> 在 2019年9月29日，07:23，Jens Axboe <axboe@xxxxxxxxx> 写道：
>
> Currently any dependent link is executed from a new workqueue context,
> which means that we'll be doing a context switch per link in the chain.
> If we are running the completion of the current request from our async
> workqueue and find that the next request is a link, then run it directly
> from the workqueue context instead of forcing another switch.
>
> This improves the performance of linked SQEs, and reduces the CPU
> overhead.
>
> Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
>
> ---
>
> 2-3x speedup doing read-write links, where the read often ends up
> blocking. Tested with examples/link-cp.c
>
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index aa8ac557493c..742d95563a54 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -667,7 +667,7 @@ static void __io_free_req(struct io_kiocb *req)
> kmem_cache_free(req_cachep, req);
> }
>
> -static void io_req_link_next(struct io_kiocb *req)
> +struct io_kiocb *io_req_link_next(struct io_kiocb *req)
> {
> struct io_kiocb *nxt;
>
> @@ -686,9 +686,19 @@ static void io_req_link_next(struct io_kiocb *req)
> }
>
> nxt->flags |= REQ_F_LINK_DONE;
> + /*
> + * If we're in async work, we can continue processing this,
> + * we can continue processing the chain in this context instead
> + * of having to queue up new async work.
> + */
> + if (current_work())
> + return nxt;
> INIT_WORK(&nxt->work, io_sq_wq_submit_work);
> io_queue_async_work(req->ctx, nxt);
> + nxt = NULL;
> }
> +
> + return nxt;
> }
>
> /*
> @@ -707,8 +717,10 @@ static void io_fail_links(struct io_kiocb *req)
> }
> }
>
> -static void io_free_req(struct io_kiocb *req)
> +static struct io_kiocb *io_free_req(struct io_kiocb *req)
> {
> + struct io_kiocb *nxt = NULL;
> +
> /*
> * If LINK is set, we have dependent requests in this chain. If we
> * didn't fail this request, queue the first one up, moving any other
> @@ -719,16 +731,30 @@ static void io_free_req(struct io_kiocb *req)
> if (req->flags & REQ_F_FAIL_LINK)
> io_fail_links(req);
> else
> - io_req_link_next(req);
> + nxt = io_req_link_next(req);
> }
>
> __io_free_req(req);
> + return nxt;
> }
>
LGTM, Reviewed-by: Jackie Liu <liuyun01@xxxxxxxxxx>
The function io_free_req has been used not only for free req, but also for the task
of finding the next link entry. I think it is possible to change a name to avoid
confusion, of course, only personal opinion.
--
BR, Jackie Liu
