On 5/25/20 1:29 AM, Pavel Begunkov wrote:
> On 23/05/2020 21:57, Jens Axboe wrote:
>> If the file is flagged with FMODE_BUF_RASYNC, then we don't have to punt
>> the buffered read to an io-wq worker. Instead we can rely on page
>> unlocking callbacks to support retry based async IO. This is a lot more
>> efficient than doing async thread offload.
>>
>> The retry is done similarly to how we handle poll based retry. From
>> the unlock callback, we simply queue the retry to a task_work based
>> handler.
>>
>> Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
>> ---
>> fs/io_uring.c | 99 +++++++++++++++++++++++++++++++++++++++++++++++++++
>> 1 file changed, 99 insertions(+)
>>
> ...
>> +
>> + init_task_work(&rw->task_work, io_async_buf_retry);
>> + /* submit ref gets dropped, acquire a new one */
>> + refcount_inc(&req->refs);
>> + tsk = req->task;
>> + ret = task_work_add(tsk, &rw->task_work, true);
>> + if (unlikely(ret)) {
>> + /* queue just for cancelation */
>> + init_task_work(&rw->task_work, io_async_buf_cancel);
>> + tsk = io_wq_get_task(req->ctx->io_wq);
>
> IIRC, task will be put somewhere around io_free_req(). Then shouldn't here be
> some juggling with reassigning req->task with task_{get,put}()?
Not sure I follow? Yes, we'll put this task again when the request
is freed, but not sure what you mean with juggling?
>> + task_work_add(tsk, &rw->task_work, true);
>> + }
>> + wake_up_process(tsk);
>> + return 1;
>> +}
> ...
>> static int io_read(struct io_kiocb *req, bool force_nonblock)
>> {
>> struct iovec inline_vecs[UIO_FASTIOV], *iovec = inline_vecs;
>> @@ -2601,6 +2696,7 @@ static int io_read(struct io_kiocb *req, bool force_nonblock)
>> if (!ret) {
>> ssize_t ret2;
>>
>> +retry:
>> if (req->file->f_op->read_iter)
>> ret2 = call_read_iter(req->file, kiocb, &iter);
>> else
>> @@ -2619,6 +2715,9 @@ static int io_read(struct io_kiocb *req, bool force_nonblock)
>> if (!(req->flags & REQ_F_NOWAIT) &&
>> !file_can_poll(req->file))
>> req->flags |= REQ_F_MUST_PUNT;
>> + if (io_rw_should_retry(req))
>
> It looks like a state machine with IOCB_WAITQ and gotos. Wouldn't it be cleaner
> to call call_read_iter()/loop_rw_iter() here directly instead of "goto retry" ?
We could, probably making that part a separate helper then. How about the
below incremental?
> BTW, can this async stuff return -EAGAIN ?
Probably? Prefer not to make any definitive calls on that being possible or
not, as it's sure to disappoint. If it does and IOCB_WAITQ is already set,
then we'll punt to a thread like before.
diff --git a/fs/io_uring.c b/fs/io_uring.c
index a5a4d9602915..669dccd81207 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -2677,6 +2677,13 @@ static bool io_rw_should_retry(struct io_kiocb *req)
return false;
}
+static int __io_read(struct io_kiocb *req, struct iov_iter *iter)
+{
+ if (req->file->f_op->read_iter)
+ return call_read_iter(req->file, &req->rw.kiocb, iter);
+ return loop_rw_iter(READ, req->file, &req->rw.kiocb, iter);
+}
+
static int io_read(struct io_kiocb *req, bool force_nonblock)
{
struct iovec inline_vecs[UIO_FASTIOV], *iovec = inline_vecs;
@@ -2710,11 +2717,7 @@ static int io_read(struct io_kiocb *req, bool force_nonblock)
if (!ret) {
ssize_t ret2;
-retry:
- if (req->file->f_op->read_iter)
- ret2 = call_read_iter(req->file, kiocb, &iter);
- else
- ret2 = loop_rw_iter(READ, req->file, kiocb, &iter);
+ ret2 = __io_read(req, &iter);
/* Catch -EAGAIN return for forced non-blocking submission */
if (!force_nonblock || ret2 != -EAGAIN) {
@@ -2729,8 +2732,11 @@ static int io_read(struct io_kiocb *req, bool force_nonblock)
if (!(req->flags & REQ_F_NOWAIT) &&
!file_can_poll(req->file))
req->flags |= REQ_F_MUST_PUNT;
- if (io_rw_should_retry(req))
- goto retry;
+ if (io_rw_should_retry(req)) {
+ ret2 = __io_read(req, &iter);
+ if (ret2 != -EAGAIN)
+ goto out_free;
+ }
kiocb->ki_flags &= ~IOCB_WAITQ;
return -EAGAIN;
}
--
Jens Axboe
