Re: [PATCH 2/2] io_uring: add irq completion work to the head of task_list

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

On 8/23/21 7:36 PM, Hao Xu wrote:
> Now we have a lot of task_work users, some are just to complete a req
> and generate a cqe. Let's put the work at the head position of the
> task_list, so that it can be handled quickly and thus to reduce
> avg req latency. an explanatory case:
> 
> origin timeline:
>     submit_sqe-->irq-->add completion task_work
>     -->run heavy work0~n-->run completion task_work
> now timeline:
>     submit_sqe-->irq-->add completion task_work
>     -->run completion task_work-->run heavy work0~n

Might be good. There are not so many hot tw users:
poll, queuing linked requests, and the new IRQ. Could be
BPF in the future.

So, for the test case I'd think about some heavy-ish
submissions linked to your IRQ req. For instance,
keeping a large QD of 

read(IRQ-based) -> linked read_pipe(PAGE_SIZE);

and running it for a while, so they get completely
out of sync and tw works really mix up. It reads
from pipes size<=PAGE_SIZE, so it completes inline,
but the copy takes enough of time.

One thing is that Jens specifically wanted tw's to
be in FIFO order, where IRQ based will be in LIFO.
I don't think it's a real problem though, the
completion handler should be brief enough.

> Signed-off-by: Hao Xu <haoxu@linux.alibaba.com>
> ---
>  fs/io-wq.h    |  9 +++++++++
>  fs/io_uring.c | 21 ++++++++++++---------
>  2 files changed, 21 insertions(+), 9 deletions(-)
> 
> diff --git a/fs/io-wq.h b/fs/io-wq.h
> index 308af3928424..51b4408fd177 100644
> --- a/fs/io-wq.h
> +++ b/fs/io-wq.h
> @@ -41,6 +41,15 @@ static inline void wq_list_add_after(struct io_wq_work_node *node,
>  		list->last = node;
>  }
>  
> +static inline void wq_list_add_head(struct io_wq_work_node *node,
> +				    struct io_wq_work_list *list)
> +{
> +	node->next = list->first;
> +	list->first = node;
> +	if (!node->next)
> +		list->last = node;
> +}
> +
>  static inline void wq_list_add_tail(struct io_wq_work_node *node,
>  				    struct io_wq_work_list *list)
>  {
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 8172f5f893ad..954cd8583945 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -2050,7 +2050,7 @@ static void tctx_task_work(struct callback_head *cb)
>  	ctx_flush_and_put(ctx);
>  }
>  
> -static void io_req_task_work_add(struct io_kiocb *req)
> +static void io_req_task_work_add(struct io_kiocb *req, bool emergency)
>  {
>  	struct task_struct *tsk = req->task;
>  	struct io_uring_task *tctx = tsk->io_uring;
> @@ -2062,7 +2062,10 @@ static void io_req_task_work_add(struct io_kiocb *req)
>  	WARN_ON_ONCE(!tctx);
>  
>  	spin_lock_irqsave(&tctx->task_lock, flags);
> -	wq_list_add_tail(&req->io_task_work.node, &tctx->task_list);
> +	if (emergency)
> +		wq_list_add_head(&req->io_task_work.node, &tctx->task_list);
> +	else
> +		wq_list_add_tail(&req->io_task_work.node, &tctx->task_list);
>  	running = tctx->task_running;
>  	if (!running)
>  		tctx->task_running = true;
> @@ -2122,19 +2125,19 @@ static void io_req_task_queue_fail(struct io_kiocb *req, int ret)
>  {
>  	req->result = ret;
>  	req->io_task_work.func = io_req_task_cancel;
> -	io_req_task_work_add(req);
> +	io_req_task_work_add(req, true);
>  }
>  
>  static void io_req_task_queue(struct io_kiocb *req)
>  {
>  	req->io_task_work.func = io_req_task_submit;
> -	io_req_task_work_add(req);
> +	io_req_task_work_add(req, false);
>  }
>  
>  static void io_req_task_queue_reissue(struct io_kiocb *req)
>  {
>  	req->io_task_work.func = io_queue_async_work;
> -	io_req_task_work_add(req);
> +	io_req_task_work_add(req, false);
>  }
>  
>  static inline void io_queue_next(struct io_kiocb *req)
> @@ -2249,7 +2252,7 @@ static inline void io_put_req_deferred(struct io_kiocb *req)
>  {
>  	if (req_ref_put_and_test(req)) {
>  		req->io_task_work.func = io_free_req;
> -		io_req_task_work_add(req);
> +		io_req_task_work_add(req, false);
>  	}
>  }
>  
> @@ -2564,7 +2567,7 @@ static void io_complete_rw(struct kiocb *kiocb, long res, long res2)
>  		return;
>  	req->result = res;
>  	req->io_task_work.func = io_req_task_complete;
> -	io_req_task_work_add(req);
> +	io_req_task_work_add(req, true);
>  }
>  
>  static void io_complete_rw_iopoll(struct kiocb *kiocb, long res, long res2)
> @@ -4881,7 +4884,7 @@ static int __io_async_wake(struct io_kiocb *req, struct io_poll_iocb *poll,
>  	 * of executing it. We can't safely execute it anyway, as we may not
>  	 * have the needed state needed for it anyway.
>  	 */
> -	io_req_task_work_add(req);
> +	io_req_task_work_add(req, false);
>  	return 1;
>  }
>  
> @@ -6430,7 +6433,7 @@ static enum hrtimer_restart io_link_timeout_fn(struct hrtimer *timer)
>  	spin_unlock_irqrestore(&ctx->timeout_lock, flags);
>  
>  	req->io_task_work.func = io_req_task_link_timeout;
> -	io_req_task_work_add(req);
> +	io_req_task_work_add(req, false);
>  	return HRTIMER_NORESTART;
>  }
>  
> 

-- 
Pavel Begunkov