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svcrdma: Avoid DMA mapping small RPC Replies

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On some platforms, DMA mapping part of a page is more costly than
copying bytes. Indeed, not involving the I/O MMU can help the
RPC/RDMA transport scale better for tiny I/Os across more RDMA
devices. This is because interaction with the I/O MMU is eliminated
for each of these small I/Os. Without the explicit unmapping, the
NIC no longer needs to do a costly internal TLB shoot down for
buffers that are just a handful of bytes.

Since pull-up is now a more a frequent operation, I've introduced a
trace point in the pull-up path. It can be used for debugging or
user-space tools that count pull-up frequency.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
This commit is contained in:
Chuck Lever 2020-03-03 13:28:14 -05:00
parent aee4b74a3f
commit 0dabe948f2
3 changed files with 31 additions and 1 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

1
include/linux/sunrpc/svc_rdma.h
View File

@ -52,6 +52,7 @@
/* Default and maximum inline threshold sizes */
enum {
RPCRDMA_PULLUP_THRESH = RPCRDMA_V1_DEF_INLINE_SIZE >> 1,
RPCRDMA_DEF_INLINE_THRESH = 4096,
RPCRDMA_MAX_INLINE_THRESH = 65536
};

18
include/trace/events/rpcrdma.h
View File

@ -1639,6 +1639,24 @@ TRACE_EVENT(svcrdma_dma_map_rwctx,
)
);
TRACE_EVENT(svcrdma_send_pullup,
TP_PROTO(
unsigned int len
),
TP_ARGS(len),
TP_STRUCT__entry(
__field(unsigned int, len)
),
TP_fast_assign(
__entry->len = len;
),
TP_printk("len=%u", __entry->len)
);
TRACE_EVENT(svcrdma_send_failed,
TP_PROTO(
const struct svc_rqst *rqst,

13
net/sunrpc/xprtrdma/svc_rdma_sendto.c
View File

@ -541,6 +541,7 @@ static int svc_rdma_dma_map_buf(struct svcxprt_rdma *rdma,
/**
* svc_rdma_pull_up_needed - Determine whether to use pull-up
* @rdma: controlling transport
* @sctxt: send_ctxt for the Send WR
* @rctxt: Write and Reply chunks provided by client
* @xdr: xdr_buf containing RPC message to transmit
*
@ -549,11 +550,20 @@ static int svc_rdma_dma_map_buf(struct svcxprt_rdma *rdma,
* %false otherwise
*/
static bool svc_rdma_pull_up_needed(struct svcxprt_rdma *rdma,
struct svc_rdma_send_ctxt *sctxt,
const struct svc_rdma_recv_ctxt *rctxt,
struct xdr_buf *xdr)
{
int elements;
/* For small messages, copying bytes is cheaper than DMA mapping.
*/
if (sctxt->sc_hdrbuf.len + xdr->len < RPCRDMA_PULLUP_THRESH)
return true;
/* Check whether the xdr_buf has more elements than can
* fit in a single RDMA Send.
*/
/* xdr->head */
elements = 1;
@ -636,6 +646,7 @@ static int svc_rdma_pull_up_reply_msg(struct svcxprt_rdma *rdma,
memcpy(dst, tailbase, taillen);
sctxt->sc_sges[0].length += xdr->len;
trace_svcrdma_send_pullup(sctxt->sc_sges[0].length);
return 0;
}
@ -675,7 +686,7 @@ int svc_rdma_map_reply_msg(struct svcxprt_rdma *rdma,
/* For pull-up, svc_rdma_send() will sync the transport header.
* No additional DMA mapping is necessary.
*/
if (svc_rdma_pull_up_needed(rdma, rctxt, xdr))
if (svc_rdma_pull_up_needed(rdma, sctxt, rctxt, xdr))
return svc_rdma_pull_up_reply_msg(rdma, sctxt, rctxt, xdr);
++sctxt->sc_cur_sge_no;