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

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

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;
 	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);
+		}
+	}
+}
+
 /*
  * 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)
-- 
2.20.1


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