Sending CQE to a different ring

Sending CQE to a different ring

[Artyom Pavlov]

Greetings!

A common approach for multi-threaded servers is to have a number of 
threads equal to a number of cores and launch a separate ring in each 
one. AFAIK currently if we want to send an event to a different ring, we 
have to write-lock this ring, create SQE, and update the index ring. 
Alternatively, we could use some kind of user-space message passing.

Such approaches are somewhat inefficient and I think it can be solved 
elegantly by updating the io_uring_sqe type to allow accepting fd of a 
ring to which CQE must be sent by kernel. It can be done by introducing 
an IOSQE_ flag and using one of currently unused padding u64s.

Such feature could be useful for load balancing and message passing 
between threads which would ride on top of io-uring, i.e. you could send 
NOP with user_data pointing to a message payload.

Best regards,
Artyom Pavlov.

Re: Sending CQE to a different ring

[Jens Axboe]

On 3/9/22 4:49 PM, Artyom Pavlov wrote:
> Greetings!
> 
> A common approach for multi-threaded servers is to have a number of
> threads equal to a number of cores and launch a separate ring in each
> one. AFAIK currently if we want to send an event to a different ring,
> we have to write-lock this ring, create SQE, and update the index
> ring. Alternatively, we could use some kind of user-space message
> passing.
> 
> Such approaches are somewhat inefficient and I think it can be solved
> elegantly by updating the io_uring_sqe type to allow accepting fd of a
> ring to which CQE must be sent by kernel. It can be done by
> introducing an IOSQE_ flag and using one of currently unused padding
> u64s.
> 
> Such feature could be useful for load balancing and message passing
> between threads which would ride on top of io-uring, i.e. you could
> send NOP with user_data pointing to a message payload.

So what you want is a NOP with 'fd' set to the fd of another ring, and
that nop posts a CQE on that other ring? I don't think we'd need IOSQE
flags for that, we just need a NOP that supports that. I see a few ways
of going about that:

1) Add a new 'NOP' that takes an fd, and validates that that fd is an
   io_uring instance. It can then grab the completion lock on that ring
   and post an empty CQE.

2) We add a FEAT flag saying NOP supports taking an 'fd' argument, where
   'fd' is another ring. Posting CQE same as above.

3) We add a specific opcode for this. Basically the same as #2, but
   maybe with a more descriptive name than NOP.

Might make sense to pair that with a CQE flag or something like that, as
there's no specific user_data that could be used as it doesn't match an
existing SQE that has been issued. IORING_CQE_F_WAKEUP for example.
Would be applicable to all the above cases.

I kind of like #3 the best. Add a IORING_OP_RING_WAKEUP command, require
that sqe->fd point to a ring (could even be the ring itself, doesn't
matter). And add IORING_CQE_F_WAKEUP as a specific flag for that.

-- 
Jens Axboe

Re: Sending CQE to a different ring

[Jens Axboe]

On 3/9/22 6:36 PM, Jens Axboe wrote:
> On 3/9/22 4:49 PM, Artyom Pavlov wrote:
>> Greetings!
>>
>> A common approach for multi-threaded servers is to have a number of
>> threads equal to a number of cores and launch a separate ring in each
>> one. AFAIK currently if we want to send an event to a different ring,
>> we have to write-lock this ring, create SQE, and update the index
>> ring. Alternatively, we could use some kind of user-space message
>> passing.
>>
>> Such approaches are somewhat inefficient and I think it can be solved
>> elegantly by updating the io_uring_sqe type to allow accepting fd of a
>> ring to which CQE must be sent by kernel. It can be done by
>> introducing an IOSQE_ flag and using one of currently unused padding
>> u64s.
>>
>> Such feature could be useful for load balancing and message passing
>> between threads which would ride on top of io-uring, i.e. you could
>> send NOP with user_data pointing to a message payload.
> 
> So what you want is a NOP with 'fd' set to the fd of another ring, and
> that nop posts a CQE on that other ring? I don't think we'd need IOSQE
> flags for that, we just need a NOP that supports that. I see a few ways
> of going about that:
> 
> 1) Add a new 'NOP' that takes an fd, and validates that that fd is an
>    io_uring instance. It can then grab the completion lock on that ring
>    and post an empty CQE.
> 
> 2) We add a FEAT flag saying NOP supports taking an 'fd' argument, where
>    'fd' is another ring. Posting CQE same as above.
> 
> 3) We add a specific opcode for this. Basically the same as #2, but
>    maybe with a more descriptive name than NOP.
> 
> Might make sense to pair that with a CQE flag or something like that, as
> there's no specific user_data that could be used as it doesn't match an
> existing SQE that has been issued. IORING_CQE_F_WAKEUP for example.
> Would be applicable to all the above cases.
> 
> I kind of like #3 the best. Add a IORING_OP_RING_WAKEUP command, require
> that sqe->fd point to a ring (could even be the ring itself, doesn't
> matter). And add IORING_CQE_F_WAKEUP as a specific flag for that.

Something like the below, totally untested. The request will complete on
the original ring with either 0, for success, or -EOVERFLOW if the
target ring was already in an overflow state. If the fd specified isn't
an io_uring context, then the request will complete with -EBADFD.

If you have any way of testing this, please do. I'll write a basic
functionality test for it as well, but not until tomorrow.

Maybe we want to include in cqe->res who the waker was? We can stuff the
pid/tid in there, for example.

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 2e04f718319d..b21f85a48224 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -1105,6 +1105,9 @@ static const struct io_op_def io_op_defs[] = {
 	[IORING_OP_MKDIRAT] = {},
 	[IORING_OP_SYMLINKAT] = {},
 	[IORING_OP_LINKAT] = {},
+	[IORING_OP_WAKEUP_RING] = {
+		.needs_file		= 1,
+	},
 };
 
 /* requests with any of those set should undergo io_disarm_next() */
@@ -4235,6 +4238,44 @@ static int io_nop(struct io_kiocb *req, unsigned int issue_flags)
 	return 0;
 }
 
+static int io_wakeup_ring_prep(struct io_kiocb *req,
+			       const struct io_uring_sqe *sqe)
+{
+	if (unlikely(sqe->addr || sqe->ioprio || sqe->buf_index || sqe->off ||
+		     sqe->len || sqe->rw_flags || sqe->splice_fd_in ||
+		     sqe->buf_index || sqe->personality))
+		return -EINVAL;
+
+	if (req->file->f_op != &io_uring_fops)
+		return -EBADFD;
+
+	return 0;
+}
+
+static int io_wakeup_ring(struct io_kiocb *req, unsigned int issue_flags)
+{
+	struct io_uring_cqe *cqe;
+	struct io_ring_ctx *ctx;
+	int ret = 0;
+
+	ctx = req->file->private_data;
+	spin_lock(&ctx->completion_lock);
+	cqe = io_get_cqe(ctx);
+	if (cqe) {
+		WRITE_ONCE(cqe->user_data, 0);
+		WRITE_ONCE(cqe->res, 0);
+		WRITE_ONCE(cqe->flags, IORING_CQE_F_WAKEUP);
+	} else {
+		ret = -EOVERFLOW;
+	}
+	io_commit_cqring(ctx);
+	spin_unlock(&ctx->completion_lock);
+	io_cqring_ev_posted(ctx);
+
+	__io_req_complete(req, issue_flags, ret, 0);
+	return 0;
+}
+
 static int io_fsync_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
 {
 	struct io_ring_ctx *ctx = req->ctx;
@@ -6568,6 +6609,8 @@ static int io_req_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
 		return io_symlinkat_prep(req, sqe);
 	case IORING_OP_LINKAT:
 		return io_linkat_prep(req, sqe);
+	case IORING_OP_WAKEUP_RING:
+		return io_wakeup_ring_prep(req, sqe);
 	}
 
 	printk_once(KERN_WARNING "io_uring: unhandled opcode %d\n",
@@ -6851,6 +6894,9 @@ static int io_issue_sqe(struct io_kiocb *req, unsigned int issue_flags)
 	case IORING_OP_LINKAT:
 		ret = io_linkat(req, issue_flags);
 		break;
+	case IORING_OP_WAKEUP_RING:
+		ret = io_wakeup_ring(req, issue_flags);
+		break;
 	default:
 		ret = -EINVAL;
 		break;
diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
index 787f491f0d2a..088232133594 100644
--- a/include/uapi/linux/io_uring.h
+++ b/include/uapi/linux/io_uring.h
@@ -143,6 +143,7 @@ enum {
 	IORING_OP_MKDIRAT,
 	IORING_OP_SYMLINKAT,
 	IORING_OP_LINKAT,
+	IORING_OP_WAKEUP_RING,
 
 	/* this goes last, obviously */
 	IORING_OP_LAST,
@@ -199,9 +200,11 @@ struct io_uring_cqe {
  *
  * IORING_CQE_F_BUFFER	If set, the upper 16 bits are the buffer ID
  * IORING_CQE_F_MORE	If set, parent SQE will generate more CQE entries
+ * IORING_CQE_F_WAKEUP	Wakeup request CQE, no link to an SQE
  */
 #define IORING_CQE_F_BUFFER		(1U << 0)
 #define IORING_CQE_F_MORE		(1U << 1)
+#define IORING_CQE_F_WAKEUP		(1U << 2)
 
 enum {
 	IORING_CQE_BUFFER_SHIFT		= 16,

-- 
Jens Axboe

Re: Sending CQE to a different ring

[Jens Axboe]

[-- Attachment #1: Type: text/plain, Size: 5923 bytes --]

On 3/9/22 6:55 PM, Jens Axboe wrote:
> On 3/9/22 6:36 PM, Jens Axboe wrote:
>> On 3/9/22 4:49 PM, Artyom Pavlov wrote:
>>> Greetings!
>>>
>>> A common approach for multi-threaded servers is to have a number of
>>> threads equal to a number of cores and launch a separate ring in each
>>> one. AFAIK currently if we want to send an event to a different ring,
>>> we have to write-lock this ring, create SQE, and update the index
>>> ring. Alternatively, we could use some kind of user-space message
>>> passing.
>>>
>>> Such approaches are somewhat inefficient and I think it can be solved
>>> elegantly by updating the io_uring_sqe type to allow accepting fd of a
>>> ring to which CQE must be sent by kernel. It can be done by
>>> introducing an IOSQE_ flag and using one of currently unused padding
>>> u64s.
>>>
>>> Such feature could be useful for load balancing and message passing
>>> between threads which would ride on top of io-uring, i.e. you could
>>> send NOP with user_data pointing to a message payload.
>>
>> So what you want is a NOP with 'fd' set to the fd of another ring, and
>> that nop posts a CQE on that other ring? I don't think we'd need IOSQE
>> flags for that, we just need a NOP that supports that. I see a few ways
>> of going about that:
>>
>> 1) Add a new 'NOP' that takes an fd, and validates that that fd is an
>>    io_uring instance. It can then grab the completion lock on that ring
>>    and post an empty CQE.
>>
>> 2) We add a FEAT flag saying NOP supports taking an 'fd' argument, where
>>    'fd' is another ring. Posting CQE same as above.
>>
>> 3) We add a specific opcode for this. Basically the same as #2, but
>>    maybe with a more descriptive name than NOP.
>>
>> Might make sense to pair that with a CQE flag or something like that, as
>> there's no specific user_data that could be used as it doesn't match an
>> existing SQE that has been issued. IORING_CQE_F_WAKEUP for example.
>> Would be applicable to all the above cases.
>>
>> I kind of like #3 the best. Add a IORING_OP_RING_WAKEUP command, require
>> that sqe->fd point to a ring (could even be the ring itself, doesn't
>> matter). And add IORING_CQE_F_WAKEUP as a specific flag for that.
> 
> Something like the below, totally untested. The request will complete on
> the original ring with either 0, for success, or -EOVERFLOW if the
> target ring was already in an overflow state. If the fd specified isn't
> an io_uring context, then the request will complete with -EBADFD.
> 
> If you have any way of testing this, please do. I'll write a basic
> functionality test for it as well, but not until tomorrow.
> 
> Maybe we want to include in cqe->res who the waker was? We can stuff the
> pid/tid in there, for example.

Made the pid change, and also wrote a test case for it. Only change
otherwise is adding a completion trace event as well. Patch below
against for-5.18/io_uring, and attached the test case for liburing.

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 2e04f718319d..b21f85a48224 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -1105,6 +1105,9 @@ static const struct io_op_def io_op_defs[] = {
 	[IORING_OP_MKDIRAT] = {},
 	[IORING_OP_SYMLINKAT] = {},
 	[IORING_OP_LINKAT] = {},
+	[IORING_OP_WAKEUP_RING] = {
+		.needs_file		= 1,
+	},
 };
 
 /* requests with any of those set should undergo io_disarm_next() */
@@ -4235,6 +4238,44 @@ static int io_nop(struct io_kiocb *req, unsigned int issue_flags)
 	return 0;
 }
 
+static int io_wakeup_ring_prep(struct io_kiocb *req,
+			       const struct io_uring_sqe *sqe)
+{
+	if (unlikely(sqe->addr || sqe->ioprio || sqe->buf_index || sqe->off ||
+		     sqe->len || sqe->rw_flags || sqe->splice_fd_in ||
+		     sqe->buf_index || sqe->personality))
+		return -EINVAL;
+
+	if (req->file->f_op != &io_uring_fops)
+		return -EBADFD;
+
+	return 0;
+}
+
+static int io_wakeup_ring(struct io_kiocb *req, unsigned int issue_flags)
+{
+	struct io_uring_cqe *cqe;
+	struct io_ring_ctx *ctx;
+	int ret = 0;
+
+	ctx = req->file->private_data;
+	spin_lock(&ctx->completion_lock);
+	cqe = io_get_cqe(ctx);
+	if (cqe) {
+		WRITE_ONCE(cqe->user_data, 0);
+		WRITE_ONCE(cqe->res, 0);
+		WRITE_ONCE(cqe->flags, IORING_CQE_F_WAKEUP);
+	} else {
+		ret = -EOVERFLOW;
+	}
+	io_commit_cqring(ctx);
+	spin_unlock(&ctx->completion_lock);
+	io_cqring_ev_posted(ctx);
+
+	__io_req_complete(req, issue_flags, ret, 0);
+	return 0;
+}
+
 static int io_fsync_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
 {
 	struct io_ring_ctx *ctx = req->ctx;
@@ -6568,6 +6609,8 @@ static int io_req_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
 		return io_symlinkat_prep(req, sqe);
 	case IORING_OP_LINKAT:
 		return io_linkat_prep(req, sqe);
+	case IORING_OP_WAKEUP_RING:
+		return io_wakeup_ring_prep(req, sqe);
 	}
 
 	printk_once(KERN_WARNING "io_uring: unhandled opcode %d\n",
@@ -6851,6 +6894,9 @@ static int io_issue_sqe(struct io_kiocb *req, unsigned int issue_flags)
 	case IORING_OP_LINKAT:
 		ret = io_linkat(req, issue_flags);
 		break;
+	case IORING_OP_WAKEUP_RING:
+		ret = io_wakeup_ring(req, issue_flags);
+		break;
 	default:
 		ret = -EINVAL;
 		break;
diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
index 787f491f0d2a..088232133594 100644
--- a/include/uapi/linux/io_uring.h
+++ b/include/uapi/linux/io_uring.h
@@ -143,6 +143,7 @@ enum {
 	IORING_OP_MKDIRAT,
 	IORING_OP_SYMLINKAT,
 	IORING_OP_LINKAT,
+	IORING_OP_WAKEUP_RING,
 
 	/* this goes last, obviously */
 	IORING_OP_LAST,
@@ -199,9 +200,11 @@ struct io_uring_cqe {
  *
  * IORING_CQE_F_BUFFER	If set, the upper 16 bits are the buffer ID
  * IORING_CQE_F_MORE	If set, parent SQE will generate more CQE entries
+ * IORING_CQE_F_WAKEUP	Wakeup request CQE, no link to an SQE
  */
 #define IORING_CQE_F_BUFFER		(1U << 0)
 #define IORING_CQE_F_MORE		(1U << 1)
+#define IORING_CQE_F_WAKEUP		(1U << 2)
 
 enum {
 	IORING_CQE_BUFFER_SHIFT		= 16,

-- 
Jens Axboe

[-- Attachment #2: wakeup-ring.patch --]
[-- Type: text/x-patch, Size: 4877 bytes --]

commit d07d17adab5b918bd0543ce78f75a4747a057379
Author: Jens Axboe <axboe@kernel.dk>
Date:   Wed Mar 9 19:31:57 2022 -0700

    test/wakeup-ring: add test cases for IORING_OP_WAKEUP_RING
    
    Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/.gitignore b/.gitignore
index c9dc77fbe162..961fd9e96dc7 100644
--- a/.gitignore
+++ b/.gitignore
@@ -128,6 +128,7 @@
 /test/timeout-overflow
 /test/unlink
 /test/wakeup-hang
+/test/wakeup-ring
 /test/multicqes_drain
 /test/poll-mshot-update
 /test/rsrc_tags
diff --git a/src/include/liburing/io_uring.h b/src/include/liburing/io_uring.h
index a7d193d0df38..8f919b42a8ea 100644
--- a/src/include/liburing/io_uring.h
+++ b/src/include/liburing/io_uring.h
@@ -147,6 +147,7 @@ enum {
 	IORING_OP_MKDIRAT,
 	IORING_OP_SYMLINKAT,
 	IORING_OP_LINKAT,
+	IORING_OP_WAKEUP_RING,
 
 	/* this goes last, obviously */
 	IORING_OP_LAST,
diff --git a/test/Makefile b/test/Makefile
index f421f536df87..4aafbae826ca 100644
--- a/test/Makefile
+++ b/test/Makefile
@@ -151,6 +151,7 @@ test_srcs := \
 	timeout-overflow.c \
 	unlink.c \
 	wakeup-hang.c \
+	wakeup-ring.c \
 	skip-cqe.c \
 	# EOL
 
@@ -221,6 +222,7 @@ ring-leak2: override LDFLAGS += -lpthread
 poll-mshot-update: override LDFLAGS += -lpthread
 exit-no-cleanup: override LDFLAGS += -lpthread
 pollfree: override LDFLAGS += -lpthread
+wakeup-ring: override LDFLAGS += -lpthread
 
 install: $(test_targets) runtests.sh runtests-loop.sh
 	$(INSTALL) -D -d -m 755 $(datadir)/liburing-test/
diff --git a/test/wakeup-ring.c b/test/wakeup-ring.c
new file mode 100644
index 000000000000..0ccae92d4c93
--- /dev/null
+++ b/test/wakeup-ring.c
@@ -0,0 +1,172 @@
+/* SPDX-License-Identifier: MIT */
+/*
+ * Description: test ring wakeup command
+ *
+ */
+#include <errno.h>
+#include <stdio.h>
+#include <unistd.h>
+#include <stdlib.h>
+#include <string.h>
+#include <fcntl.h>
+#include <pthread.h>
+
+#include "liburing.h"
+
+static int no_wakeup;
+
+static int test_own(struct io_uring *ring)
+{
+	struct io_uring_cqe *cqe;
+	struct io_uring_sqe *sqe;
+	int ret, i;
+
+	sqe = io_uring_get_sqe(ring);
+	if (!sqe) {
+		fprintf(stderr, "get sqe failed\n");
+		goto err;
+	}
+
+	sqe->opcode = IORING_OP_WAKEUP_RING;
+	sqe->fd = ring->ring_fd;
+	sqe->user_data = 1;
+
+	ret = io_uring_submit(ring);
+	if (ret <= 0) {
+		fprintf(stderr, "sqe submit failed: %d\n", ret);
+		goto err;
+	}
+
+	for (i = 0; i < 2; i++) {
+		ret = io_uring_peek_cqe(ring, &cqe);
+		if (ret < 0) {
+			fprintf(stderr, "wait completion %d\n", ret);
+			goto err;
+		}
+		switch (cqe->user_data) {
+		case 1:
+			if (cqe->res == -EINVAL || cqe->res == -EOPNOTSUPP) {
+				no_wakeup = 1;
+				return 0;
+			}
+			if (cqe->res != 0) {
+				fprintf(stderr, "wakeup res %d\n", cqe->res);
+				return -1;
+			}
+			break;
+		case 0:
+			if (!(cqe->flags & (1U << 2))) {
+				fprintf(stderr, "invalid flags %x\n", cqe->flags);
+				return -1;
+			}
+			break;
+		}
+		io_uring_cqe_seen(ring, cqe);
+	}
+
+	return 0;
+err:
+	return 1;
+}
+
+static void *wait_cqe_fn(void *data)
+{
+	struct io_uring *ring = data;
+	struct io_uring_cqe *cqe;
+	int ret;
+
+	ret = io_uring_wait_cqe(ring, &cqe);
+	if (ret) {
+		fprintf(stderr, "wait cqe %d\n", ret);
+		goto err;
+	}
+
+	if (!(cqe->flags & (1U << 2))) {
+		fprintf(stderr, "invalid flags %x\n", cqe->flags);
+		goto err;
+	}
+
+	return NULL;
+err:
+	return (void *) (unsigned long) 1;
+}
+
+static int test_remote(struct io_uring *ring, struct io_uring *target)
+{
+	struct io_uring_cqe *cqe;
+	struct io_uring_sqe *sqe;
+	int ret;
+
+	sqe = io_uring_get_sqe(ring);
+	if (!sqe) {
+		fprintf(stderr, "get sqe failed\n");
+		goto err;
+	}
+
+	sqe->opcode = IORING_OP_WAKEUP_RING;
+	sqe->fd = target->ring_fd;
+	sqe->user_data = 1;
+
+	ret = io_uring_submit(ring);
+	if (ret <= 0) {
+		fprintf(stderr, "sqe submit failed: %d\n", ret);
+		goto err;
+	}
+
+	ret = io_uring_peek_cqe(ring, &cqe);
+	if (ret < 0) {
+		fprintf(stderr, "wait completion %d\n", ret);
+		goto err;
+	}
+	if (cqe->res != 0) {
+		fprintf(stderr, "wakeup res %d\n", cqe->res);
+		return -1;
+	}
+
+	io_uring_cqe_seen(ring, cqe);
+	return 0;
+err:
+	return 1;
+}
+
+int main(int argc, char *argv[])
+{
+	struct io_uring ring, ring2;
+	pthread_t thread;
+	void *tret;
+	int ret;
+
+	if (argc > 1)
+		return 0;
+
+	ret = io_uring_queue_init(8, &ring, 0);
+	if (ret) {
+		fprintf(stderr, "ring setup failed: %d\n", ret);
+		return 1;
+	}
+	ret = io_uring_queue_init(8, &ring2, 0);
+	if (ret) {
+		fprintf(stderr, "ring setup failed: %d\n", ret);
+		return 1;
+	}
+
+	pthread_create(&thread, NULL, wait_cqe_fn, &ring2);
+
+	ret = test_own(&ring);
+	if (ret) {
+		fprintf(stderr, "test_own failed\n");
+		return ret;
+	}
+	if (no_wakeup)
+		return 0;
+
+	ret = test_remote(&ring, &ring2);
+	if (ret) {
+		fprintf(stderr, "test_remote failed\n");
+		return ret;
+	}
+
+	pthread_join(thread, &tret);
+
+	return 0;
+}

Re: Sending CQE to a different ring

[Jens Axboe]

On 3/9/22 7:11 PM, Artyom Pavlov wrote:
> 10.03.2022 04:36, Jens Axboe wrote:
>> On 3/9/22 4:49 PM, Artyom Pavlov wrote:
>>> Greetings!
>>>
>>> A common approach for multi-threaded servers is to have a number of
>>> threads equal to a number of cores and launch a separate ring in each
>>> one. AFAIK currently if we want to send an event to a different ring,
>>> we have to write-lock this ring, create SQE, and update the index
>>> ring. Alternatively, we could use some kind of user-space message
>>> passing.
>>>
>>> Such approaches are somewhat inefficient and I think it can be solved
>>> elegantly by updating the io_uring_sqe type to allow accepting fd of a
>>> ring to which CQE must be sent by kernel. It can be done by
>>> introducing an IOSQE_ flag and using one of currently unused padding
>>> u64s.
>>>
>>> Such feature could be useful for load balancing and message passing
>>> between threads which would ride on top of io-uring, i.e. you could
>>> send NOP with user_data pointing to a message payload.
>>
>> So what you want is a NOP with 'fd' set to the fd of another ring, and
>> that nop posts a CQE on that other ring? I don't think we'd need IOSQE
>> flags for that, we just need a NOP that supports that. I see a few ways
>> of going about that:
>>
>> 1) Add a new 'NOP' that takes an fd, and validates that that fd is an
>>     io_uring instance. It can then grab the completion lock on that ring
>>     and post an empty CQE.
>>
>> 2) We add a FEAT flag saying NOP supports taking an 'fd' argument, where
>>     'fd' is another ring. Posting CQE same as above.
>>
>> 3) We add a specific opcode for this. Basically the same as #2, but
>>     maybe with a more descriptive name than NOP.
>>
>> Might make sense to pair that with a CQE flag or something like that, as
>> there's no specific user_data that could be used as it doesn't match an
>> existing SQE that has been issued. IORING_CQE_F_WAKEUP for example.
>> Would be applicable to all the above cases.
>>
>> I kind of like #3 the best. Add a IORING_OP_RING_WAKEUP command, require
>> that sqe->fd point to a ring (could even be the ring itself, doesn't
>> matter). And add IORING_CQE_F_WAKEUP as a specific flag for that.
>>
> 
> No, ideally I would like to be able to send any type of SQE to a
> different ring. For example, if I see that the current ring is
> overloaded, I can create exactly the same SQEs as during usual
> operation, but with a changed recipient ring.
> 
> Your approach with a new "sendable" NOP will allow to emulate it in
> user-space, but it will involve unnecessary ring round-trip and will
> be a bit less pleasant in user code, e.g. we would need to encode a
> separate state "the task is being sent to a different ring" instead of
> simply telling io-uring "read data and report CQE on this ring"
> without any intermediate states.

OK, so what you're asking is to be able to submit an sqe to ring1, but
have the completion show up in ring2? With the idea being that the rings
are setup so that you're basing this on which thread should ultimately
process the request when it completes, which is why you want it to
target another ring?

It'd certainly be doable, but it's a bit of a strange beast. My main
concern with that would be:

1) It's a fast path code addition to every request, we'd need to check
   some new field (sqe->completion_ring_fd) and then also grab a
   reference to that file for use at completion time.

2) Completions are protected by the completion lock, and it isn't
   trivial to nest these. What happens if ring1 submits an sqe with
   ring2 as the cqe target, and ring2 submits an sqe with ring1 as the
   cqe target? We can't safely nest these, as we could easily introduce
   deadlocks that way.

My knee jerk reaction is that it'd be both simpler and cheaper to
implement this in userspace... Unless there's an elegant solution to it,
which I don't immediately see.

-- 
Jens Axboe

Re: Sending CQE to a different ring

[Artyom Pavlov]

After thinking about it a bit, I think this approach has one serious 
disadvantage: you lose successful result value of the initial request. 
Imagine we submit IORING_OP_READ and link IORING_OP_WAKEUP_RING to it. 
If the request is completed successfully, both ring1 and ring2 will lose 
number of read bytes.

10.03.2022 07:14, Jens Axboe wrote:
> On 3/9/22 9:03 PM, Jens Axboe wrote:
>> I'll mull over this a bit...
> 
> One idea... You issue the request as you normally would for ring1, and
> you mark that request A with IOSQE_CQE_SKIP_SUCCESS. Then you link an
> IORING_OP_WAKEUP_RING to request A, with the fd for it set to ring2, and
> also mark that with IOSQE_CQE_SKIP_SUCCESS.
> 
> We'd need to have sqe->addr (or whatever field) in the WAKEUP_RING
> request be set to the user_data of request A, so we can propagate it.
> 
> The end result is that ring1 won't see any completions for request A or
> the linked WAKEUP, unless one of them fails. If that happens, then you
> get to process things locally still, but given that this is a weird
> error case, shouldn't affect things in practice. ring2 will get the CQE
> posted once request A has completed, with the user_data filled in from
> request A. Which is really what we care about here, as far as I
> understand.
> 
> This basically works right now with what I posted, and without needing
> to rearchitect a bunch of stuff. And it's pretty efficient. Only thing
> we'd need to add is passing in the target cqe user_data for the WAKEUP
> request. Would just need to be wrapped in something that allows you to
> do this easily, as it would be useful for others too potentially.
> 

Re: Sending CQE to a different ring

[Jens Axboe]

On 3/9/22 7:11 PM, Artyom Pavlov wrote:
> No, ideally I would like to be able to send any type of SQE to a
> different ring. For example, if I see that the current ring is
> overloaded, I can create exactly the same SQEs as during usual
> operation, but with a changed recipient ring.

And since I could still be misunderstanding your intent here, if you
mean that you want to submit sqe to ring1 with ring2 being the target,
then I'm not sure that would be very helpful. The submitting task is the
owner of the request, and will ultimately be the one that ends up
running eg task_work associated with the request. It's not really a good
way to shift work from one ring to another, if the setup is such that
the rings are tied to a thread and the threads are in turn mostly tied
to a CPU or group of CPUs.

-- 
Jens Axboe