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

[Ming Lei <ming.lei@redhat.com>]

On Tue, Nov 12, 2019 at 11:11:40AM -0700, Nadolski, Edmund wrote:
> 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?

Yeah, cq_poll_lock is only used for poll queue, and cq_lock is only for
irq queue. And one queue can either be poll or irq queue.

> 
> >   	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.

This function will be inline, and the above may help to avoid
unnecessary stack variable allocation. 


Thanks,
Ming


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