Re: [PATCH v5 3/5] io_uring: count CQEs in io_iopoll_check()

[Caleb Sander Mateos <csander@purestorage.com>]

On Wed, Mar 4, 2026 at 2:33 AM Ming Lei <ming.lei@redhat.com> wrote:
>
> On Mon, Mar 02, 2026 at 10:29:12AM -0700, Caleb Sander Mateos wrote:
> > A subsequent commit will allow uring_cmds that don't use iopoll on
> > IORING_SETUP_IOPOLL io_urings. As a result, CQEs can be posted without
> > setting the iopoll_completed flag for a request in iopoll_list or going
> > through task work. For example, a UBLK_U_IO_FETCH_IO_CMDS command could
> > call io_uring_mshot_cmd_post_cqe() to directly post a CQE. The
> > io_iopoll_check() loop currently only counts completions posted in
> > io_do_iopoll() when determining whether the min_events threshold has
> > been met. It also exits early if there are any existing CQEs before
> > polling, or if any CQEs are posted while running task work. CQEs posted
> > via io_uring_mshot_cmd_post_cqe() or other mechanisms won't be counted
> > against min_events.
> >
> > Explicitly check the available CQEs in each io_iopoll_check() loop
> > iteration to account for CQEs posted in any fashion.
> >
> > Signed-off-by: Caleb Sander Mateos <csander@purestorage.com>
> > ---
> >  io_uring/io_uring.c | 9 ++-------
> >  1 file changed, 2 insertions(+), 7 deletions(-)
> >
> > diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
> > index 46f39831d27c..b4625695bb3a 100644
> > --- a/io_uring/io_uring.c
> > +++ b/io_uring/io_uring.c
> > @@ -1184,11 +1184,10 @@ __cold void io_iopoll_try_reap_events(struct io_ring_ctx *ctx)
> >               io_move_task_work_from_local(ctx);
> >  }
> >
> >  static int io_iopoll_check(struct io_ring_ctx *ctx, unsigned int min_events)
> >  {
> > -     unsigned int nr_events = 0;
> >       unsigned long check_cq;
> >
> >       min_events = min(min_events, ctx->cq_entries);
> >
> >       lockdep_assert_held(&ctx->uring_lock);
> > @@ -1227,34 +1226,30 @@ static int io_iopoll_check(struct io_ring_ctx *ctx, unsigned int min_events)
> >                * the poll to the issued list. Otherwise we can spin here
> >                * forever, while the workqueue is stuck trying to acquire the
> >                * very same mutex.
> >                */
> >               if (list_empty(&ctx->iopoll_list) || io_task_work_pending(ctx)) {
> > -                     u32 tail = ctx->cached_cq_tail;
> > -
> >                       (void) io_run_local_work_locked(ctx, min_events);
> >
> >                       if (task_work_pending(current) || list_empty(&ctx->iopoll_list)) {
> >                               mutex_unlock(&ctx->uring_lock);
> >                               io_run_task_work();
> >                               mutex_lock(&ctx->uring_lock);
> >                       }
> >                       /* some requests don't go through iopoll_list */
> > -                     if (tail != ctx->cached_cq_tail || list_empty(&ctx->iopoll_list))
> > +                     if (list_empty(&ctx->iopoll_list))
> >                               break;
> >               }
> >               ret = io_do_iopoll(ctx, !min_events);
> >               if (unlikely(ret < 0))
> >                       return ret;
> >
> >               if (task_sigpending(current))
> >                       return -EINTR;
> >               if (need_resched())
> >                       break;
> > -
> > -             nr_events += ret;
> > -     } while (nr_events < min_events);
> > +     } while (io_cqring_events(ctx) < min_events);
>
> Before entering the loop, if io_cqring_events() finds any queued CQE,
> io_iopoll_check() returns immediately without polling.
>
> If the queued CQE is originated from non-iopoll uring_cmd, iopoll request
> will not be polled, may this be one issue?

I also noticed that logic and thought it seemed odd. I would think
we'd always want to wait for min_events CQEs (and iopoll once even if
min_events is 0). Looks like Jens added the early return in commit
a3a0e43fd770 ("io_uring: don't enter poll loop if we have CQEs
pending"), perhaps he can shed some light on it?

Thanks,
Caleb