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SubjectRe: [PATCH] nvme: Acknowledge completion queue on each iteration
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> 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 <okaya@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).

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