> 在 2019年9月30日，08:42，Jens Axboe <ax...@kernel.dk> 写道：
>
> On 9/30/19 2:37 AM, Jackie Liu wrote:
>>
>>
>>> 在 2019年9月29日，22:54，Jens Axboe <ax...@kernel.dk> 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 <ax...@kernel.dk>
>>>
>>> ---
>>>
>>> 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 <ax...@kernel.dk>
> 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 <ax...@kernel.dk>
>
Cool performance improvement, Reviewed-by: Jackie Liu <liuyu...@kylinos.cn>
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
