Re: [PATCH 3/3] io_uring/msg_ring: improve handling of target CQE posting

[Pavel Begunkov <asml.silence@gmail.com>]

On 3/29/24 13:32, Jens Axboe wrote:
> On 3/29/24 6:54 AM, Pavel Begunkov wrote:
>> On 3/28/24 18:52, Jens Axboe wrote:
>>> Use the exported helper for queueing task_work, rather than rolling our
>>> own.
>>>
>>> This improves peak performance of message passing by about 5x in some
>>> basic testing, with 2 threads just sending messages to each other.
>>> Before this change, it was capped at around 700K/sec, with the change
>>> it's at over 4M/sec.
>>>
>>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>>> ---
>>>    io_uring/msg_ring.c | 27 ++++++++++-----------------
>>>    1 file changed, 10 insertions(+), 17 deletions(-)
>>>
>>> diff --git a/io_uring/msg_ring.c b/io_uring/msg_ring.c
>>> index d1f66a40b4b4..e12a9e8a910a 100644
>>> --- a/io_uring/msg_ring.c
>>> +++ b/io_uring/msg_ring.c
>>> @@ -11,9 +11,9 @@
>>>    #include "io_uring.h"
>>>    #include "rsrc.h"
>>>    #include "filetable.h"
>>> +#include "refs.h"
>>>    #include "msg_ring.h"
>>>    -
>>>    /* All valid masks for MSG_RING */
>>>    #define IORING_MSG_RING_MASK        (IORING_MSG_RING_CQE_SKIP | \
>>>                        IORING_MSG_RING_FLAGS_PASS)
>>> @@ -21,7 +21,6 @@
>>>    struct io_msg {
>>>        struct file            *file;
>>>        struct file            *src_file;
>>> -    struct callback_head        tw;
>>>        u64 user_data;
>>>        u32 len;
>>>        u32 cmd;
>>> @@ -73,26 +72,20 @@ static inline bool io_msg_need_remote(struct io_ring_ctx *target_ctx)
>>>        return current != target_ctx->submitter_task;
>>>    }
>>>    -static int io_msg_exec_remote(struct io_kiocb *req, task_work_func_t func)
>>> +static int io_msg_exec_remote(struct io_kiocb *req, io_req_tw_func_t func)
>>>    {
>>>        struct io_ring_ctx *ctx = req->file->private_data;
>>> -    struct io_msg *msg = io_kiocb_to_cmd(req, struct io_msg);
>>>        struct task_struct *task = READ_ONCE(ctx->submitter_task);
>>>    -    if (unlikely(!task))
>>> -        return -EOWNERDEAD;
>>> -
>>> -    init_task_work(&msg->tw, func);
>>> -    if (task_work_add(ctx->submitter_task, &msg->tw, TWA_SIGNAL))
>>> -        return -EOWNERDEAD;
>>> -
>>> +    __io_req_set_refcount(req, 2);
>>
>> I'd argue it's better avoid any more req refcount users, I'd be more
>> happy it it dies out completely at some point.
>>
>> Why it's even needed here? You pass it via tw to post a CQE/etc and
>> then pass it back via another tw hop to complete IIRC, the ownership
>> is clear. At least it worth a comment.
> 
> It's not, it was more documentation than anything else. But I agree that
> we should just avoid it, I'll kill it.

Great, it was confusing and I don't think it's even correct. In case
it comes with refcounting enabled you'd get only 1 ref instead of
desired 2. See how io_wq_submit_work() does it. Probably it's better
to kill the "__" set refs helper.

-- 
Pavel Begunkov