Re: [PATCH v2] io_uring: run dependent links inline if possible

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> 在 2019年9月30日,08:42,Jens Axboe <axboe@xxxxxxxxx> 写道:
> 
> On 9/30/19 2:37 AM, Jackie Liu wrote:
>> 
>> 
>>> 在 2019年9月29日,22:54,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>
>>> 
>>> ---
>>> 
>>> v2:
>>> - Improve naming
>>> - Improve async detection
>>> - Harden cases where we could miss req return
>>> - Add support for fsync/sync_file_range/recvmsg/sendmsg
>>> 
>>> 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;
>>> }
>>> 
>>> -static void io_put_req(struct io_kiocb *req)
>>> +static struct io_kiocb *__io_put_req(struct io_kiocb *req)
>>> {
>>> 	if (refcount_dec_and_test(&req->refs))
>>> -		io_free_req(req);
>>> +		return io_free_req(req);
>>> +
>>> +	return NULL;
>>> +}
>>> +
>>> +static void io_put_req(struct io_kiocb *req)
>>> +{
>>> +	struct io_kiocb *nxt;
>>> +
>>> +	nxt = __io_put_req(req);
>>> +	if (nxt) {
>>> +		INIT_WORK(&nxt->work, io_sq_wq_submit_work);
>>> +		io_queue_async_work(nxt->ctx, nxt);
>>> +	}
>>> }
>>> 
>>> static unsigned io_cqring_events(struct io_rings *rings)
>>> @@ -934,7 +960,7 @@ static void kiocb_end_write(struct kiocb *kiocb)
>>> 	}
>>> }
>>> 
>>> -static void io_complete_rw(struct kiocb *kiocb, long res, long res2)
>>> +static void io_complete_rw_common(struct kiocb *kiocb, long res)
>>> {
>>> 	struct io_kiocb *req = container_of(kiocb, struct io_kiocb, rw);
>>> 
>>> @@ -943,9 +969,24 @@ static void io_complete_rw(struct kiocb *kiocb, long res, long res2)
>>> 	if ((req->flags & REQ_F_LINK) && res != req->result)
>>> 		req->flags |= REQ_F_FAIL_LINK;
>>> 	io_cqring_add_event(req->ctx, req->user_data, res);
>>> +}
>>> +
>>> +static void io_complete_rw(struct kiocb *kiocb, long res, long res2)
>>> +{
>>> +	struct io_kiocb *req = container_of(kiocb, struct io_kiocb, rw);
>>> +
>>> +	io_complete_rw_common(kiocb, res);
>>> 	io_put_req(req);
>>> }
>>> 
>>> +static struct io_kiocb *__io_complete_rw(struct kiocb *kiocb, long res)
>>> +{
>>> +	struct io_kiocb *req = container_of(kiocb, struct io_kiocb, rw);
>>> +
>>> +	io_complete_rw_common(kiocb, res);
>>> +	return __io_put_req(req);
>>> +}
>>> +
>>> static void io_complete_rw_iopoll(struct kiocb *kiocb, long res, long res2)
>>> {
>>> 	struct io_kiocb *req = container_of(kiocb, struct io_kiocb, rw);
>>> @@ -1128,6 +1169,15 @@ static inline void io_rw_done(struct kiocb *kiocb, ssize_t ret)
>>> 	}
>>> }
>>> 
>>> +static void call_io_rw_done(struct kiocb *kiocb, ssize_t ret,
>>> +			    struct io_kiocb **nxt, bool in_async)
>>> +{
>>> +	if (in_async && ret >= 0 && nxt && kiocb->ki_complete == io_complete_rw)
>>> +		*nxt = __io_complete_rw(kiocb, ret);
>>> +	else
>>> +		io_rw_done(kiocb, ret);
>>> +}
>>> +
>>> static int io_import_fixed(struct io_ring_ctx *ctx, int rw,
>>> 			   const struct io_uring_sqe *sqe,
>>> 			   struct iov_iter *iter)
>>> @@ -1344,7 +1394,7 @@ static ssize_t loop_rw_iter(int rw, struct file *file, struct kiocb *kiocb,
>>> }
>>> 
>>> static int io_read(struct io_kiocb *req, const struct sqe_submit *s,
>>> -		   bool force_nonblock)
>>> +		   bool force_nonblock, struct io_kiocb **nxt)
>>> {
>>> 	struct iovec inline_vecs[UIO_FASTIOV], *iovec = inline_vecs;
>>> 	struct kiocb *kiocb = &req->rw;
>>> @@ -1391,7 +1441,7 @@ static int io_read(struct io_kiocb *req, const struct sqe_submit *s,
>>> 			ret2 = -EAGAIN;
>>> 		/* Catch -EAGAIN return for forced non-blocking submission */
>>> 		if (!force_nonblock || ret2 != -EAGAIN) {
>>> -			io_rw_done(kiocb, ret2);
>>> +			call_io_rw_done(kiocb, ret2, nxt, s->needs_lock);
>>> 		} else {
>>> 			/*
>>> 			 * If ->needs_lock is true, we're already in async
>>> @@ -1407,7 +1457,7 @@ static int io_read(struct io_kiocb *req, const struct sqe_submit *s,
>>> }
>>> 
>>> static int io_write(struct io_kiocb *req, const struct sqe_submit *s,
>>> -		    bool force_nonblock)
>>> +		    bool force_nonblock, struct io_kiocb **nxt)
>>> {
>>> 	struct iovec inline_vecs[UIO_FASTIOV], *iovec = inline_vecs;
>>> 	struct kiocb *kiocb = &req->rw;
>>> @@ -1465,7 +1515,7 @@ static int io_write(struct io_kiocb *req, const struct sqe_submit *s,
>>> 		else
>>> 			ret2 = loop_rw_iter(WRITE, file, kiocb, &iter);
>>> 		if (!force_nonblock || ret2 != -EAGAIN) {
>>> -			io_rw_done(kiocb, ret2);
>>> +			call_io_rw_done(kiocb, ret2, nxt, s->needs_lock);
>>> 		} else {
>>> 			/*
>>> 			 * If ->needs_lock is true, we're already in async
>>> @@ -1968,7 +2018,8 @@ static int io_req_defer(struct io_ring_ctx *ctx, struct io_kiocb *req,
>>> }
>>> 
>>> static int __io_submit_sqe(struct io_ring_ctx *ctx, struct io_kiocb *req,
>>> -			   const struct sqe_submit *s, bool force_nonblock)
>>> +			   const struct sqe_submit *s, bool force_nonblock,
>>> +			   struct io_kiocb **nxt)
>>> {
>>> 	int ret, opcode;
>>> 
>>> @@ -1985,18 +2036,18 @@ static int __io_submit_sqe(struct io_ring_ctx *ctx, struct io_kiocb *req,
>>> 	case IORING_OP_READV:
>>> 		if (unlikely(s->sqe->buf_index))
>>> 			return -EINVAL;
>>> -		ret = io_read(req, s, force_nonblock);
>>> +		ret = io_read(req, s, force_nonblock, nxt);
>>> 		break;
>>> 	case IORING_OP_WRITEV:
>>> 		if (unlikely(s->sqe->buf_index))
>>> 			return -EINVAL;
>>> -		ret = io_write(req, s, force_nonblock);
>>> +		ret = io_write(req, s, force_nonblock, nxt);
>>> 		break;
>>> 	case IORING_OP_READ_FIXED:
>>> -		ret = io_read(req, s, force_nonblock);
>>> +		ret = io_read(req, s, force_nonblock, nxt);
>>> 		break;
>>> 	case IORING_OP_WRITE_FIXED:
>>> -		ret = io_write(req, s, force_nonblock);
>>> +		ret = io_write(req, s, force_nonblock, nxt);
>>> 		break;
>>> 	case IORING_OP_FSYNC:
>>> 		ret = io_fsync(req, s->sqe, force_nonblock);
>>> @@ -2081,6 +2132,7 @@ static void io_sq_wq_submit_work(struct work_struct *work)
>>> 		struct sqe_submit *s = &req->submit;
>>> 		const struct io_uring_sqe *sqe = s->sqe;
>>> 		unsigned int flags = req->flags;
>>> +		struct io_kiocb *nxt = NULL;
>>> 
>>> 		/* Ensure we clear previously set non-block flag */
>>> 		req->rw.ki_flags &= ~IOCB_NOWAIT;
>>> @@ -2101,7 +2153,7 @@ static void io_sq_wq_submit_work(struct work_struct *work)
>>> 			s->has_user = cur_mm != NULL;
>>> 			s->needs_lock = true;
>>> 			do {
>>> -				ret = __io_submit_sqe(ctx, req, s, false);
>>> +				ret = __io_submit_sqe(ctx, req, s, false, &nxt);
>>> 				/*
>>> 				 * We can get EAGAIN for polled IO even though
>>> 				 * we're forcing a sync submission from here,
>>> @@ -2125,6 +2177,12 @@ static void io_sq_wq_submit_work(struct work_struct *work)
>>> 		/* async context always use a copy of the sqe */
>>> 		kfree(sqe);
>>> 
>>> +		/* if a dependent link is ready, do that as the next one */
>>> +		if (!ret && nxt) {
>>> +			req = nxt;
>>> +			continue;
>>> +		}
>>> +
>>> 		/* req from defer and link list needn't decrease async cnt */
>>> 		if (flags & (REQ_F_IO_DRAINED | REQ_F_LINK_DONE))
>>> 			goto out;
>>> @@ -2271,7 +2329,7 @@ static int __io_queue_sqe(struct io_ring_ctx *ctx, struct io_kiocb *req,
>>> {
>>> 	int ret;
>>> 
>>> -	ret = __io_submit_sqe(ctx, req, s, force_nonblock);
>>> +	ret = __io_submit_sqe(ctx, req, s, force_nonblock, NULL);
>>> 	if (ret == -EAGAIN && !(req->flags & REQ_F_NOWAIT)) {
>>> 		struct io_uring_sqe *sqe_copy;
>>> 
>>> -- 
>>> Jens Axboe
>>> 
>>> 
>> 
>> Hi Jens, are you sure this is version 2, why is it the same as v1?
>> Is Link [1] is the correct one?
>> 
>> Link: [1] http://git.kernel.dk/cgit/linux-block/patch/?id=39b0f9f8e295b98bbcfd448709fa298f5545e28c
> 
> Yeah the link is the right one, that's odd. Below for reference!
> 
> 
> commit 98bb8de9e72fc61210976db3368dd3ad2549fa3c
> Author: Jens Axboe <axboe@xxxxxxxxx>
> Date:   Sat Sep 28 11:36:45 2019 -0600
> 
>    io_uring: run dependent links inline if possible
> 
>    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>
> 

Cool performance improvement, Reviewed-by: Jackie Liu <liuyun01@xxxxxxxxxx>

BTW, we always use s->needs_lock to determine if it is in async. Is it possible
to consider replacing it directly with s->in_async?

--
BR, Jackie Liu






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