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xprtrdma: Initialize separate RPC call and reply buffers

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RPC-over-RDMA needs to separate its RPC call and reply buffers.

 o When an RPC Call is sent, rq_snd_buf is DMA mapped for an RDMA
   Send operation using DMA_TO_DEVICE

 o If the client expects a large RPC reply, it DMA maps rq_rcv_buf
   as part of a Reply chunk using DMA_FROM_DEVICE

The two mappings are for data movement in opposite directions.

DMA-API.txt suggests that if these mappings share a DMA cacheline,
bad things can happen. This could occur in the final bytes of
rq_snd_buf and the first bytes of rq_rcv_buf if the two buffers
happen to share a DMA cacheline.

On x86_64 the cacheline size is typically 8 bytes, and RPC call
messages are usually much smaller than the send buffer, so this
hasn't been a noticeable problem. But the DMA cacheline size can be
larger on other platforms.

Also, often rq_rcv_buf starts most of the way into a page, thus
an additional RDMA segment is needed to map and register the end of
that buffer. Try to avoid that scenario to reduce the cost of
registering and invalidating Reply chunks.

Instead of carrying a single regbuf that covers both rq_snd_buf and
rq_rcv_buf, each struct rpcrdma_req now carries one regbuf for
rq_snd_buf and one regbuf for rq_rcv_buf.

Some incidental changes worth noting:

- To clear out some spaghetti, refactor xprt_rdma_allocate.
- The value stored in rg_size is the same as the value stored in
  the iov.length field, so eliminate rg_size

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
This commit is contained in:
Chuck Lever
2016-09-15 10:55:53 -04:00
committed by Anna Schumaker
parent 5a6d1db455
commit 9c40c49f14
3 changed files with 99 additions and 59 deletions
Download Patch File Download Diff File Expand all files Collapse all files

6
net/sunrpc/xprtrdma/xprt_rdma.h
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@@ -112,7 +112,6 @@ struct rpcrdma_ep {
*/
struct rpcrdma_regbuf {
size_t rg_size;
struct ib_sge rg_iov;
__be32 rg_base[0] __attribute__ ((aligned(256)));
};
@@ -285,8 +284,9 @@ struct rpcrdma_req {
struct rpcrdma_buffer *rl_buffer;
struct rpcrdma_rep *rl_reply;/* holder for reply buffer */
struct ib_sge rl_send_iov[RPCRDMA_MAX_IOVS];
struct rpcrdma_regbuf *rl_rdmabuf;
struct rpcrdma_regbuf *rl_sendbuf;
struct rpcrdma_regbuf *rl_rdmabuf; /* xprt header */
struct rpcrdma_regbuf *rl_sendbuf; /* rq_snd_buf */
struct rpcrdma_regbuf *rl_recvbuf; /* rq_rcv_buf */
struct ib_cqe rl_cqe;
struct list_head rl_all;