From: Israel Rukshin <israelr@mellanox.com>

Date: Sun, 24 Nov 2019 18:38:30 +0200

Subject: [PATCH] nvme-rdma: Avoid preallocating big SGL for data

Git-commit: 38e1800275d3af607e4df92ff49dc2cf442586a4

Patch-mainline: v5.5-rc2

References: bsc#1169045

nvme_rdma_alloc_tagset() preallocates a big buffer for the IO SGL based

on SG_CHUNK_SIZE.

Modern DMA engines are often capable of dealing with very big segments so

the SG_CHUNK_SIZE is often too big. SG_CHUNK_SIZE results in a static 4KB

SGL allocation per command.

If a controller has lots of deep queues, preallocation for the sg list can

consume substantial amounts of memory. For nvme-rdma, nr_hw_queues can be

128 and each queue's depth 128. This means the resulting preallocation

for the data SGL is 128*128*4K = 64MB per controller.

Switch to runtime allocation for SGL for lists longer than 2 entries. This

is the approach used by NVMe PCI so it should be reasonable for NVMeOF as

well. Runtime SGL allocation has always been the case for the legacy I/O

path so this is nothing new.

The preallocated small SGL depends on SG_CHAIN so if the ARCH doesn't

support SG_CHAIN, use only runtime allocation for the SGL.

We didn't notice of a performance degradation, since for small IOs we'll

use the inline SG and for the bigger IOs the allocation of a bigger SGL

from slab is fast enough.

Suggested-by: Christoph Hellwig <hch@lst.de>

Reviewed-by: Max Gurtovoy <maxg@mellanox.com>

Reviewed-by: Christoph Hellwig <hch@lst.de>

Signed-off-by: Israel Rukshin <israelr@mellanox.com>

Signed-off-by: Keith Busch <kbusch@kernel.org>

Acked-by: Hannes Reinecke <hare@suse.com>

---

 drivers/nvme/host/nvme.h |  6 ++++++

 drivers/nvme/host/rdma.c | 10 +++++-----

 2 files changed, 11 insertions(+), 5 deletions(-)

diff --git a/drivers/nvme/host/nvme.h b/drivers/nvme/host/nvme.h

index 3b9cbe0668fa..1024fec7914c 100644

--- a/drivers/nvme/host/nvme.h

+++ b/drivers/nvme/host/nvme.h

@@ -28,6 +28,12 @@ extern unsigned int admin_timeout;

 #define NVME_DEFAULT_KATO	5

 #define NVME_KATO_GRACE		10

+#ifdef CONFIG_ARCH_NO_SG_CHAIN

+#define  NVME_INLINE_SG_CNT  0

+#else

+#define  NVME_INLINE_SG_CNT  2

+#endif

+

 extern struct workqueue_struct *nvme_wq;

 extern struct workqueue_struct *nvme_reset_wq;

 extern struct workqueue_struct *nvme_delete_wq;

diff --git a/drivers/nvme/host/rdma.c b/drivers/nvme/host/rdma.c

index dce59459ed41..2a47c6c5007e 100644

--- a/drivers/nvme/host/rdma.c

+++ b/drivers/nvme/host/rdma.c

@@ -731,7 +731,7 @@ static struct blk_mq_tag_set *nvme_rdma_alloc_tagset(struct nvme_ctrl *nctrl,

 		set->reserved_tags = 2; /* connect + keep-alive */

 		set->numa_node = nctrl->numa_node;

 		set->cmd_size = sizeof(struct nvme_rdma_request) +

-			SG_CHUNK_SIZE * sizeof(struct scatterlist);

+			NVME_INLINE_SG_CNT * sizeof(struct scatterlist);

 		set->driver_data = ctrl;

 		set->nr_hw_queues = 1;

 		set->timeout = ADMIN_TIMEOUT;

@@ -745,7 +745,7 @@ static struct blk_mq_tag_set *nvme_rdma_alloc_tagset(struct nvme_ctrl *nctrl,

 		set->numa_node = nctrl->numa_node;

 		set->flags = BLK_MQ_F_SHOULD_MERGE;

 		set->cmd_size = sizeof(struct nvme_rdma_request) +

-			SG_CHUNK_SIZE * sizeof(struct scatterlist);

+			NVME_INLINE_SG_CNT * sizeof(struct scatterlist);

 		set->driver_data = ctrl;

 		set->nr_hw_queues = nctrl->queue_count - 1;

 		set->timeout = NVME_IO_TIMEOUT;

@@ -1160,7 +1160,7 @@ static void nvme_rdma_unmap_data(struct nvme_rdma_queue *queue,

 	}

 	ib_dma_unmap_sg(ibdev, req->sg_table.sgl, req->nents, rq_dma_dir(rq));

-	sg_free_table_chained(&req->sg_table, SG_CHUNK_SIZE);

+	sg_free_table_chained(&req->sg_table, NVME_INLINE_SG_CNT);

 }

 static int nvme_rdma_set_sg_null(struct nvme_command *c)

@@ -1276,7 +1276,7 @@ static int nvme_rdma_map_data(struct nvme_rdma_queue *queue,

 	req->sg_table.sgl = req->first_sgl;

 	ret = sg_alloc_table_chained(&req->sg_table,

 			blk_rq_nr_phys_segments(rq), req->sg_table.sgl,

-			SG_CHUNK_SIZE);

+			NVME_INLINE_SG_CNT);

 	if (ret)

 		return -ENOMEM;

@@ -1314,7 +1314,7 @@ static int nvme_rdma_map_data(struct nvme_rdma_queue *queue,

 out_unmap_sg:

 	ib_dma_unmap_sg(ibdev, req->sg_table.sgl, req->nents, rq_dma_dir(rq));

 out_free_table:

-	sg_free_table_chained(&req->sg_table, SG_CHUNK_SIZE);

+	sg_free_table_chained(&req->sg_table, NVME_INLINE_SG_CNT);

 	return ret;

 }

-- 

2.16.4