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

[Jens Axboe <axboe@kernel.dk>]

On 3/4/26 8:46 AM, Caleb Sander Mateos wrote:
> 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?

I don't  recall the bug in question, it's been a while... But it always
makes sense to return events that are ready, and skip polling. It should
only be done if there are no ready events to reap.

-- 
Jens Axboe