Subject: Re: [PATCH] nvme: Acknowledge completion queue on each iteration
To: Sinan Kaya <okaya@codeaurora.org>, linux-nvme@lists.infradead.org,
        timur@codeaurora.org
Cc: linux-arm-msm@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        Keith Busch <keith.busch@intel.com>, Jens Axboe <axboe@fb.com>,
        Christoph Hellwig <hch@lst.de>, linux-kernel@vger.kernel.org
References: <1500330983-27501-1-git-send-email-okaya@codeaurora.org>
From: Sagi Grimberg <sagi@grimberg.me>
Message-ID: <5595ca25-f616-c0f8-fb2c-241a951e8848@grimberg.me>
Date: Wed, 19 Jul 2017 12:20:57 +0300
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101
 Thunderbird/52.2.1
MIME-Version: 1.0
In-Reply-To: <1500330983-27501-1-git-send-email-okaya@codeaurora.org>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1124
Lines: 34

> 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).