linux-kernel - Re: [PATCH v4] io_uring: reduce latency by reissueing the operation

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Date:   Tue, 22 Jun 2021 15:05:55 -0400
From:   Olivier Langlois <olivier@...llion01.com>
To:     Pavel Begunkov <asml.silence@...il.com>,
        Jens Axboe <axboe@...nel.dk>, io-uring@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4] io_uring: reduce latency by reissueing the operation

On Tue, 2021-06-22 at 19:01 +0100, Pavel Begunkov wrote:
> On 6/22/21 6:54 PM, Pavel Begunkov wrote:
> > On 6/22/21 1:17 PM, Olivier Langlois wrote:
> > > 
> > 
> > >  static bool __io_poll_remove_one(struct io_kiocb *req,
> > > @@ -6437,6 +6445,7 @@ static void __io_queue_sqe(struct io_kiocb
> > > *req)
> > >         struct io_kiocb *linked_timeout =
> > > io_prep_linked_timeout(req);
> > >         int ret;
> > >  
> > > +issue_sqe:
> > >         ret = io_issue_sqe(req,
> > > IO_URING_F_NONBLOCK|IO_URING_F_COMPLETE_DEFER);
> > >  
> > >         /*
> > > @@ -6456,12 +6465,16 @@ static void __io_queue_sqe(struct
> > > io_kiocb *req)
> > >                         io_put_req(req);
> > >                 }
> > >         } else if (ret == -EAGAIN && !(req->flags &
> > > REQ_F_NOWAIT)) {
> > > -               if (!io_arm_poll_handler(req)) {
> > > +               switch (io_arm_poll_handler(req)) {
> > > +               case IO_APOLL_READY:
> > > +                       goto issue_sqe;
> > > +               case IO_APOLL_ABORTED:
> > >                         /*
> > >                          * Queued up for async execution, worker
> > > will release
> > >                          * submit reference when the iocb is
> > > actually submitted.
> > >                          */
> > >                         io_queue_async_work(req);
> > > +                       break;
> > 
> > Hmm, why there is a new break here? It will miscount
> > @linked_timeout
> > if you do that. Every io_prep_linked_timeout() should be matched
> > with
> > io_queue_linked_timeout().
> 
> Never mind, I said some nonsense and apparently need some coffee

but this is a pertinant question, imho. I guess that you could get away
without it since it is the last case of the switch statement... I am
not sure what kernel coding standard says about that.

However, I can tell you that there was also a break statement at the
end of the case for IO_APOLL_READY and checkpatch.pl did complain about
it saying that it was useless since it was following a goto statement.
Therefore, I did remove that one.

checkpatch.pl did remain silent about the other remaining break. Hence
this is why I left it there.

Greetings,