Re: [PATCH v1 03/18] xprtrdma: Remove completion polling budgets

[Sagi Grimberg <sagig@dev.mellanox.co.il>]

>> It is possible that in a given poll_cq
>> call you end up getting on 1 completion, the other completion is
>> delayed due to some reason.
>
> If a CQE is allowed to be delayed, how does polling
> again guarantee that the consumer can retrieve it?
>
> What happens if a signal occurs, there is only one CQE,
> but it is delayed? ib_poll_cq would return 0 in that
> case, and the consumer would never call again, thinking
> the CQ is empty. There's no way the consumer can know
> for sure when a CQ is drained.
>
> If the delayed CQE happens only when there is more
> than one CQE, how can polling multiple WCs ever work
> reliably?
>
> Maybe I don't understand what is meant by delayed.
>

If I'm not mistaken, Devesh meant that if between ib_poll_cq (where you
polled the last 2 wcs) until the while statement another CQE was
generated then you lost a bit of efficiency. Correct?

>
>> Would it be better to poll for 1 in every
>> poll call Or
>> otherwise have this
>> while ( rc <= ARRAY_SIZE(wcs) && rc);
>>