Commit 38e18002 authored by Israel Rukshin's avatar Israel Rukshin Committed by Keith Busch
Browse files

nvme-rdma: Avoid preallocating big SGL for data 



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: default avatarChristoph Hellwig <hch@lst.de>
Reviewed-by: default avatarMax Gurtovoy <maxg@mellanox.com>
Reviewed-by: default avatarChristoph Hellwig <hch@lst.de>
Signed-off-by: default avatarIsrael Rukshin <israelr@mellanox.com>
Signed-off-by: default avatarKeith Busch <kbusch@kernel.org>
parent be2eca94

drivers/nvme/host/nvme.h

+6 −0
Original line number Diff line number Diff line
@@ -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;

drivers/nvme/host/rdma.c

+5 −5
Original line number Diff line number Diff line
@@ -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 @@ out: 
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;

}

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