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
