[PATCH for-next 0/4] singly linked list for chains

[PATCH for-next 0/4] singly linked list for chains

[Pavel Begunkov]

see [3/4] and [4/4] for motivation, the other two are just preps.
I wasn't expecting to find performance difference, but my naive nop
test yeilds 5030 vs 5160 KIOPS, before and after [3/4] correspondingly.

The test is submitting 32 linked nops and waits for them to complete.
The testing is tuned for consistentcy, and the results are consistent
across reboots.

Pavel Begunkov (4):
  io_uring: inline io_fail_links()
  io_uring: simplify linked timeout cancel
  io_uring: link requests with singly linked list
  io_uring: toss io_kiocb fields for better caching
 
 fs/io_uring.c | 202 +++++++++++++++++++++-----------------------------
 1 file changed, 83 insertions(+), 119 deletions(-)

-- 
2.24.0

[PATCH 1/4] io_uring: inline io_fail_links()

[Pavel Begunkov]

Inline io_fail_links() and kill extra io_cqring_ev_posted().

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---
 fs/io_uring.c | 13 +++----------
 1 file changed, 3 insertions(+), 10 deletions(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 2c83c2688ec5..373b67a252df 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -1795,10 +1795,12 @@ static struct io_kiocb *io_req_link_next(struct io_kiocb *req)
 /*
  * Called if REQ_F_LINK_HEAD is set, and we fail the head request
  */
-static void __io_fail_links(struct io_kiocb *req)
+static void io_fail_links(struct io_kiocb *req)
 {
 	struct io_ring_ctx *ctx = req->ctx;
+	unsigned long flags;
 
+	spin_lock_irqsave(&ctx->completion_lock, flags);
 	while (!list_empty(&req->link_list)) {
 		struct io_kiocb *link = list_first_entry(&req->link_list,
 						struct io_kiocb, link_list);
@@ -1820,15 +1822,6 @@ static void __io_fail_links(struct io_kiocb *req)
 	}
 
 	io_commit_cqring(ctx);
-}
-
-static void io_fail_links(struct io_kiocb *req)
-{
-	struct io_ring_ctx *ctx = req->ctx;
-	unsigned long flags;
-
-	spin_lock_irqsave(&ctx->completion_lock, flags);
-	__io_fail_links(req);
 	spin_unlock_irqrestore(&ctx->completion_lock, flags);
 
 	io_cqring_ev_posted(ctx);
-- 
2.24.0

[PATCH 2/4] io_uring: simplify linked timeout cancel

[Pavel Begunkov]

As was done with general timeouts, don't remove linked timeouts from
link_list in io_link_cancel_timeout() unless they were successefully
cancelled with hrtimer_try_to_cancel(). With that it's not needed to
check if there was a race and the timeout got removed before getting
into io_link_timeout_fn().

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---
 fs/io_uring.c | 32 +++++++++++---------------------
 1 file changed, 11 insertions(+), 21 deletions(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 373b67a252df..8065df90ce98 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -1732,6 +1732,7 @@ static bool io_link_cancel_timeout(struct io_kiocb *req)
 
 	ret = hrtimer_try_to_cancel(&io->timer);
 	if (ret != -1) {
+		list_del_init(&req->link_list);
 		io_cqring_fill_event(req, -ECANCELED);
 		io_commit_cqring(ctx);
 		req->flags &= ~REQ_F_LINK_HEAD;
@@ -1747,13 +1748,11 @@ static bool __io_kill_linked_timeout(struct io_kiocb *req)
 	struct io_kiocb *link;
 	bool wake_ev;
 
-	if (list_empty(&req->link_list))
-		return false;
-	link = list_first_entry(&req->link_list, struct io_kiocb, link_list);
-	if (link->opcode != IORING_OP_LINK_TIMEOUT)
+	link = list_first_entry_or_null(&req->link_list, struct io_kiocb,
+					link_list);
+	if (!link || link->opcode != IORING_OP_LINK_TIMEOUT)
 		return false;
 
-	list_del_init(&link->link_list);
 	wake_ev = io_link_cancel_timeout(link);
 	req->flags &= ~REQ_F_LINK_TIMEOUT;
 	return wake_ev;
@@ -5970,27 +5969,18 @@ static enum hrtimer_restart io_link_timeout_fn(struct hrtimer *timer)
 {
 	struct io_timeout_data *data = container_of(timer,
 						struct io_timeout_data, timer);
-	struct io_kiocb *req = data->req;
+	struct io_kiocb *prev, *req = data->req;
 	struct io_ring_ctx *ctx = req->ctx;
-	struct io_kiocb *prev = NULL;
 	unsigned long flags;
 
 	spin_lock_irqsave(&ctx->completion_lock, flags);
+	prev = list_entry(req->link_list.prev, struct io_kiocb, link_list);
 
-	/*
-	 * We don't expect the list to be empty, that will only happen if we
-	 * race with the completion of the linked work.
-	 */
-	if (!list_empty(&req->link_list)) {
-		prev = list_entry(req->link_list.prev, struct io_kiocb,
-				  link_list);
-		if (refcount_inc_not_zero(&prev->refs)) {
-			list_del_init(&req->link_list);
-			prev->flags &= ~REQ_F_LINK_TIMEOUT;
-		} else
-			prev = NULL;
-	}
-
+	if (refcount_inc_not_zero(&prev->refs)) {
+		list_del_init(&req->link_list);
+		prev->flags &= ~REQ_F_LINK_TIMEOUT;
+	} else
+		prev = NULL;
 	spin_unlock_irqrestore(&ctx->completion_lock, flags);
 
 	if (prev) {
-- 
2.24.0

[PATCH 3/4] io_uring: link requests with singly linked list

[Pavel Begunkov]

We don't need a doubly linked list to keep a list of linked requests,
it's not traversed backwards (almost) and removes doesn't happen
arbitrary. The exception is linked_timeout, that needs to keep track
of a previous request, which is also the head of the link. Linked
timeout are anyway a one big corner case, it's not difficult to put a
back reference into timeout struct.

There are 4 main steps here:
- replace link_list with a singly linked io_kiocb *link.
- kills REQ_F_LINK_HEAD and just checks ->link for NULL.
- adds a back reference to linked timeouts.
- adjusts io_submit_sqe() because it needs to know the last request in
a link to append new ones.

The benefits:
- fight back 8 bytes per request
- these 8B are in the 2nd cacheline, moving fixed_file_refs there will
  finally make the 3rd cacheline cold.
- it doesn't touch the last request every time we advance a link, and so
  makes better use of cache
- it looks better and for_each is shorter

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---
 fs/io_uring.c | 164 ++++++++++++++++++++++----------------------------
 1 file changed, 73 insertions(+), 91 deletions(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 8065df90ce98..fd5360a238a7 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -437,6 +437,8 @@ struct io_timeout {
 	u32				off;
 	u32				target_seq;
 	struct list_head		list;
+	/* head of the link, used by linked timeouts only */
+	struct io_kiocb			*head;
 };
 
 struct io_timeout_rem {
@@ -567,7 +569,6 @@ enum {
 	REQ_F_FORCE_ASYNC_BIT	= IOSQE_ASYNC_BIT,
 	REQ_F_BUFFER_SELECT_BIT	= IOSQE_BUFFER_SELECT_BIT,
 
-	REQ_F_LINK_HEAD_BIT,
 	REQ_F_FAIL_LINK_BIT,
 	REQ_F_INFLIGHT_BIT,
 	REQ_F_CUR_POS_BIT,
@@ -598,8 +599,6 @@ enum {
 	/* IOSQE_BUFFER_SELECT */
 	REQ_F_BUFFER_SELECT	= BIT(REQ_F_BUFFER_SELECT_BIT),
 
-	/* head of a link */
-	REQ_F_LINK_HEAD		= BIT(REQ_F_LINK_HEAD_BIT),
 	/* fail rest of links */
 	REQ_F_FAIL_LINK		= BIT(REQ_F_FAIL_LINK_BIT),
 	/* on inflight list */
@@ -675,7 +674,7 @@ struct io_kiocb {
 	struct task_struct		*task;
 	u64				user_data;
 
-	struct list_head		link_list;
+	struct io_kiocb			*link;
 
 	/*
 	 * 1. used with ctx->iopoll_list with reads/writes
@@ -998,6 +997,9 @@ struct sock *io_uring_get_socket(struct file *file)
 }
 EXPORT_SYMBOL(io_uring_get_socket);
 
+#define io_for_each_link(pos, head) \
+	for (pos = (head); pos; pos = pos->link)
+
 static inline void io_clean_op(struct io_kiocb *req)
 {
 	if (req->flags & (REQ_F_NEED_CLEANUP | REQ_F_BUFFER_SELECTED |
@@ -1274,10 +1276,8 @@ static void io_prep_async_link(struct io_kiocb *req)
 {
 	struct io_kiocb *cur;
 
-	io_prep_async_work(req);
-	if (req->flags & REQ_F_LINK_HEAD)
-		list_for_each_entry(cur, &req->link_list, link_list)
-			io_prep_async_work(cur);
+	io_for_each_link(cur, req)
+		io_prep_async_work(cur);
 }
 
 static struct io_kiocb *__io_queue_async_work(struct io_kiocb *req)
@@ -1724,6 +1724,14 @@ static void __io_free_req(struct io_kiocb *req)
 	percpu_ref_put(&ctx->refs);
 }
 
+static inline void io_remove_next_linked(struct io_kiocb *req)
+{
+	struct io_kiocb *nxt = req->link;
+
+	req->link = nxt->link;
+	nxt->link = NULL;
+}
+
 static bool io_link_cancel_timeout(struct io_kiocb *req)
 {
 	struct io_timeout_data *io = req->async_data;
@@ -1732,10 +1740,10 @@ static bool io_link_cancel_timeout(struct io_kiocb *req)
 
 	ret = hrtimer_try_to_cancel(&io->timer);
 	if (ret != -1) {
-		list_del_init(&req->link_list);
+		io_remove_next_linked(req->timeout.head);
+		req->timeout.head = NULL;
 		io_cqring_fill_event(req, -ECANCELED);
 		io_commit_cqring(ctx);
-		req->flags &= ~REQ_F_LINK_HEAD;
 		io_put_req_deferred(req, 1);
 		return true;
 	}
@@ -1745,11 +1753,9 @@ static bool io_link_cancel_timeout(struct io_kiocb *req)
 
 static bool __io_kill_linked_timeout(struct io_kiocb *req)
 {
-	struct io_kiocb *link;
+	struct io_kiocb *link = req->link;
 	bool wake_ev;
 
-	link = list_first_entry_or_null(&req->link_list, struct io_kiocb,
-					link_list);
 	if (!link || link->opcode != IORING_OP_LINK_TIMEOUT)
 		return false;
 
@@ -1772,41 +1778,21 @@ static void io_kill_linked_timeout(struct io_kiocb *req)
 		io_cqring_ev_posted(ctx);
 }
 
-static struct io_kiocb *io_req_link_next(struct io_kiocb *req)
-{
-	struct io_kiocb *nxt;
-
-	/*
-	 * The list should never be empty when we are called here. But could
-	 * potentially happen if the chain is messed up, check to be on the
-	 * safe side.
-	 */
-	if (unlikely(list_empty(&req->link_list)))
-		return NULL;
-
-	nxt = list_first_entry(&req->link_list, struct io_kiocb, link_list);
-	list_del_init(&req->link_list);
-	if (!list_empty(&nxt->link_list))
-		nxt->flags |= REQ_F_LINK_HEAD;
-	return nxt;
-}
-
-/*
- * Called if REQ_F_LINK_HEAD is set, and we fail the head request
- */
 static void io_fail_links(struct io_kiocb *req)
 {
+	struct io_kiocb *link, *nxt;
 	struct io_ring_ctx *ctx = req->ctx;
 	unsigned long flags;
 
 	spin_lock_irqsave(&ctx->completion_lock, flags);
-	while (!list_empty(&req->link_list)) {
-		struct io_kiocb *link = list_first_entry(&req->link_list,
-						struct io_kiocb, link_list);
+	link = req->link;
+	req->link = NULL;
 
-		list_del_init(&link->link_list);
-		trace_io_uring_fail_link(req, link);
+	while (link) {
+		nxt = link->link;
+		link->link = NULL;
 
+		trace_io_uring_fail_link(req, link);
 		io_cqring_fill_event(link, -ECANCELED);
 
 		/*
@@ -1818,8 +1804,8 @@ static void io_fail_links(struct io_kiocb *req)
 			io_put_req_deferred(link, 2);
 		else
 			io_double_put_req(link);
+		link = nxt;
 	}
-
 	io_commit_cqring(ctx);
 	spin_unlock_irqrestore(&ctx->completion_lock, flags);
 
@@ -1828,7 +1814,6 @@ static void io_fail_links(struct io_kiocb *req)
 
 static struct io_kiocb *__io_req_find_next(struct io_kiocb *req)
 {
-	req->flags &= ~REQ_F_LINK_HEAD;
 	if (req->flags & REQ_F_LINK_TIMEOUT)
 		io_kill_linked_timeout(req);
 
@@ -1838,15 +1823,19 @@ static struct io_kiocb *__io_req_find_next(struct io_kiocb *req)
 	 * dependencies to the next request. In case of failure, fail the rest
 	 * of the chain.
 	 */
-	if (likely(!(req->flags & REQ_F_FAIL_LINK)))
-		return io_req_link_next(req);
+	if (likely(!(req->flags & REQ_F_FAIL_LINK))) {
+		struct io_kiocb *nxt = req->link;
+
+		req->link = NULL;
+		return nxt;
+	}
 	io_fail_links(req);
 	return NULL;
 }
 
-static struct io_kiocb *io_req_find_next(struct io_kiocb *req)
+static inline struct io_kiocb *io_req_find_next(struct io_kiocb *req)
 {
-	if (likely(!(req->flags & REQ_F_LINK_HEAD)))
+	if (likely(!(req->link)))
 		return NULL;
 	return __io_req_find_next(req);
 }
@@ -1940,7 +1929,7 @@ static void io_req_task_queue(struct io_kiocb *req)
 	}
 }
 
-static void io_queue_next(struct io_kiocb *req)
+static inline void io_queue_next(struct io_kiocb *req)
 {
 	struct io_kiocb *nxt = io_req_find_next(req);
 
@@ -1995,8 +1984,7 @@ static void io_req_free_batch(struct req_batch *rb, struct io_kiocb *req)
 		io_free_req(req);
 		return;
 	}
-	if (req->flags & REQ_F_LINK_HEAD)
-		io_queue_next(req);
+	io_queue_next(req);
 
 	if (req->task != rb->task) {
 		if (rb->task) {
@@ -5633,11 +5621,10 @@ static u32 io_get_sequence(struct io_kiocb *req)
 {
 	struct io_kiocb *pos;
 	struct io_ring_ctx *ctx = req->ctx;
-	u32 total_submitted, nr_reqs = 1;
+	u32 total_submitted, nr_reqs = 0;
 
-	if (req->flags & REQ_F_LINK_HEAD)
-		list_for_each_entry(pos, &req->link_list, link_list)
-			nr_reqs++;
+	io_for_each_link(pos, req)
+		nr_reqs++;
 
 	total_submitted = ctx->cached_sq_head - ctx->cached_sq_dropped;
 	return total_submitted - nr_reqs;
@@ -5974,10 +5961,11 @@ static enum hrtimer_restart io_link_timeout_fn(struct hrtimer *timer)
 	unsigned long flags;
 
 	spin_lock_irqsave(&ctx->completion_lock, flags);
-	prev = list_entry(req->link_list.prev, struct io_kiocb, link_list);
+	prev = req->timeout.head;
 
 	if (refcount_inc_not_zero(&prev->refs)) {
-		list_del_init(&req->link_list);
+		io_remove_next_linked(prev);
+		req->timeout.head = NULL;
 		prev->flags &= ~REQ_F_LINK_TIMEOUT;
 	} else
 		prev = NULL;
@@ -5996,10 +5984,10 @@ static enum hrtimer_restart io_link_timeout_fn(struct hrtimer *timer)
 static void __io_queue_linked_timeout(struct io_kiocb *req)
 {
 	/*
-	 * If the list is now empty, then our linked request finished before
-	 * we got a chance to setup the timer
+	 * If the back reference is NULL, then our linked request finished
+	 * before we got a chance to setup the timer
 	 */
-	if (!list_empty(&req->link_list)) {
+	if (req->timeout.head) {
 		struct io_timeout_data *data = req->async_data;
 
 		data->timer.function = io_link_timeout_fn;
@@ -6022,18 +6010,14 @@ static void io_queue_linked_timeout(struct io_kiocb *req)
 
 static struct io_kiocb *io_prep_linked_timeout(struct io_kiocb *req)
 {
-	struct io_kiocb *nxt;
+	struct io_kiocb *nxt = req->link;
 
-	if (!(req->flags & REQ_F_LINK_HEAD))
+	if (!nxt || nxt->opcode != IORING_OP_LINK_TIMEOUT)
 		return NULL;
 	if (req->flags & REQ_F_LINK_TIMEOUT)
 		return NULL;
 
-	nxt = list_first_entry_or_null(&req->link_list, struct io_kiocb,
-					link_list);
-	if (!nxt || nxt->opcode != IORING_OP_LINK_TIMEOUT)
-		return NULL;
-
+	nxt->timeout.head = req;
 	req->flags |= REQ_F_LINK_TIMEOUT;
 	return nxt;
 }
@@ -6152,8 +6136,13 @@ static inline void io_queue_link_head(struct io_kiocb *req,
 		io_queue_sqe(req, NULL, cs);
 }
 
+struct io_link {
+	struct io_kiocb *head;
+	struct io_kiocb *last;
+};
+
 static int io_submit_sqe(struct io_kiocb *req, const struct io_uring_sqe *sqe,
-			 struct io_kiocb **link, struct io_comp_state *cs)
+			 struct io_link *link, struct io_comp_state *cs)
 {
 	struct io_ring_ctx *ctx = req->ctx;
 	int ret;
@@ -6165,8 +6154,8 @@ static int io_submit_sqe(struct io_kiocb *req, const struct io_uring_sqe *sqe,
 	 * submitted sync once the chain is complete. If none of those
 	 * conditions are true (normal request), then just queue it.
 	 */
-	if (*link) {
-		struct io_kiocb *head = *link;
+	if (link->head) {
+		struct io_kiocb *head = link->head;
 
 		/*
 		 * Taking sequential execution of a link, draining both sides
@@ -6186,12 +6175,13 @@ static int io_submit_sqe(struct io_kiocb *req, const struct io_uring_sqe *sqe,
 			return ret;
 		}
 		trace_io_uring_link(ctx, req, head);
-		list_add_tail(&req->link_list, &head->link_list);
+		link->last->link = req;
+		link->last = req;
 
 		/* last request of a link, enqueue the link */
 		if (!(req->flags & (REQ_F_LINK | REQ_F_HARDLINK))) {
 			io_queue_link_head(head, cs);
-			*link = NULL;
+			link->head = NULL;
 		}
 	} else {
 		if (unlikely(ctx->drain_next)) {
@@ -6199,13 +6189,11 @@ static int io_submit_sqe(struct io_kiocb *req, const struct io_uring_sqe *sqe,
 			ctx->drain_next = 0;
 		}
 		if (req->flags & (REQ_F_LINK | REQ_F_HARDLINK)) {
-			req->flags |= REQ_F_LINK_HEAD;
-			INIT_LIST_HEAD(&req->link_list);
-
 			ret = io_req_defer_prep(req, sqe);
 			if (unlikely(ret))
 				req->flags |= REQ_F_FAIL_LINK;
-			*link = req;
+			link->head = req;
+			link->last = req;
 		} else {
 			io_queue_sqe(req, sqe, cs);
 		}
@@ -6333,6 +6321,7 @@ static int io_init_req(struct io_ring_ctx *ctx, struct io_kiocb *req,
 	req->file = NULL;
 	req->ctx = ctx;
 	req->flags = 0;
+	req->link = NULL;
 	/* one is dropped after submission, the other at completion */
 	refcount_set(&req->refs, 2);
 	req->task = current;
@@ -6378,8 +6367,8 @@ static int io_init_req(struct io_ring_ctx *ctx, struct io_kiocb *req,
 
 static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr)
 {
+	struct io_link link;
 	struct io_submit_state state;
-	struct io_kiocb *link = NULL;
 	int i, submitted = 0;
 
 	/* if we have a backlog and couldn't flush it all, return BUSY */
@@ -6399,6 +6388,7 @@ static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr)
 	refcount_add(nr, &current->usage);
 
 	io_submit_state_start(&state, ctx, nr);
+	link.head = NULL;
 
 	for (i = 0; i < nr; i++) {
 		const struct io_uring_sqe *sqe;
@@ -6442,8 +6432,8 @@ static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr)
 		atomic_long_sub(nr - ref_used, &current->io_uring->req_issue);
 		put_task_struct_many(current, nr - ref_used);
 	}
-	if (link)
-		io_queue_link_head(link, &state.comp);
+	if (link.head)
+		io_queue_link_head(link.head, &state.comp);
 	io_submit_state_end(&state);
 
 	 /* Commit SQ ring head once we've consumed and submitted all SQEs */
@@ -8240,29 +8230,21 @@ static bool io_match_link(struct io_kiocb *preq, struct io_kiocb *req)
 {
 	struct io_kiocb *link;
 
-	if (!(preq->flags & REQ_F_LINK_HEAD))
-		return false;
-
-	list_for_each_entry(link, &preq->link_list, link_list) {
+	io_for_each_link(link, preq->link) {
 		if (link == req)
 			return true;
 	}
-
 	return false;
 }
 
-static bool io_match_link_files(struct io_kiocb *req,
+static bool io_match_link_files(struct io_kiocb *head,
 				struct files_struct *files)
 {
-	struct io_kiocb *link;
+	struct io_kiocb *req;
 
-	if (io_match_files(req, files))
-		return true;
-	if (req->flags & REQ_F_LINK_HEAD) {
-		list_for_each_entry(link, &req->link_list, link_list) {
-			if (io_match_files(link, files))
-				return true;
-		}
+	io_for_each_link(req, head) {
+		if (io_match_files(req, files))
+			return true;
 	}
 	return false;
 }
-- 
2.24.0

[PATCH 4/4] io_uring: toss io_kiocb fields for better caching

[Pavel Begunkov]

We've got extra 8 bytes in the 2nd cacheline, put ->fixed_file_refs
there, so inline execution path mostly doesn't touch the 3rd cacheline
for fixed_file requests as well.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---
 fs/io_uring.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index fd5360a238a7..f3f4cb538d43 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -675,14 +675,13 @@ struct io_kiocb {
 	u64				user_data;
 
 	struct io_kiocb			*link;
+	struct percpu_ref		*fixed_file_refs;
 
 	/*
 	 * 1. used with ctx->iopoll_list with reads/writes
 	 * 2. to track reqs with ->files (see io_op_def::file_table)
 	 */
 	struct list_head		inflight_entry;
-
-	struct percpu_ref		*fixed_file_refs;
 	struct callback_head		task_work;
 	/* for polled requests, i.e. IORING_OP_POLL_ADD and async armed poll */
 	struct hlist_node		hash_node;
-- 
2.24.0

Re: [PATCH for-next 0/4] singly linked list for chains

[Pavel Begunkov]

On 15/10/2020 22:11, Pavel Begunkov wrote:
> see [3/4] and [4/4] for motivation, the other two are just preps.
> I wasn't expecting to find performance difference, but my naive nop
> test yeilds 5030 vs 5160 KIOPS, before and after [3/4] correspondingly.

Forgot to add that I want to review it thoroughly on silly bugs,
so until then it's more of RFC. 

> The test is submitting 32 linked nops and waits for them to complete.
> The testing is tuned for consistentcy, and the results are consistent
> across reboots.

The test was basically like the diff below.


diff --git a/tools/io_uring/io_uring-bench.c b/tools/io_uring/io_uring-bench.c
index 7703f0118385..84c4487c4d4e 100644
--- a/tools/io_uring/io_uring-bench.c
+++ b/tools/io_uring/io_uring-bench.c
@@ -96,13 +96,13 @@ static volatile int finish;
 /*
  * OPTIONS: Set these to test the various features of io_uring.
  */
-static int polled = 1;		/* use IO polling */
+static int polled = 0;		/* use IO polling */
 static int fixedbufs = 1;	/* use fixed user buffers */
 static int register_files = 1;	/* use fixed files */
 static int buffered = 0;	/* use buffered IO, not O_DIRECT */
 static int sq_thread_poll = 0;	/* use kernel submission/poller thread */
 static int sq_thread_cpu = -1;	/* pin above thread to this CPU */
-static int do_nop = 0;		/* no-op SQ ring commands */
+static int do_nop = 1;		/* no-op SQ ring commands */
 
 static int io_uring_register_buffers(struct submitter *s)
 {
@@ -149,6 +149,7 @@ static void init_io(struct submitter *s, unsigned index)
 
 	if (do_nop) {
 		sqe->opcode = IORING_OP_NOP;
+		sqe->flags = IOSQE_IO_LINK;
 		return;
 	}
 
-- 
Pavel Begunkov