Date:	Wed, 09 Oct 2013 09:12:47 +0200
From:	Matias Bjørling <m@...rling.me>
To:	Keith Busch <keith.busch@...el.com>
CC:	axboe@...nel.dk, willy@...ux.intel.com,
	linux-kernel@...r.kernel.org, linux-nvme@...ts.infradead.org
Subject: Re: [PATCH RFC 2/2] NVMe: rfc blk-mq support

Thanks for the feedback. I'll make a v2 and report back measurements of 
gain/loss for the machines I have available.

On 10/08/2013 10:59 PM, Keith Busch wrote:
> On Tue, 8 Oct 2013, Matias Bjørling wrote:
>> Convert the driver to blk mq.
>>
>> The patch consists of:
>>
>> * Initializion of mq data structures.
>> * Convert function calls from bio to request data structures.
>> * IO queues are split into an admin queue and io queues.
>> * bio splits are removed as it should be handled by block layer.
>>
>> Signed-off-by: Matias Bjørling <m@...rling.me>
>
> I have no opinion right now if this is a good idea or not. I'll just
> comment on a couple issues on this implementation. Otherwise I think
> it's pretty neat and gave me a reason to explore multiqueues!
>
> First a couple minor suggestions:
>
> You might want to use "REQ_END" from the rq->cmd_flags to know if you
> should write the queue doorbell or not. Aggregating these would help
> most devices but we didn't have a good way of knowing before if this
> was the last request or not.
>
> Maybe change nvme_submit_bio_queue to return a BLK_MQ_RQ_QUEUE_ status
> so you don't need that switch statement after calling it.
>
> Must do something about requests that don't align to PRPs. I think you
> mentioned this in the outstanding work in [0/2].
>
>> struct nvme_queue *get_nvmeq(struct nvme_dev *dev)
>> {
>> -    return dev->queues[get_cpu() + 1];
>> +    get_cpu();
>> +    return dev->admin_queue;
>> }
>
> get_nvmeq now returns only the admin queue when it used to return only
> IO queues. This breaks NVME_IO and SG_IO ioctl handling.
>
>> +static int nvme_init_hctx(struct blk_mq_hw_ctx *hctx, void *data,
>> +              unsigned int i)
>> +{
>> +    struct nvme_ns *ns = data;
>> +    struct nvme_dev *dev = ns->dev;
>> +    struct nvme_queue *nq;
>> +
>> +    nq = nvme_create_queue(dev, i + 1, hctx->queue_depth, i);
>> +    if (IS_ERR(nq))
>> +        return PTR_ERR(nq);
>> +
>> +    hctx->driver_data = nq;
>> +
>> +    return 0;
>> +}
>
> This right here is the biggest problem with the implemenation. It is
> going to fail for every namespace but the first one since each namespace
> registers a multiqueue and each mulitqueue requires a hw context to
> work. The number of queues is for the device, not namespace, so only
> the first namespace is going to successfully return from nvme_init_hctx;
> the rest will be unable to create an NVMe IO queue for trying to create
> one with already allocated QID.
>
> You should instead create the IO queues on the device like how it was
> done before then just set the hctx->driver_data to dev->queues[i + 1]
> or something like.
>
>> +static enum blk_eh_timer_return nvme_timeout(struct request *rq)
>> +{
>> +    /* Currently the driver handle timeouts by itself */
>> +    return BLK_EH_NOT_HANDLED;
>> +}
>
> Need do something with the command timeouts here or somewhere. You've
> changed the driver to poll only on the admin queue for timed out commands,
> and left the multiqueue timeout a no-op.
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