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

[Hao Xu <haoxu@xxxxxxxxxxxxxxxxx>]

在 2021/8/24 下午8:57, Pavel Begunkov 写道:
> 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.
async buffered reads as well, regarding buffered reads is
hot operation.
> 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.
Thanks Pavel, previously I tried
direct read-->buffered read(async buffered read)
didn't see much difference. I'll try the above case
you offered.
> 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.In my latest code, the IRQ based tw are also FIFO,
only LIFO between IRQ based tw and other tw:
timeline: tw1 tw2 irq1 irq2
task_list: irq1 irq2 tw1 tw2
> > Signed-off-by: Hao Xu <haoxu@xxxxxxxxxxxxxxxxx>
> > ---
> >   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;
> >   }
> >