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Subject: Re: [PATCH v1 6/6] xprtrdma: Pull up sometimes
To:     Chuck Lever <chuck.lever@oracle.com>, linux-rdma@vger.kernel.org,
        linux-nfs@vger.kernel.org
References: <20191017182811.2517.25676.stgit@oracle-102.nfsv4bat.org>
 <20191017183152.2517.67599.stgit@oracle-102.nfsv4bat.org>
From:   Tom Talpey <tom@talpey.com>
Message-ID: <f5075a20-7c9a-773f-e76b-11cba1ab0f16@talpey.com>
Date:   Fri, 18 Oct 2019 16:17:01 -0400
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On 10/17/2019 2:31 PM, Chuck Lever wrote:
> On some platforms, DMA mapping part of a page is more costly than
> copying bytes. Restore the pull-up code and use that when we
> think it's going to be faster. The heuristic for now is to pull-up
> when the size of the RPC message body fits in the buffer underlying
> the head iovec.
> 
> 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, as is handling a Send
> completion, 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.

This is good stuff. Do you have any performance data for the new
strategy, especially latencies and local CPU cycles per byte?

Tom.

> Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
> ---
>   net/sunrpc/xprtrdma/backchannel.c |    2 -
>   net/sunrpc/xprtrdma/rpc_rdma.c    |   82 +++++++++++++++++++++++++++++++++++--
>   net/sunrpc/xprtrdma/verbs.c       |    2 -
>   net/sunrpc/xprtrdma/xprt_rdma.h   |    2 +
>   4 files changed, 81 insertions(+), 7 deletions(-)
> 
> diff --git a/include/trace/events/rpcrdma.h b/include/trace/events/rpcrdma.h
> index 16572ae7..336a65d 100644
> --- a/include/trace/events/rpcrdma.h
> +++ b/include/trace/events/rpcrdma.h
> @@ -532,6 +532,8 @@
>   DEFINE_WRCH_EVENT(reply);
>   
>   TRACE_DEFINE_ENUM(rpcrdma_noch);
> +TRACE_DEFINE_ENUM(rpcrdma_noch_pullup);
> +TRACE_DEFINE_ENUM(rpcrdma_noch_mapped);
>   TRACE_DEFINE_ENUM(rpcrdma_readch);
>   TRACE_DEFINE_ENUM(rpcrdma_areadch);
>   TRACE_DEFINE_ENUM(rpcrdma_writech);
> @@ -540,6 +542,8 @@
>   #define xprtrdma_show_chunktype(x)					\
>   		__print_symbolic(x,					\
>   				{ rpcrdma_noch, "inline" },		\
> +				{ rpcrdma_noch_pullup, "pullup" },	\
> +				{ rpcrdma_noch_mapped, "mapped" },	\
>   				{ rpcrdma_readch, "read list" },	\
>   				{ rpcrdma_areadch, "*read list" },	\
>   				{ rpcrdma_writech, "write list" },	\
> diff --git a/net/sunrpc/xprtrdma/backchannel.c b/net/sunrpc/xprtrdma/backchannel.c
> index 50e075f..1168524 100644
> --- a/net/sunrpc/xprtrdma/backchannel.c
> +++ b/net/sunrpc/xprtrdma/backchannel.c
> @@ -79,7 +79,7 @@ static int rpcrdma_bc_marshal_reply(struct rpc_rqst *rqst)
>   	*p = xdr_zero;
>   
>   	if (rpcrdma_prepare_send_sges(r_xprt, req, RPCRDMA_HDRLEN_MIN,
> -				      &rqst->rq_snd_buf, rpcrdma_noch))
> +				      &rqst->rq_snd_buf, rpcrdma_noch_pullup))
>   		return -EIO;
>   
>   	trace_xprtrdma_cb_reply(rqst);
> diff --git a/net/sunrpc/xprtrdma/rpc_rdma.c b/net/sunrpc/xprtrdma/rpc_rdma.c
> index a441dbf..4ad8889 100644
> --- a/net/sunrpc/xprtrdma/rpc_rdma.c
> +++ b/net/sunrpc/xprtrdma/rpc_rdma.c
> @@ -392,7 +392,7 @@ static int rpcrdma_encode_read_list(struct rpcrdma_xprt *r_xprt,
>   	unsigned int pos;
>   	int nsegs;
>   
> -	if (rtype == rpcrdma_noch)
> +	if (rtype == rpcrdma_noch_pullup || rtype == rpcrdma_noch_mapped)
>   		goto done;
>   
>   	pos = rqst->rq_snd_buf.head[0].iov_len;
> @@ -691,6 +691,72 @@ static bool rpcrdma_prepare_tail_iov(struct rpcrdma_req *req,
>   	return false;
>   }
>   
> +/* Copy the tail to the end of the head buffer.
> + */
> +static void rpcrdma_pullup_tail_iov(struct rpcrdma_xprt *r_xprt,
> +				    struct rpcrdma_req *req,
> +				    struct xdr_buf *xdr)
> +{
> +	unsigned char *dst;
> +
> +	dst = (unsigned char *)xdr->head[0].iov_base;
> +	dst += xdr->head[0].iov_len + xdr->page_len;
> +	memmove(dst, xdr->tail[0].iov_base, xdr->tail[0].iov_len);
> +	r_xprt->rx_stats.pullup_copy_count += xdr->tail[0].iov_len;
> +}
> +
> +/* Copy pagelist content into the head buffer.
> + */
> +static void rpcrdma_pullup_pagelist(struct rpcrdma_xprt *r_xprt,
> +				    struct rpcrdma_req *req,
> +				    struct xdr_buf *xdr)
> +{
> +	unsigned int len, page_base, remaining;
> +	struct page **ppages;
> +	unsigned char *src, *dst;
> +
> +	dst = (unsigned char *)xdr->head[0].iov_base;
> +	dst += xdr->head[0].iov_len;
> +	ppages = xdr->pages + (xdr->page_base >> PAGE_SHIFT);
> +	page_base = offset_in_page(xdr->page_base);
> +	remaining = xdr->page_len;
> +	while (remaining) {
> +		src = page_address(*ppages);
> +		src += page_base;
> +		len = min_t(unsigned int, PAGE_SIZE - page_base, remaining);
> +		memcpy(dst, src, len);
> +		r_xprt->rx_stats.pullup_copy_count += len;
> +
> +		ppages++;
> +		dst += len;
> +		remaining -= len;
> +		page_base = 0;
> +	}
> +}
> +
> +/* Copy the contents of @xdr into @rl_sendbuf and DMA sync it.
> + * When the head, pagelist, and tail are small, a pull-up copy
> + * is considerably less costly than DMA mapping the components
> + * of @xdr.
> + *
> + * Assumptions:
> + *  - the caller has already verified that the total length
> + *    of the RPC Call body will fit into @rl_sendbuf.
> + */
> +static bool rpcrdma_prepare_noch_pullup(struct rpcrdma_xprt *r_xprt,
> +					struct rpcrdma_req *req,
> +					struct xdr_buf *xdr)
> +{
> +	if (unlikely(xdr->tail[0].iov_len))
> +		rpcrdma_pullup_tail_iov(r_xprt, req, xdr);
> +
> +	if (unlikely(xdr->page_len))
> +		rpcrdma_pullup_pagelist(r_xprt, req, xdr);
> +
> +	/* The whole RPC message resides in the head iovec now */
> +	return rpcrdma_prepare_head_iov(r_xprt, req, xdr->len);
> +}
> +
>   static bool rpcrdma_prepare_noch_mapped(struct rpcrdma_xprt *r_xprt,
>   					struct rpcrdma_req *req,
>   					struct xdr_buf *xdr)
> @@ -779,7 +845,11 @@ inline int rpcrdma_prepare_send_sges(struct rpcrdma_xprt *r_xprt,
>   		goto out_unmap;
>   
>   	switch (rtype) {
> -	case rpcrdma_noch:
> +	case rpcrdma_noch_pullup:
> +		if (!rpcrdma_prepare_noch_pullup(r_xprt, req, xdr))
> +			goto out_unmap;
> +		break;
> +	case rpcrdma_noch_mapped:
>   		if (!rpcrdma_prepare_noch_mapped(r_xprt, req, xdr))
>   			goto out_unmap;
>   		break;
> @@ -827,6 +897,7 @@ inline int rpcrdma_prepare_send_sges(struct rpcrdma_xprt *r_xprt,
>   	struct rpcrdma_req *req = rpcr_to_rdmar(rqst);
>   	struct xdr_stream *xdr = &req->rl_stream;
>   	enum rpcrdma_chunktype rtype, wtype;
> +	struct xdr_buf *buf = &rqst->rq_snd_buf;
>   	bool ddp_allowed;
>   	__be32 *p;
>   	int ret;
> @@ -884,8 +955,9 @@ inline int rpcrdma_prepare_send_sges(struct rpcrdma_xprt *r_xprt,
>   	 */
>   	if (rpcrdma_args_inline(r_xprt, rqst)) {
>   		*p++ = rdma_msg;
> -		rtype = rpcrdma_noch;
> -	} else if (ddp_allowed && rqst->rq_snd_buf.flags & XDRBUF_WRITE) {
> +		rtype = buf->len < rdmab_length(req->rl_sendbuf) ?
> +			rpcrdma_noch_pullup : rpcrdma_noch_mapped;
> +	} else if (ddp_allowed && buf->flags & XDRBUF_WRITE) {
>   		*p++ = rdma_msg;
>   		rtype = rpcrdma_readch;
>   	} else {
> @@ -927,7 +999,7 @@ inline int rpcrdma_prepare_send_sges(struct rpcrdma_xprt *r_xprt,
>   		goto out_err;
>   
>   	ret = rpcrdma_prepare_send_sges(r_xprt, req, req->rl_hdrbuf.len,
> -					&rqst->rq_snd_buf, rtype);
> +					buf, rtype);
>   	if (ret)
>   		goto out_err;
>   
> diff --git a/net/sunrpc/xprtrdma/verbs.c b/net/sunrpc/xprtrdma/verbs.c
> index 2f46582..a514e2c 100644
> --- a/net/sunrpc/xprtrdma/verbs.c
> +++ b/net/sunrpc/xprtrdma/verbs.c
> @@ -1165,7 +1165,7 @@ int rpcrdma_buffer_create(struct rpcrdma_xprt *r_xprt)
>   	for (i = 0; i < buf->rb_max_requests; i++) {
>   		struct rpcrdma_req *req;
>   
> -		req = rpcrdma_req_create(r_xprt, RPCRDMA_V1_DEF_INLINE_SIZE,
> +		req = rpcrdma_req_create(r_xprt, RPCRDMA_V1_DEF_INLINE_SIZE * 2,
>   					 GFP_KERNEL);
>   		if (!req)
>   			goto out;
> diff --git a/net/sunrpc/xprtrdma/xprt_rdma.h b/net/sunrpc/xprtrdma/xprt_rdma.h
> index cdd6a3d..5d15140 100644
> --- a/net/sunrpc/xprtrdma/xprt_rdma.h
> +++ b/net/sunrpc/xprtrdma/xprt_rdma.h
> @@ -554,6 +554,8 @@ struct rpcrdma_mr_seg *frwr_map(struct rpcrdma_xprt *r_xprt,
>   
>   enum rpcrdma_chunktype {
>   	rpcrdma_noch = 0,
> +	rpcrdma_noch_pullup,
> +	rpcrdma_noch_mapped,
>   	rpcrdma_readch,
>   	rpcrdma_areadch,
>   	rpcrdma_writech,
> 
> 
>