Re: [PATCH v2] block/io_uring: resubmit when result is -EAGAIN

[Kevin Wolf <kwolf@redhat.com>]

Am 04.08.2021 um 16:50 hat Stefano Garzarella geschrieben:
> On Mon, Aug 02, 2021 at 02:40:36PM +0200, Kevin Wolf wrote:
> > Am 29.07.2021 um 11:10 hat Fabian Ebner geschrieben:
> > > Linux SCSI can throw spurious -EAGAIN in some corner cases in its
> > > completion path, which will end up being the result in the completed
> > > io_uring request.
> > > 
> > > Resubmitting such requests should allow block jobs to complete, even
> > > if such spurious errors are encountered.
> > > 
> > > Co-authored-by: Stefan Hajnoczi <stefanha@gmail.com>
> > > Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
> > > Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>
> > > ---
> > > 
> > > Changes from v1:
> > >     * Focus on what's relevant for the patch itself in the commit
> > >       message.
> > >     * Add Stefan's comment.
> > >     * Add Stefano's R-b tag (I hope that's fine, since there was no
> > >       change code-wise).
> > > 
> > >  block/io_uring.c | 16 +++++++++++++++-
> > >  1 file changed, 15 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/block/io_uring.c b/block/io_uring.c
> > > index 00a3ee9fb8..dfa475cc87 100644
> > > --- a/block/io_uring.c
> > > +++ b/block/io_uring.c
> > > @@ -165,7 +165,21 @@ static void luring_process_completions(LuringState *s)
> > >          total_bytes = ret + luringcb->total_read;
> > > 
> > >          if (ret < 0) {
> > > -            if (ret == -EINTR) {
> > > +            /*
> > > +             * Only writev/readv/fsync requests on regular files or host block
> > > +             * devices are submitted. Therefore -EAGAIN is not expected but it's
> > > +             * known to happen sometimes with Linux SCSI. Submit again and hope
> > > +             * the request completes successfully.
> > > +             *
> > > +             * For more information, see:
> > > +             * https://lore.kernel.org/io-uring/20210727165811.284510-3-axboe@kernel.dk/T/#u
> > > +             *
> > > +             * If the code is changed to submit other types of requests in the
> > > +             * future, then this workaround may need to be extended to deal with
> > > +             * genuine -EAGAIN results that should not be resubmitted
> > > +             * immediately.
> > > +             */
> > > +            if (ret == -EINTR || ret == -EAGAIN) {
> > >                  luring_resubmit(s, luringcb);
> > >                  continue;
> > >              }
> > 
> > Reviewed-by: Kevin Wolf <kwolf@redhat.com>
> > 
> > Question about the preexisting code, though: luring_resubmit() requires
> > that the caller calls ioq_submit() later so that the request doesn't
> > just sit in a queue without getting any attention, but actually gets
> > submitted to the kernel.
> > 
> > In the call chain ioq_submit() -> luring_process_completions() ->
> > luring_resubmit(), who takes care of that?
> 
> Mmm, good point.
> There should be the same problem with ioq_submit() ->
> luring_process_completions() -> luring_resubmit_short_read() ->
> luring_resubmit().
> 
> Should we schedule a BH for example in luring_resubmit() to make sure that
> ioq_submit() is invoked after a resubmission?

Or just loop in ioq_submit() after calling luring_process_completions()
if new requests were added to the queue?

Kevin