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: 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>
diff --git a/drivers/nvme/host/rdma.c b/drivers/nvme/host/rdma.c
index dce5945..2a47c6c 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;
}