[PATCH for-next v2 4/6] io_uring: add IORING_SETUP_DEFER_TASKRUN

[Dylan Yudaken <dylany@fb.com>]

Allow deferring async tasks until the user calls io_uring_enter(2) with
the IORING_ENTER_GETEVENTS flag. Enable this mode with a flag at
io_uring_setup time. This functionality requires that the later
io_uring_enter will be called from the same submission task, and therefore
restrict this flag to work only when IORING_SETUP_SINGLE_ISSUER is also
set.

Being able to hand pick when tasks are run prevents the problem where
there is current work to be done, however task work runs anyway.

For example, a common workload would obtain a batch of CQEs, and process
each one. Interrupting this to additional taskwork would add latency but
not gain anything. If instead task work is deferred to just before more
CQEs are obtained then no additional latency is added.

The way this is implemented is by trying to keep task work local to a
io_ring_ctx, rather than to the submission task. This is required, as the
application will want to wake up only a single io_ring_ctx at a time to
process work, and so the lists of work have to be kept separate.

This has some other benefits like not having to check the task continually
in handle_tw_list (and potentially unlocking/locking those), and reducing
locks in the submit & process completions path.

There are networking cases where using this option can reduce request
latency by 50%. For example a contrived example using [1] where the client
sends 2k data and receives the same data back while doing some system
calls (to trigger task work) shows this reduction. The reason ends up
being that if sending responses is delayed by processing task work, then
the client side sits idle. Whereas reordering the sends first means that
the client runs it's workload in parallel with the local task work.

[1]:
Using https://github.com/DylanZA/netbench/tree/defer_run
Client:
./netbench  --client_only 1 --control_port 10000 --host <host> --tx "epoll --threads 16 --per_thread 1 --size 2048 --resp 2048 --workload 1000"
Server:
./netbench  --server_only 1 --control_port 10000  --rx "io_uring --defer_taskrun 0 --workload 100"   --rx "io_uring  --defer_taskrun 1 --workload 100"

Signed-off-by: Dylan Yudaken <dylany@fb.com>
---
 include/linux/io_uring_types.h |   2 +
 include/uapi/linux/io_uring.h  |   7 ++
 io_uring/cancel.c              |   2 +-
 io_uring/io_uring.c            | 119 +++++++++++++++++++++++++++++----
 io_uring/io_uring.h            |  30 ++++++++-
 io_uring/rsrc.c                |   2 +-
 6 files changed, 146 insertions(+), 16 deletions(-)

diff --git a/include/linux/io_uring_types.h b/include/linux/io_uring_types.h
index 677a25d44d7f..d56ff2185168 100644
--- a/include/linux/io_uring_types.h
+++ b/include/linux/io_uring_types.h
@@ -301,6 +301,8 @@ struct io_ring_ctx {
 		struct io_hash_table	cancel_table;
 		bool			poll_multi_queue;
 
+		struct llist_head	work_llist;
+
 		struct list_head	io_buffers_comp;
 	} ____cacheline_aligned_in_smp;
 
diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
index 1463cfecb56b..be8d1801bf4a 100644
--- a/include/uapi/linux/io_uring.h
+++ b/include/uapi/linux/io_uring.h
@@ -153,6 +153,13 @@ enum {
  */
 #define IORING_SETUP_SINGLE_ISSUER	(1U << 12)
 
+/*
+ * Defer running task work to get events.
+ * Rather than running bits of task work whenever the task transitions
+ * try to do it just before it is needed.
+ */
+#define IORING_SETUP_DEFER_TASKRUN	(1U << 13)
+
 enum io_uring_op {
 	IORING_OP_NOP,
 	IORING_OP_READV,
diff --git a/io_uring/cancel.c b/io_uring/cancel.c
index e4e1dc0325f0..db6180b62e41 100644
--- a/io_uring/cancel.c
+++ b/io_uring/cancel.c
@@ -292,7 +292,7 @@ int io_sync_cancel(struct io_ring_ctx *ctx, void __user *arg)
 			break;
 
 		mutex_unlock(&ctx->uring_lock);
-		ret = io_run_task_work_sig();
+		ret = io_run_task_work_sig(ctx);
 		if (ret < 0) {
 			mutex_lock(&ctx->uring_lock);
 			break;
diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index 53696dd90626..6b16da0be712 100644
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -316,6 +316,7 @@ static __cold struct io_ring_ctx *io_ring_ctx_alloc(struct io_uring_params *p)
 	INIT_LIST_HEAD(&ctx->rsrc_ref_list);
 	INIT_DELAYED_WORK(&ctx->rsrc_put_work, io_rsrc_put_work);
 	init_llist_head(&ctx->rsrc_put_llist);
+	init_llist_head(&ctx->work_llist);
 	INIT_LIST_HEAD(&ctx->tctx_list);
 	ctx->submit_state.free_list.next = NULL;
 	INIT_WQ_LIST(&ctx->locked_free_list);
@@ -1047,12 +1048,30 @@ void tctx_task_work(struct callback_head *cb)
 	trace_io_uring_task_work_run(tctx, count, loops);
 }
 
-void io_req_task_work_add(struct io_kiocb *req)
+static void io_req_local_work_add(struct io_kiocb *req)
+{
+	struct io_ring_ctx *ctx = req->ctx;
+
+	if (!llist_add(&req->io_task_work.node, &ctx->work_llist))
+		return;
+
+	if (ctx->flags & IORING_SETUP_TASKRUN_FLAG)
+		atomic_or(IORING_SQ_TASKRUN, &ctx->rings->sq_flags);
+
+	io_cqring_wake(ctx);
+}
+
+static inline void __io_req_task_work_add(struct io_kiocb *req, bool allow_local)
 {
 	struct io_uring_task *tctx = req->task->io_uring;
 	struct io_ring_ctx *ctx = req->ctx;
 	struct llist_node *node;
 
+	if (allow_local && ctx->flags & IORING_SETUP_DEFER_TASKRUN) {
+		io_req_local_work_add(req);
+		return;
+	}
+
 	/* task_work already pending, we're done */
 	if (!llist_add(&req->io_task_work.node, &tctx->task_list))
 		return;
@@ -1074,6 +1093,55 @@ void io_req_task_work_add(struct io_kiocb *req)
 	}
 }
 
+void io_req_task_work_add(struct io_kiocb *req)
+{
+	__io_req_task_work_add(req, true);
+}
+
+bool io_run_local_work(struct io_ring_ctx *ctx, bool locked)
+{
+	struct llist_node *node;
+	struct llist_node fake;
+	struct llist_node *current_final = NULL;
+	unsigned int count;
+
+	if (!locked)
+		locked = mutex_trylock(&ctx->uring_lock);
+
+	node = io_llist_xchg(&ctx->work_llist, &fake);
+	count = 0;
+again:
+	while (node != current_final) {
+		struct llist_node *next = node->next;
+		struct io_kiocb *req = container_of(node, struct io_kiocb,
+						    io_task_work.node);
+		prefetch(container_of(next, struct io_kiocb, io_task_work.node));
+		if (unlikely(req->task != current)) {
+			__io_req_task_work_add(req, false);
+		} else {
+			req->io_task_work.func(req, &locked);
+			count++;
+		}
+		node = next;
+	}
+
+	if (ctx->flags & IORING_SETUP_TASKRUN_FLAG)
+		atomic_andnot(IORING_SQ_TASKRUN, &ctx->rings->sq_flags);
+
+	node = io_llist_cmpxchg(&ctx->work_llist, &fake, NULL);
+	if (node != &fake) {
+		current_final = &fake;
+		node = io_llist_xchg(&ctx->work_llist, &fake);
+		goto again;
+	}
+
+	if (locked) {
+		io_submit_flush_completions(ctx);
+		mutex_unlock(&ctx->uring_lock);
+	}
+	return count > 0;
+}
+
 static void io_req_tw_post(struct io_kiocb *req, bool *locked)
 {
 	io_req_complete_post(req);
@@ -1284,8 +1352,7 @@ static int io_iopoll_check(struct io_ring_ctx *ctx, long min)
 		if (wq_list_empty(&ctx->iopoll_list)) {
 			u32 tail = ctx->cached_cq_tail;
 
-			mutex_unlock(&ctx->uring_lock);
-			io_run_task_work();
+			io_run_task_work_unlock_ctx(ctx);
 			mutex_lock(&ctx->uring_lock);
 
 			/* some requests don't go through iopoll_list */
@@ -2146,7 +2213,9 @@ struct io_wait_queue {
 
 static inline bool io_has_work(struct io_ring_ctx *ctx)
 {
-	return test_bit(IO_CHECK_CQ_OVERFLOW_BIT, &ctx->check_cq);
+	return test_bit(IO_CHECK_CQ_OVERFLOW_BIT, &ctx->check_cq) ||
+	       ((ctx->flags & IORING_SETUP_DEFER_TASKRUN) &&
+		!llist_empty(&ctx->work_llist));
 }
 
 static inline bool io_should_wake(struct io_wait_queue *iowq)
@@ -2178,9 +2247,9 @@ static int io_wake_function(struct wait_queue_entry *curr, unsigned int mode,
 	return -1;
 }
 
-int io_run_task_work_sig(void)
+int io_run_task_work_sig(struct io_ring_ctx *ctx)
 {
-	if (io_run_task_work())
+	if (io_run_task_work_ctx(ctx, true))
 		return 1;
 	if (task_sigpending(current))
 		return -EINTR;
@@ -2196,7 +2265,7 @@ static inline int io_cqring_wait_schedule(struct io_ring_ctx *ctx,
 	unsigned long check_cq;
 
 	/* make sure we run task_work before checking for signals */
-	ret = io_run_task_work_sig();
+	ret = io_run_task_work_sig(ctx);
 	if (ret || io_should_wake(iowq))
 		return ret;
 
@@ -2230,7 +2299,7 @@ static int io_cqring_wait(struct io_ring_ctx *ctx, int min_events,
 		io_cqring_overflow_flush(ctx);
 		if (io_cqring_events(ctx) >= min_events)
 			return 0;
-		if (!io_run_task_work())
+		if (!io_run_task_work_ctx(ctx, false))
 			break;
 	} while (1);
 
@@ -2768,6 +2837,8 @@ static __cold bool io_uring_try_cancel_requests(struct io_ring_ctx *ctx,
 		}
 	}
 
+	if (ctx->flags & IORING_SETUP_DEFER_TASKRUN)
+		ret |= io_run_local_work(ctx, false);
 	ret |= io_cancel_defer_files(ctx, task, cancel_all);
 	mutex_lock(&ctx->uring_lock);
 	ret |= io_poll_remove_all(ctx, task, cancel_all);
@@ -2837,7 +2908,7 @@ __cold void io_uring_cancel_generic(bool cancel_all, struct io_sq_data *sqd)
 		}
 
 		prepare_to_wait(&tctx->wait, &wait, TASK_INTERRUPTIBLE);
-		io_run_task_work();
+		io_run_task_work_ctx(ctx, true);
 		io_uring_drop_tctx_refs(current);
 
 		/*
@@ -3057,10 +3128,17 @@ SYSCALL_DEFINE6(io_uring_enter, unsigned int, fd, u32, to_submit,
 		}
 		if ((flags & IORING_ENTER_GETEVENTS) && ctx->syscall_iopoll)
 			goto iopoll_locked;
+		if ((flags & IORING_ENTER_GETEVENTS) &&
+			(ctx->flags & IORING_SETUP_DEFER_TASKRUN)) {
+			io_run_local_work(ctx, true);
+			goto getevents_ran_local;
+		}
 		mutex_unlock(&ctx->uring_lock);
 	}
+
 	if (flags & IORING_ENTER_GETEVENTS) {
 		int ret2;
+
 		if (ctx->syscall_iopoll) {
 			/*
 			 * We disallow the app entering submit/complete with
@@ -3081,6 +3159,9 @@ SYSCALL_DEFINE6(io_uring_enter, unsigned int, fd, u32, to_submit,
 			const sigset_t __user *sig;
 			struct __kernel_timespec __user *ts;
 
+			if (ctx->flags & IORING_SETUP_DEFER_TASKRUN)
+				io_run_local_work(ctx, false);
+getevents_ran_local:
 			ret2 = io_get_ext_arg(flags, argp, &argsz, &ts, &sig);
 			if (likely(!ret2)) {
 				min_complete = min(min_complete,
@@ -3289,17 +3370,29 @@ static __cold int io_uring_create(unsigned entries, struct io_uring_params *p,
 	if (ctx->flags & IORING_SETUP_SQPOLL) {
 		/* IPI related flags don't make sense with SQPOLL */
 		if (ctx->flags & (IORING_SETUP_COOP_TASKRUN |
-				  IORING_SETUP_TASKRUN_FLAG))
+				  IORING_SETUP_TASKRUN_FLAG |
+				  IORING_SETUP_DEFER_TASKRUN))
 			goto err;
 		ctx->notify_method = TWA_SIGNAL_NO_IPI;
 	} else if (ctx->flags & IORING_SETUP_COOP_TASKRUN) {
 		ctx->notify_method = TWA_SIGNAL_NO_IPI;
 	} else {
-		if (ctx->flags & IORING_SETUP_TASKRUN_FLAG)
+		if (ctx->flags & IORING_SETUP_TASKRUN_FLAG &&
+		    !(ctx->flags & IORING_SETUP_DEFER_TASKRUN))
 			goto err;
 		ctx->notify_method = TWA_SIGNAL;
 	}
 
+	/*
+	 * For DEFER_TASKRUN we require the completion task to be the same as the
+	 * submission task. This implies that there is only one submitter, so enforce
+	 * that.
+	 */
+	if (ctx->flags & IORING_SETUP_DEFER_TASKRUN &&
+	    !(ctx->flags & IORING_SETUP_SINGLE_ISSUER)) {
+		goto err;
+	}
+
 	/*
 	 * This is just grabbed for accounting purposes. When a process exits,
 	 * the mm is exited and dropped before the files, hence we need to hang
@@ -3400,7 +3493,7 @@ static long io_uring_setup(u32 entries, struct io_uring_params __user *params)
 			IORING_SETUP_R_DISABLED | IORING_SETUP_SUBMIT_ALL |
 			IORING_SETUP_COOP_TASKRUN | IORING_SETUP_TASKRUN_FLAG |
 			IORING_SETUP_SQE128 | IORING_SETUP_CQE32 |
-			IORING_SETUP_SINGLE_ISSUER))
+			IORING_SETUP_SINGLE_ISSUER | IORING_SETUP_DEFER_TASKRUN))
 		return -EINVAL;
 
 	return io_uring_create(entries, &p, params);
@@ -3872,7 +3965,7 @@ SYSCALL_DEFINE4(io_uring_register, unsigned int, fd, unsigned int, opcode,
 
 	ctx = f.file->private_data;
 
-	io_run_task_work();
+	io_run_task_work_ctx(ctx, true);
 
 	mutex_lock(&ctx->uring_lock);
 	ret = __io_uring_register(ctx, opcode, arg, nr_args);
diff --git a/io_uring/io_uring.h b/io_uring/io_uring.h
index 2f73f83af960..4d0e9c2e00d0 100644
--- a/io_uring/io_uring.h
+++ b/io_uring/io_uring.h
@@ -26,7 +26,8 @@ enum {
 
 struct io_uring_cqe *__io_get_cqe(struct io_ring_ctx *ctx);
 bool io_req_cqe_overflow(struct io_kiocb *req);
-int io_run_task_work_sig(void);
+int io_run_task_work_sig(struct io_ring_ctx *ctx);
+bool io_run_local_work(struct io_ring_ctx *ctx, bool locked);
 void io_req_complete_failed(struct io_kiocb *req, s32 res);
 void __io_req_complete(struct io_kiocb *req, unsigned issue_flags);
 void io_req_complete_post(struct io_kiocb *req);
@@ -234,6 +235,33 @@ static inline bool io_run_task_work(void)
 	return false;
 }
 
+static inline bool io_run_task_work_ctx(struct io_ring_ctx *ctx, bool all)
+{
+	bool ret = false;
+
+	if (ctx->flags & IORING_SETUP_DEFER_TASKRUN) {
+		ret = io_run_local_work(ctx, false);
+		if (!all)
+			return ret;
+	}
+	ret  |= io_run_task_work();
+	return ret;
+}
+
+static inline bool io_run_task_work_unlock_ctx(struct io_ring_ctx *ctx)
+{
+	bool ret;
+
+	if (ctx->flags & IORING_SETUP_DEFER_TASKRUN) {
+		ret = io_run_local_work(ctx, true);
+	} else {
+		mutex_unlock(&ctx->uring_lock);
+		ret = io_run_task_work();
+	}
+
+	return ret;
+}
+
 static inline void io_tw_lock(struct io_ring_ctx *ctx, bool *locked)
 {
 	if (!*locked) {
diff --git a/io_uring/rsrc.c b/io_uring/rsrc.c
index 71359a4d0bd4..80cda6e2067f 100644
--- a/io_uring/rsrc.c
+++ b/io_uring/rsrc.c
@@ -343,7 +343,7 @@ __cold static int io_rsrc_ref_quiesce(struct io_rsrc_data *data,
 		flush_delayed_work(&ctx->rsrc_put_work);
 		reinit_completion(&data->done);
 
-		ret = io_run_task_work_sig();
+		ret = io_run_task_work_sig(ctx);
 		mutex_lock(&ctx->uring_lock);
 	} while (ret >= 0);
 	data->quiesce = false;
-- 
2.30.2