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

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Date:   Wed, 19 Jul 2017 12:20:57 +0300
From:   Sagi Grimberg <sagi@...mberg.me>
To:     Sinan Kaya <okaya@...eaurora.org>, linux-nvme@...ts.infradead.org,
        timur@...eaurora.org
Cc:     linux-arm-msm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        Keith Busch <keith.busch@...el.com>, Jens Axboe <axboe@...com>,
        Christoph Hellwig <hch@....de>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] nvme: Acknowledge completion queue on each iteration

> 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@...eaurora.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).