[PATCH v2] nvme-rdma: support devices with queue size < 32

[Marta Rybczynska]

> On 4/11/2017 11:52 AM, Marta Rybczynska wrote:
>>> On Mon, 2017-04-10 at 17:12 +0200, Marta Rybczynska wrote:
>>>> In the case of small NVMe-oF queue size (<32) we may enter
>>>> a deadlock caused by the fact that the IB completions aren't sent
>>>> waiting for 32 and the send queue will fill up.
>>>>
>>>> The error is seen as (using mlx5):
>>>> [ 2048.693355] mlx5_0:mlx5_ib_post_send:3765:(pid 7273):
>>>> [ 2048.693360] nvme nvme1: nvme_rdma_post_send failed with error code -12
>>>>
>>>> This patch changes the way the signaling is done so
>>>> that it depends on the queue depth now. The magic define has
>>>> been removed completely.
>>>>
>>>> Signed-off-by: Marta Rybczynska <marta.rybczynska at kalray.eu>
>>>> Signed-off-by: Samuel Jones <sjones at kalray.eu>
>>>> ---
>>>> Changes from v1:
>>>> * signal by queue size/2, remove hardcoded 32
>>>> * support queue depth of 1
>>>>
>>>>  drivers/nvme/host/rdma.c | 17 +++++++++++++----
>>>>  1 file changed, 13 insertions(+), 4 deletions(-)
>>>>
>>>> diff --git a/drivers/nvme/host/rdma.c b/drivers/nvme/host/rdma.c
>>>> index 47a479f..4de1b92 100644
>>>> --- a/drivers/nvme/host/rdma.c
>>>> +++ b/drivers/nvme/host/rdma.c
>>>> @@ -1029,6 +1029,18 @@ static void nvme_rdma_send_done(struct ib_cq *cq, struct
>>>> ib_wc *wc)
>>>>                 nvme_rdma_wr_error(cq, wc, "SEND");
>>>>  }
>>>>
>>>> +static inline nvme_rdma_queue_sig_limit(struct nvme_rdma_queue *queue)
>>>> +{
>>>> +       int sig_limit;
>>>> +
>>>> +       /* We signal completion every queue depth/2 and also
>>>> +        * handle the case of possible device with queue_depth=1,
>>>> +        * where we would need to signal every message.
>>>> +        */
>>>> +       sig_limit = max(queue->queue_size / 2, 1);
>>>> +       return (++queue->sig_count % sig_limit) == 0;
>>>> +}
>>>> +
>>>>  static int nvme_rdma_post_send(struct nvme_rdma_queue *queue,
>>>>                 struct nvme_rdma_qe *qe, struct ib_sge *sge, u32 num_sge,
>>>>                 struct ib_send_wr *first, bool flush)
>>>> @@ -1056,9 +1068,6 @@ static int nvme_rdma_post_send(struct nvme_rdma_queue
>>>> *queue,
>>>>          * Would have been way to obvious to handle this in hardware or
>>>>          * at least the RDMA stack..
>>>>          *
>>>> -        * This messy and racy code sniplet is copy and pasted from the iSER
>>>> -        * initiator, and the magic '32' comes from there as well.
>>>> -        *
>>>>          * Always signal the flushes. The magic request used for the flush
>>>>          * sequencer is not allocated in our driver's tagset and it's
>>>>          * triggered to be freed by blk_cleanup_queue(). So we need to
>>>> @@ -1066,7 +1075,7 @@ static int nvme_rdma_post_send(struct nvme_rdma_queue
>>>> *queue,
>>>>          * embedded in request's payload, is not freed when __ib_process_cq()
>>>>          * calls wr_cqe->done().
>>>>          */
>>>> -       if ((++queue->sig_count % 32) == 0 || flush)
>>>> +       if (nvme_rdma_queue_sig_limit(queue) || flush)
>>>>                 wr.send_flags |= IB_SEND_SIGNALED;
>>>>
>>>>         if (first)
>>>
>>> Hello Marta,
>>>
>>> The approach of this patch is suboptimal from a performance point of view.
>>> If the number of WRs that have been submitted since the last signaled WR
>>> was submitted would be tracked in a member variable that would allow to
>>> get rid of the (relatively slow) division operation.
>>>
>>
>> Hello Bart,
>> I think that we can remove the division (the modulo sig_limit). The sig_count
>> is an u8 so it is really a type of variable you propose. It isn't used anywhere
>> it seems so we can change the way it is used in the snippet to count until the
>> signaling moment. It will give something like:
>>
>> static inline nvme_rdma_queue_sig_limit(struct nvme_rdma_queue *queue)
>> {
>>        int sig_limit;
>>
>>        /* We signal completion every queue depth/2 and also
>>         * handle the case of possible device with queue_depth=1,
>>         * where we would need to signal every message.
>>         */
>>        sig_limit = max(queue->queue_size / 2, 1);
>>        queue->sig_count++;
>>        if (queue->sig_count < sig_limit)
>>            return 0;
>>        queue->sig_count = 0;
>>        return 1;
>> }
>>
>>
>> Do you like it better?
> 
> Hi Marta,
> I think that Bart (and I agree) meant to avoid doing the division in the
> fast path. You can set a new variable to rdma_ctrl called sig_limit and
> set once it during initialization stage. Then just do:
> 
> if ((++queue->sig_count % queue->ctrl->sig_limit) == 0 || flush)
> 
> in nvme_rdma_post_send function.
> 
> Also, as you mentioned sig_count is u8 so you need to avoid setting the
> sig_limit to a bigger value than 255. So please take a minimum of 32 (or
> any other suggestions ?? ) and the result of max(queue->queue_size / 2, 1);
> 

Hello Max,
Do you mean the division by 2 or the modulo? I would say that the division
by two should be compiled as a shift operation on most architectures, won't it?

Marta