Commit 0dabe948 authored by Chuck Lever's avatar Chuck Lever
Browse files

svcrdma: Avoid DMA mapping small RPC Replies 



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: default avatarChuck Lever <chuck.lever@oracle.com>
parent aee4b74a

include/linux/sunrpc/svc_rdma.h

+1 −0
Original line number Diff line number Diff line
@@ -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

};

include/trace/events/rpcrdma.h

+18 −0
Original line number Diff line number Diff line
@@ -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,

net/sunrpc/xprtrdma/svc_rdma_sendto.c

+12 −1
Original line number Diff line number Diff line
@@ -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;

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