linux-kernel - Re: [PATCH 07/13] IB: add a proper completion queue abstraction

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Date:	Sun, 17 Jan 2016 16:31:46 +0530
From:	Parav Pandit <pandit.parav@...il.com>
To:	Sagi Grimberg <sagig@....mellanox.co.il>
Cc:	Christoph Hellwig <hch@....de>, linux-rdma@...r.kernel.org,
	Bart Van Assche <bart.vanassche@...disk.com>, axboe@...com,
	linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 07/13] IB: add a proper completion queue abstraction

Hi Sagi,

On Sun, Jan 17, 2016 at 2:54 PM, Sagi Grimberg <sagig@....mellanox.co.il> wrote:
>
>> Hi Christoph, Sagi,
>>
>> On Tue, Dec 8, 2015 at 2:21 AM, Christoph Hellwig <hch@....de> wrote:
>>
>>> +static void ib_cq_poll_work(struct work_struct *work)
>>> +{
>>> +       struct ib_cq *cq = container_of(work, struct ib_cq, work);
>>> +       int completed;
>>> +
>>> +       completed = __ib_process_cq(cq, IB_POLL_BUDGET_WORKQUEUE);
>>> +       if (completed >= IB_POLL_BUDGET_WORKQUEUE ||
>>> +           ib_req_notify_cq(cq, IB_POLL_FLAGS) > 0)
>>> +               queue_work(ib_comp_wq, &cq->work);
>>> +}
>>
>>
>> In above code, Let says completion is added in a time window where
>> ib_process_cq is completed (CQ is diarmed in hw at that point) and
>> ib_req_notify_cq is yet to be called.
>> Provider vendor driver say mlx4 or mlx5 as specific case always
>> returns ib_req_notify_cq = 0.
>> Will it result into a missed notification? (so queue_work is not done).
>
>
> If we have not drained the CQ (consumed budget or more) the second
> condition (ib_req_notify_cq) will not be invoked. We are only rearming
> the CQ when we drained it completely. So I don't see how we can end up
> with missed notifications.
We drain the CQ completely for whatever CQEs available at that time,
say for example,
33 CQEs drained at time t1. So now req_notify_cq will be invoked at time t2.
During time delta t2-t1, CQ in hardware remains unarmed.
If cqes are added during that time delta, Will event/interrupt raised
for it, for CQ in unarmed state?

At time time t2, CQ is armed containing pending CQEs. Will
event/interrupt raised for those pending CQEs on next arming?