Re: [PATCH] io_uring/uring_cmd: push IRQ based completions through task_work

[Jens Axboe <axboe@kernel.dk>]

On 3/20/23 9:06?AM, Kanchan Joshi wrote:
> On Sun, Mar 19, 2023 at 8:51?PM Jens Axboe <axboe@kernel.dk> wrote:
>>
>> This is similar to what we do on the non-passthrough read/write side,
>> and helps take advantage of the completion batching we can do when we
>> post CQEs via task_work. On top of that, this avoids a uring_lock
>> grab/drop for every completion.
>>
>> In the normal peak IRQ based testing, this increases performance in
>> my testing from ~75M to ~77M IOPS, or an increase of 2-3%.
>>
>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>>
>> ---
>>
>> diff --git a/io_uring/uring_cmd.c b/io_uring/uring_cmd.c
>> index 2e4c483075d3..b4fba5f0ab0d 100644
>> --- a/io_uring/uring_cmd.c
>> +++ b/io_uring/uring_cmd.c
>> @@ -45,18 +45,21 @@ static inline void io_req_set_cqe32_extra(struct io_kiocb *req,
>>  void io_uring_cmd_done(struct io_uring_cmd *ioucmd, ssize_t ret, ssize_t res2)
>>  {
>>         struct io_kiocb *req = cmd_to_io_kiocb(ioucmd);
>> +       struct io_ring_ctx *ctx = req->ctx;
>>
>>         if (ret < 0)
>>                 req_set_fail(req);
>>
>>         io_req_set_res(req, ret, 0);
>> -       if (req->ctx->flags & IORING_SETUP_CQE32)
>> +       if (ctx->flags & IORING_SETUP_CQE32)
>>                 io_req_set_cqe32_extra(req, res2, 0);
>> -       if (req->ctx->flags & IORING_SETUP_IOPOLL)
>> +       if (ctx->flags & IORING_SETUP_IOPOLL) {
>>                 /* order with io_iopoll_req_issued() checking ->iopoll_complete */
>>                 smp_store_release(&req->iopoll_completed, 1);
>> -       else
>> -               io_req_complete_post(req, 0);
>> +               return;
>> +       }
>> +       req->io_task_work.func = io_req_task_complete;
>> +       io_req_task_work_add(req);
>>  }
> 
> Since io_uring_cmd_done itself would be executing in task-work often
> (always in case of nvme), can this be further optimized by doing
> directly what this new task-work (that is being set up here) would
> have done?
> Something like below on top of your patch -
> 
> diff --git a/io_uring/uring_cmd.c b/io_uring/uring_cmd.c
> index e1929f6e5a24..7a764e04f309 100644
> --- a/io_uring/uring_cmd.c
> +++ b/io_uring/uring_cmd.c
> @@ -58,8 +58,12 @@ void io_uring_cmd_done(struct io_uring_cmd *ioucmd,
> ssize_t ret, ssize_t res2)
>                 smp_store_release(&req->iopoll_completed, 1);
>                 return;
>         }
> -       req->io_task_work.func = io_req_task_complete;
> -       io_req_task_work_add(req);
> +       if (in_task()) {
> +               io_req_complete_defer(req);
> +       } else {
> +               req->io_task_work.func = io_req_task_complete;
> +               io_req_task_work_add(req);
> +       }
>  }
>  EXPORT_SYMBOL_GPL(io_uring_cmd_done);

Good point, though I do think we should rework to pass in the flags
instead. I'll take a look.

-- 
Jens Axboe