct io_ring_ctx *ctx, struct sqe_submit *s)
 	return false;
 }
 
+static int io_submit_sqes(struct io_ring_ctx *ctx, struct sqe_submit *sqes,
+			  unsigned int nr, bool mm_fault)
+{
+	struct io_submit_state state, *statep = NULL;
+	int ret, i, submitted = 0;
+
+	if (nr > IO_PLUG_THRESHOLD) {
+		io_submit_state_start(&state, ctx, nr);
+		statep = &state;
+	}
+
+	for (i = 0; i < nr; i++) {
+		if (unlikely(mm_fault))
+			ret = -EFAULT;
+		else
+			ret = io_submit_sqe(ctx, &sqes[i], statep);
+		if (!ret) {
+			submitted++;
+			continue;
+		}
+
+		io_fill_cq_error(ctx, &sqes[i], ret);
+	}
+
+	if (statep)
+		io_submit_state_end(&state);
+
+	return submitted;
+}
+
+static int io_sq_thread(void *data)
+{
+	struct sqe_submit sqes[IO_IOPOLL_BATCH];
+	struct io_ring_ctx *ctx = data;
+	struct mm_struct *cur_mm = NULL;
+	struct files_struct *old_files;
+	mm_segment_t old_fs;
+	DEFINE_WAIT(wait);
+	unsigned inflight;
+	unsigned long timeout;
+
+	old_files = current->files;
+	current->files = ctx->sqo_files;
+
+	old_fs = get_fs();
+	set_fs(USER_DS);
+
+	timeout = inflight = 0;
+	while (!kthread_should_stop()) {
+		bool all_fixed, mm_fault = false;
+		int i;
+
+		if (inflight) {
+			unsigned int nr_events = 0;
+
+			/*
+			 * Normal IO, just pretend everything completed.
+			 * We don't have to poll completions for that.
+			 */
+			if (ctx->flags & IORING_SETUP_IOPOLL) {
+				/*
+				 * App should not use IORING_ENTER_GETEVENTS
+				 * with thread polling, but if it does, then
+				 * ensure we are mutually exclusive.
+				 */
+				if (mutex_trylock(&ctx->uring_lock)) {
+					io_iopoll_check(ctx, &nr_events, 0);
+					mutex_unlock(&ctx->uring_lock);
+				}
+			} else {
+				nr_events = inflight;
+			}
+
+			inflight -= nr_events;
+			if (!inflight)
+				timeout = jiffies + HZ;
+		}
+
+		if (!io_peek_sqring(ctx, &sqes[0])) {
+			/*
+			 * We're polling, let us spin for a second without
+			 * work before going to sleep.
+			 */
+			if (inflight || !time_after(jiffies, timeout)) {
+				cpu_relax();
+				continue;
+			}
+
+			/*
+			 * Drop cur_mm before scheduling, we can't hold it for
+			 * long periods (or over schedule()). Do this before
+			 * adding ourselves to the waitqueue, as the unuse/drop
+			 * may sleep.
+			 */
+			if (cur_mm) {
+				unuse_mm(cur_mm);
+				mmput(cur_mm);
+				cur_mm = NULL;
+			}
+
+			prepare_to_wait(&ctx->sqo_wait, &wait,
+						TASK_INTERRUPTIBLE);
+
+			/* Tell userspace we may need a wakeup call */
+			ctx->sq_ring->flags |= IORING_SQ_NEED_WAKEUP;
+			smp_wmb();
+
+			if (!io_peek_sqring(ctx, &sqes[0])) {
+				if (kthread_should_park())
+					kthread_parkme();
+				if (kthread_should_stop()) {
+					finish_wait(&ctx->sqo_wait, &wait);
+					break;
+				}
+				if (signal_pending(current))
+					flush_signals(current);
+				schedule();
+				finish_wait(&ctx->sqo_wait, &wait);
+
+				ctx->sq_ring->flags &= ~IORING_SQ_NEED_WAKEUP;
+				smp_wmb();
+				continue;
+			}
+			finish_wait(&ctx->sqo_wait, &wait);
+
+			ctx->sq_ring->flags &= ~IORING_SQ_NEED_WAKEUP;
+			smp_wmb();
+		}
+
+		i = 0;
+		all_fixed = true;
+		do {
+			if (sqes[i].sqe->opcode != IORING_OP_READ_FIXED &&
+			    sqes[i].sqe->opcode != IORING_OP_WRITE_FIXED)
+				all_fixed = false;
+			if (i + 1 == ARRAY_SIZE(sqes))
+				break;
+			i++;
+			io_inc_sqring(ctx);
+		} while (io_peek_sqring(ctx, &sqes[i]));
+
+		/* Unless all new commands are FIXED regions, grab mm */
+		if (!all_fixed && !cur_mm) {
+			mm_fault = !mmget_not_zero(ctx->sqo_mm);
+			if (!mm_fault) {
+				use_mm(ctx->sqo_mm);
+				cur_mm = ctx->sqo_mm;
+			}
+		}
+
+		inflight += io_submit_sqes(ctx, sqes, i, mm_fault);
+	}
+	current->files = old_files;
+	set_fs(old_fs);
+	if (cur_mm) {
+		unuse_mm(cur_mm);
+		mmput(cur_mm);
+	}
+	return 0;
+}
+
 static int io_ring_submit(struct io_ring_ctx *ctx, unsigned int to_submit)
 {
 	struct io_submit_state state, *statep = NULL;
@@ -1183,9 +1347,14 @@ static int __io_uring_enter(struct io_ring_ctx *ctx, unsigned to_submit,
 	int ret = 0;
 
 	if (to_submit) {
-		ret = io_ring_submit(ctx, to_submit);
-		if (ret < 0)
-			return ret;
+		if (ctx->flags & IORING_SETUP_SQPOLL) {
+			wake_up(&ctx->sqo_wait);
+			ret = to_submit;
+		} else {
+			ret = io_ring_submit(ctx, to_submit);
+			if (ret < 0)
+				return ret;
+		}
 	}
 	if (flags & IORING_ENTER_GETEVENTS) {
 		unsigned nr_events = 0;
@@ -1206,10 +1375,12 @@ static int __io_uring_enter(struct io_ring_ctx *ctx, unsigned to_submit,
 	return ret;
 }
 
-static int io_sq_offload_start(struct io_ring_ctx *ctx)
+static int io_sq_offload_start(struct io_ring_ctx *ctx,
+			       struct io_uring_params *p)
 {
 	int ret;
 
+	init_waitqueue_head(&ctx->sqo_wait);
 	ctx->sqo_mm = current->mm;
 
 	/*
@@ -1223,6 +1394,27 @@ static int io_sq_offload_start(struct io_ring_ctx *ctx)
 	if (!ctx->sqo_files)
 		goto err;
 
+	if (ctx->flags & IORING_SETUP_SQPOLL) {
+		if (p->flags & IORING_SETUP_SQ_AFF) {
+			ctx->sqo_thread = kthread_create_on_cpu(io_sq_thread,
+							ctx, p->sq_thread_cpu,
+							"io_uring-sq");
+		} else {
+			ctx->sqo_thread = kthread_create(io_sq_thread, ctx,
+							"io_uring-sq");
+		}
+		if (IS_ERR(ctx->sqo_thread)) {
+			ret = PTR_ERR(ctx->sqo_thread);
+			ctx->sqo_thread = NULL;
+			goto err;
+		}
+		wake_up_process(ctx->sqo_thread);
+	} else if (p->flags & IORING_SETUP_SQ_AFF) {
+		/* Can't have SQ_AFF without SQPOLL */
+		ret = -EINVAL;
+		goto err;
+	}
+
 	/* Do QD, or 2 * CPUS, whatever is smallest */
 	ctx->sqo_wq = alloc_workqueue("io_ring-wq", WQ_UNBOUND | WQ_FREEZABLE,
 			min(ctx->sq_entries - 1, 2 * num_online_cpus()));
@@ -1233,6 +1425,11 @@ static int io_sq_offload_start(struct io_ring_ctx *ctx)
 
 	return 0;
 err:
+	if (ctx->sqo_thread) {
+		kthread_park(ctx->sqo_thread);
+		kthread_stop(ctx->sqo_thread);
+		ctx->sqo_thread = NULL;
+	}
 	if (ctx->sqo_files)
 		ctx->sqo_files = NULL;
 	ctx->sqo_mm = NULL;
@@ -1241,6 +1438,11 @@ static int io_sq_offload_start(struct io_ring_ctx *ctx)
 
 static void io_sq_offload_stop(struct io_ring_ctx *ctx)
 {
+	if (ctx->sqo_thread) {
+		kthread_park(ctx->sqo_thread);
+		kthread_stop(ctx->sqo_thread);
+		ctx->sqo_thread = NULL;
+	}
 	if (ctx->sqo_wq) {
 		destroy_workqueue(ctx->sqo_wq);
 		ctx->sqo_wq = NULL;
@@ -1631,7 +1833,7 @@ static int io_uring_create(unsigned entries, struct io_uring_params *p,
 	if (ret)
 		goto err;
 
-	ret = io_sq_offload_start(ctx);
+	ret = io_sq_offload_start(ctx, p);
 	if (ret)
 		goto err;
 
@@ -1666,7 +1868,8 @@ static long io_uring_setup(u32 entries, struct io_uring_params __user *params,
 			return -EINVAL;
 	}
 
-	if (p.flags & ~IORING_SETUP_IOPOLL)
+	if (p.flags & ~(IORING_SETUP_IOPOLL | IORING_SETUP_SQPOLL |
+			IORING_SETUP_SQ_AFF))
 		return -EINVAL;
 
 	ret = io_uring_create(entries, &p, compat);
diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
index fda25d09c8a1..cb075971d8fb 100644
--- a/include/uapi/linux/io_uring.h
+++ b/include/uapi/linux/io_uring.h
@@ -40,6 +40,8 @@ struct io_uring_sqe {
  * io_uring_setup() flags
  */
 #define IORING_SETUP_IOPOLL	(1 << 0)	/* io_context is polled */
+#define IORING_SETUP_SQPOLL	(1 << 1)	/* SQ poll thread */
+#define IORING_SETUP_SQ_AFF	(1 << 2)	/* sq_thread_cpu is valid */
 
 #define IORING_OP_NOP		0
 #define IORING_OP_READV		1
@@ -83,6 +85,11 @@ struct io_sqring_offsets {
 	__u32 resv[3];
 };
 
+/*
+ * sq_ring->flags
+ */
+#define IORING_SQ_NEED_WAKEUP	(1 << 0) /* needs io_uring_enter wakeup */
+
 struct io_cqring_offsets {
 	__u32 head;
 	__u32 tail;
@@ -105,7 +112,8 @@ struct io_uring_params {
 	__u32 sq_entries;
 	__u32 cq_entries;
 	__u32 flags;
-	__u16 resv[10];
+	__u16 sq_thread_cpu;
+	__u16 resv[9];
 	struct io_sqring_offsets sq_off;
 	struct io_cqring_offsets cq_off;
 };
-- 
2.17.1