Re: [PATCH] nvme-rdma: remove race conditions from IB signalling

[Marta Rybczynska]

> On 6/5/2017 12:45 PM, Marta Rybczynska wrote:
>> This patch improves the way the RDMA IB signalling is done
>> by using atomic operations for the signalling variable. This
>> avoids race conditions on sig_count.
>>
>> The signalling interval changes slightly and is now the
>> largest power of two not larger than queue depth / 2.
>>
>> ilog() usage idea by Bart Van Assche.
>>
>> Signed-off-by: Marta Rybczynska <marta.rybczynska@xxxxxxxxx>
>> ---
>>  drivers/nvme/host/rdma.c | 31 +++++++++++++++++++++----------
>>  1 file changed, 21 insertions(+), 10 deletions(-)
>>
>> diff --git a/drivers/nvme/host/rdma.c b/drivers/nvme/host/rdma.c
>> index 28bd255..80682f7 100644
>> --- a/drivers/nvme/host/rdma.c
>> +++ b/drivers/nvme/host/rdma.c
>> @@ -88,7 +88,7 @@ enum nvme_rdma_queue_flags {
>>
>>  struct nvme_rdma_queue {
>>         struct nvme_rdma_qe     *rsp_ring;
>> -       u8                      sig_count;
>> +       atomic_t                sig_count;
>>         int                     queue_size;
>>         size_t                  cmnd_capsule_len;
>>         struct nvme_rdma_ctrl   *ctrl;
>> @@ -554,6 +554,8 @@ static int nvme_rdma_init_queue(struct nvme_rdma_ctrl *ctrl,
>>
>>         queue->queue_size = queue_size;
>>
>> +       atomic_set(&queue->sig_count, 0);
>> +
>>         queue->cm_id = rdma_create_id(&init_net, nvme_rdma_cm_handler, queue,
>>                         RDMA_PS_TCP, IB_QPT_RC);
>>         if (IS_ERR(queue->cm_id)) {
>> @@ -1038,17 +1040,26 @@ static void nvme_rdma_send_done(struct ib_cq *cq, struct
>> ib_wc *wc)
>>                 nvme_rdma_wr_error(cq, wc, "SEND");
>>  }
>>
>> -static inline int nvme_rdma_queue_sig_limit(struct nvme_rdma_queue *queue)
>> +static inline int nvme_rdma_init_sig_count(int queue_size)
>>  {
>> -       int sig_limit;
>> -
>> -       /*
>> -        * We signal completion every queue depth/2 and also handle the
>> -        * degenerated case of a  device with queue_depth=1, where we
>> -        * would need to signal every message.
>> +       /* We want to signal completion at least every queue depth/2.
>> +        * This returns the largest power of two that is not above half
>> +        * of (queue size + 1) to optimize (avoid divisions).
>>          */
>> -       sig_limit = max(queue->queue_size / 2, 1);
>> -       return (++queue->sig_count % sig_limit) == 0;
>> +       return 1 << ilog2((queue_size + 1) / 2);
>> +}
>> +
>> +static inline bool nvme_rdma_queue_sig_limit(struct nvme_rdma_queue *queue)
>> +{
>> +       int count, limit;
>> +
>> +       limit = nvme_rdma_init_sig_count(queue->queue_size);
> 
> Maybe I'm missing something, but why we need to calc limit each time in
> data path (not a big deal but we can avoid that)?
> Can you add queue->sig_limit that will be calculated once at queue
> allocation ?

We've said last time that ilog() is cheap enough so that we do not
need to add this variable. It maps to fls() or a specific instruction
of a platform.

Marta