Re: [PATCH] nvme: Acknowledge completion queue on each iteration

[Sagi Grimberg]

Code is moving the completion queue doorbell after processing all completed
events and sending callbacks to the block layer on each iteration.

This is causing a performance drop when a lot of jobs are queued towards
the HW. Move the completion queue doorbell on each loop instead and allow new
jobs to be queued by the HW.

Signed-off-by: Sinan Kaya <ok...@codeaurora.org>
---
  drivers/nvme/host/pci.c | 5 ++---
  1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/drivers/nvme/host/pci.c b/drivers/nvme/host/pci.c
index d10d2f2..33d9b5b 100644
--- a/drivers/nvme/host/pci.c
+++ b/drivers/nvme/host/pci.c
@@ -810,13 +810,12 @@ static void nvme_process_cq(struct nvme_queue *nvmeq)

while (nvme_read_cqe(nvmeq, &cqe)) {

                nvme_handle_cqe(nvmeq, &cqe);
+               nvme_ring_cq_doorbell(nvmeq);
                consumed++;
        }

- if (consumed) {

-               nvme_ring_cq_doorbell(nvmeq);
+       if (consumed)
                nvmeq->cqe_seen = 1;
-       }
  }

Agree with Keith that this is definitely not the way to go, it
adds mmio operations in the hot path with very little gain (if
at all).