Re: [PATCH 2/2] nvme-pci: poll IO after batch submission for multi-mapping queue

["Nadolski, Edmund" <edmund.nadolski@intel.com>]

On 11/7/2019 8:55 PM, Ming Lei wrote:
> f9dde187fa92("nvme-pci: remove cq check after submission") removes
> cq check after submission, this change actually causes performance
> regression on some NVMe drive in which single nvmeq handles requests
> originated from more than one blk-mq sw queues(call it multi-mapping
> queue).
> 
> Actually polling IO after submission can handle IO more efficiently,
> especially for multi-mapping queue:
> 
> 1) the poll itself is very cheap, and lockless check on cq is enough,
> see nvme_cqe_pending(). Especially the check can be done after batch
> submission is done.
> 
> 2) when IO completion is observed via the poll in submission, the requst
> may be completed without interrupt involved, or the interrupt handler
> overload can be decreased.
> 
> 3) when single sw queue is submiting IOs to this hw queue, if IOs completion
> is observed via this poll, the IO can be completed locally, which is
> cheaper than remote completion.
> 
> Follows test result done on Azure L80sv2 guest with NVMe drive(
> Microsoft Corporation Device b111). This guest has 80 CPUs and 10
> numa nodes, and each NVMe drive supports 8 hw queues.
> 
> 1) test script:
> fio --bs=4k --ioengine=libaio --iodepth=64 --filename=/dev/nvme0n1 \
> 	--iodepth_batch_submit=16 --iodepth_batch_complete_min=16 \
> 	--direct=1 --runtime=30 --numjobs=1 --rw=randread \
> 	--name=test --group_reporting --gtod_reduce=1
> 
> 2) test result:
>       | v5.3       | v5.3 with this patchset
> -------------------------------------------
> IOPS | 130K       | 424K
> 
> Given IO is handled more efficiently in this way, the original report
> of CPU lockup[1] on Hyper-V can't be observed any more after this patch
> is applied. This issue is usually triggered when running IO from all
> CPUs concurrently.
> 
> I also run test on Optane(32 hw queues) in big machine(96 cores, 2 numa
> nodes), small improvement is observed on running the above fio over two
> NVMe drive with batch 1.
> 
> [1] https://lore.kernel.org/lkml/1566281669-48212-1-git-send-email-longli@linuxonhyperv.com
> 
> Cc: Keith Busch <kbusch@kernel.org>
> Cc: Jens Axboe <axboe@fb.com>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Sagi Grimberg <sagi@grimberg.me>
> Cc: Long Li <longli@microsoft.com>
> Fixes: f9dde187fa92("nvme-pci: remove cq check after submission")
> Signed-off-by: Ming Lei <ming.lei@redhat.com>
> ---
>   drivers/nvme/host/pci.c | 90 ++++++++++++++++++++++++++++++++++++-----
>   1 file changed, 80 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/nvme/host/pci.c b/drivers/nvme/host/pci.c
> index 5b20ab4d21d3..880376f43897 100644
> --- a/drivers/nvme/host/pci.c
> +++ b/drivers/nvme/host/pci.c
> @@ -83,6 +83,7 @@ struct nvme_queue;
>   
>   static void nvme_dev_disable(struct nvme_dev *dev, bool shutdown);
>   static bool __nvme_disable_io_queues(struct nvme_dev *dev, u8 opcode);
> +static void nvme_poll_in_submission(struct nvme_queue *nvmeq);
>   
>   /*
>    * Represents an NVM Express device.  Each nvme_dev is a PCI function.
> @@ -165,7 +166,10 @@ struct nvme_queue {
>   	spinlock_t sq_lock;
>   	void *sq_cmds;
>   	 /* only used for poll queues: */
> -	spinlock_t cq_poll_lock ____cacheline_aligned_in_smp;
> +	union {
> +		spinlock_t cq_poll_lock;
> +		spinlock_t cq_lock;
> +	}____cacheline_aligned_in_smp;

Is the new lock intended to protect anything differently than the old lock?

>   	volatile struct nvme_completion *cqes;
>   	struct blk_mq_tags **tags;
>   	dma_addr_t sq_dma_addr;
> @@ -185,6 +189,7 @@ struct nvme_queue {
>   #define NVMEQ_SQ_CMB		1
>   #define NVMEQ_DELETE_ERROR	2
>   #define NVMEQ_POLLED		3
> +#define NVMEQ_MULTI_MAPPING	4
>   	u32 *dbbuf_sq_db;
>   	u32 *dbbuf_cq_db;
>   	u32 *dbbuf_sq_ei;
> @@ -911,6 +916,10 @@ static blk_status_t nvme_queue_rq(struct blk_mq_hw_ctx *hctx,
>   
>   	blk_mq_start_request(req);
>   	nvme_submit_cmd(nvmeq, &cmnd, bd->last);
> +
> +	if (bd->last)
> +		nvme_poll_in_submission(nvmeq);
> +
>   	return BLK_STS_OK;
>   out_unmap_data:
>   	nvme_unmap_data(dev, req);
> @@ -1016,6 +1025,19 @@ static inline int nvme_process_cq(struct nvme_queue *nvmeq, u16 *start,
>   	return found;
>   }
>   
> +static inline irqreturn_t
> +nvme_update_cq(struct nvme_queue *nvmeq, u16 *start, u16 *end)
> +{
> +	irqreturn_t ret = IRQ_NONE;
> +
> +	if (nvmeq->cq_head != nvmeq->last_cq_head)
> +		ret = IRQ_HANDLED;
> +	nvme_process_cq(nvmeq, start, end, -1);
> +	nvmeq->last_cq_head = nvmeq->cq_head;
> +
> +	return ret;
> +}
> +
>   static irqreturn_t nvme_irq(int irq, void *data)
>   {
>   	struct nvme_queue *nvmeq = data;
> @@ -1027,10 +1049,7 @@ static irqreturn_t nvme_irq(int irq, void *data)
>   	 * the irq handler, even if that was on another CPU.
>   	 */
>   	rmb();
> -	if (nvmeq->cq_head != nvmeq->last_cq_head)
> -		ret = IRQ_HANDLED;
> -	nvme_process_cq(nvmeq, &start, &end, -1);
> -	nvmeq->last_cq_head = nvmeq->cq_head;
> +	ret = nvme_update_cq(nvmeq, &start, &end);
>   	wmb();
>   
>   	if (start != end) {
> @@ -1041,6 +1060,24 @@ static irqreturn_t nvme_irq(int irq, void *data)
>   	return ret;
>   }
>   
> +static irqreturn_t nvme_irq_multi_mapping(int irq, void *data)
> +{
> +	struct nvme_queue *nvmeq = data;
> +	irqreturn_t ret = IRQ_NONE;
> +	u16 start, end;
> +
> +	spin_lock(&nvmeq->cq_lock);
> +	ret = nvme_update_cq(nvmeq, &start, &end);
> +	spin_unlock(&nvmeq->cq_lock);
> +
> +	if (start != end) {
> +		nvme_complete_cqes(nvmeq, start, end);
> +		return IRQ_HANDLED;
> +	}
> +
> +	return ret;
> +}
> +
>   static irqreturn_t nvme_irq_check(int irq, void *data)
>   {
>   	struct nvme_queue *nvmeq = data;
> @@ -1049,6 +1086,24 @@ static irqreturn_t nvme_irq_check(int irq, void *data)
>   	return IRQ_NONE;
>   }
>   
> +static void nvme_poll_in_submission(struct nvme_queue *nvmeq)
> +{
> +	if (test_bit(NVMEQ_MULTI_MAPPING, &nvmeq->flags) &&
> +			nvme_cqe_pending(nvmeq)) {
> +		unsigned long flags;
> +
> +		if (spin_trylock_irqsave(&nvmeq->cq_lock, flags)) {
> +			u16 start, end;
> +
> +			nvme_update_cq(nvmeq, &start, &end);
> +			spin_unlock_irqrestore(&nvmeq->cq_lock, flags);
> +
> +			if (start != end)
> +				nvme_complete_cqes(nvmeq, start, end);
> +		}
> +	}
> +}

Just a nit, to me it reads simpler to return right away when the first test is 
false, rather than enclose the true path in an additional nesting level.

Thanks,
Ed

>   /*
>    * Poll for completions any queue, including those not dedicated to polling.
>    * Can be called from any context.
> @@ -1499,12 +1554,14 @@ static int queue_request_irq(struct nvme_queue *nvmeq)
>   {
>   	struct pci_dev *pdev = to_pci_dev(nvmeq->dev->dev);
>   	int nr = nvmeq->dev->ctrl.instance;
> +	irq_handler_t handler = test_bit(NVMEQ_MULTI_MAPPING, &nvmeq->flags) ?
> +		nvme_irq_multi_mapping : nvme_irq;
>   
>   	if (use_threaded_interrupts) {
>   		return pci_request_irq(pdev, nvmeq->cq_vector, nvme_irq_check,
> -				nvme_irq, nvmeq, "nvme%dq%d", nr, nvmeq->qid);
> +				handler, nvmeq, "nvme%dq%d", nr, nvmeq->qid);
>   	} else {
> -		return pci_request_irq(pdev, nvmeq->cq_vector, nvme_irq,
> +		return pci_request_irq(pdev, nvmeq->cq_vector, handler,
>   				NULL, nvmeq, "nvme%dq%d", nr, nvmeq->qid);
>   	}
>   }
> @@ -1514,7 +1571,13 @@ static void nvme_init_queue(struct nvme_queue *nvmeq, u16 qid)
>   	struct nvme_dev *dev = nvmeq->dev;
>   
>   	spin_lock_init(&nvmeq->sq_lock);
> -	spin_lock_init(&nvmeq->cq_poll_lock);
> +
> +	if (test_bit(NVMEQ_MULTI_MAPPING, &nvmeq->flags)) {
> +		WARN_ON_ONCE(test_bit(NVMEQ_POLLED, &nvmeq->flags));
> +		spin_lock_init(&nvmeq->cq_lock);
> +	} else {
> +		spin_lock_init(&nvmeq->cq_poll_lock);
> +	}
>   
>   	nvmeq->sq_tail = 0;
>   	nvmeq->last_sq_tail = 0;
> @@ -1534,15 +1597,22 @@ static int nvme_create_queue(struct nvme_queue *nvmeq, int qid, bool polled)
>   	u16 vector = 0;
>   
>   	clear_bit(NVMEQ_DELETE_ERROR, &nvmeq->flags);
> +	clear_bit(NVMEQ_MULTI_MAPPING, &nvmeq->flags);
>   
>   	/*
>   	 * A queue's vector matches the queue identifier unless the controller
>   	 * has only one vector available.
>   	 */
> -	if (!polled)
> +	if (!polled) {
> +		struct pci_dev *pdev = to_pci_dev(dev->dev);
> +
>   		vector = dev->num_vecs == 1 ? 0 : qid;
> -	else
> +		if (vector && cpumask_weight(pci_irq_get_affinity(pdev,
> +						vector)) > 1)
> +			set_bit(NVMEQ_MULTI_MAPPING, &nvmeq->flags);
> +	} else {
>   		set_bit(NVMEQ_POLLED, &nvmeq->flags);
> +	}
>   
>   	result = adapter_alloc_cq(dev, qid, nvmeq, vector);
>   	if (result)
> 


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