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

From: Marta Rybczynska
Date: Mon Jun 05 2017 - 10:26:02 EST


> 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