Re: [LSF/MM/BPF ATTEND][LSF/MM/BPF Topic] Non-block IO

[Ming Lei <ming.lei@redhat.com>]

On Wed, Apr 12, 2023 at 06:56:15PM +0530, Kanchan Joshi wrote:
> On Wed, Apr 12, 2023 at 10:33:40AM +0800, Ming Lei wrote:
> > On Wed, Apr 12, 2023 at 04:18:16AM +0530, Kanchan Joshi wrote:
> > > > > 4. Direct NVMe queues - will there be interest in having io_uring
> > > > > managed NVMe queues?  Sort of a new ring, for which I/O is destaged from
> > > > > io_uring SQE to NVMe SQE without having to go through intermediate
> > > > > constructs (i.e., bio/request). Hopefully,that can further amp up the
> > > > > efficiency of IO.
> > > >
> > > > This is interesting, and I've pondered something like that before too. I
> > > > think it's worth investigating and hacking up a prototype. I recently
> > > > had one user of IOPOLL assume that setting up a ring with IOPOLL would
> > > > automatically create a polled queue on the driver side and that is what
> > > > would be used for IO. And while that's not how it currently works, it
> > > > definitely does make sense and we could make some things faster like
> > > > that. It would also potentially easier enable cancelation referenced in
> > > > #1 above, if it's restricted to the queue(s) that the ring "owns".
> > > 
> > > So I am looking at prototyping it, exclusively for the polled-io case.
> > > And for that, is there already a way to ensure that there are no
> > > concurrent submissions to this ring (set with IORING_SETUP_IOPOLL
> > > flag)?
> > > That will be the case generally (and submissions happen under
> > > uring_lock mutex), but submission may still get punted to io-wq
> > > worker(s) which do not take that mutex.
> > > So the original task and worker may get into doing concurrent submissions.
> > 
> > It seems one defect for uring command support, since io_ring_ctx and
> > io_ring_submit_lock() can't be exported for driver.
> 
> Sorry, did not follow the defect part.
> io-wq not acquring uring_lock in case of uring-cmd - is a defect? The same
> happens for direct block-io too.
> Or do you mean anything else here?

Maybe defect isn't one accurate word here.

I meant ->uring_cmd() is the only driver/fs callback in which
issue_flags is exposed, so IO_URING_F_UNLOCKED is visible to
driver, but io_ring_submit_lock() can't be done inside driver.

No such problem for direct io since the above io_uring details
isn't exposed to direct io code.


Thanks, 
Ming