From b541dd554bc0442f7ff8c6cab6c5460c044913c8 Mon Sep 17 00:00:00 2001 From: Chuck Lever Date: Tue, 21 Nov 2023 11:40:13 -0500 Subject: svcrdma: Eliminate allocation of recv_ctxt objects in backchannel The svc_rdma_recv_ctxt free list uses a lockless list to avoid the need for a spin lock in the fast path. llist_del_first(), which is used by svc_rdma_recv_ctxt_get(), requires serialization, however, when there are multiple list producers that are unserialized. I mistakenly thought there was only one caller of svc_rdma_recv_ctxt_get() (svc_rdma_refresh_recvs()), thus explicit serialization would not be necessary. But there is another caller: svc_rdma_bc_sendto(), and these two are not serialized against each other. I haven't seen ill effects that I could directly ascribe to a lack of serialization. It's just an observation based on code audit. When DMA-mapping before sending a Reply, the passed-in struct svc_rdma_recv_ctxt is used only for its write and reply PCLs. These are currently always empty in the backchannel case. So, instead of passing a full svc_rdma_recv_ctxt object to svc_rdma_map_reply_msg(), let's pass in just the Write and Reply PCLs. This change makes it unnecessary for the backchannel to acquire a dummy svc_rdma_recv_ctxt object when sending an RPC Call. The need for svc_rdma_recv_ctxt free list serialization is now completely avoided. Signed-off-by: Chuck Lever --- net/sunrpc/xprtrdma/svc_rdma_backchannel.c | 11 ++++------- net/sunrpc/xprtrdma/svc_rdma_sendto.c | 31 ++++++++++++++++-------------- 2 files changed, 21 insertions(+), 21 deletions(-) (limited to 'net/sunrpc/xprtrdma') diff --git a/net/sunrpc/xprtrdma/svc_rdma_backchannel.c b/net/sunrpc/xprtrdma/svc_rdma_backchannel.c index 7420a2c990c7..c9be6778643b 100644 --- a/net/sunrpc/xprtrdma/svc_rdma_backchannel.c +++ b/net/sunrpc/xprtrdma/svc_rdma_backchannel.c @@ -76,15 +76,12 @@ static int svc_rdma_bc_sendto(struct svcxprt_rdma *rdma, struct rpc_rqst *rqst, struct svc_rdma_send_ctxt *sctxt) { - struct svc_rdma_recv_ctxt *rctxt; + struct svc_rdma_pcl empty_pcl; int ret; - rctxt = svc_rdma_recv_ctxt_get(rdma); - if (!rctxt) - return -EIO; - - ret = svc_rdma_map_reply_msg(rdma, sctxt, rctxt, &rqst->rq_snd_buf); - svc_rdma_recv_ctxt_put(rdma, rctxt); + pcl_init(&empty_pcl); + ret = svc_rdma_map_reply_msg(rdma, sctxt, &empty_pcl, &empty_pcl, + &rqst->rq_snd_buf); if (ret < 0) return -EIO; diff --git a/net/sunrpc/xprtrdma/svc_rdma_sendto.c b/net/sunrpc/xprtrdma/svc_rdma_sendto.c index c6644cca52c5..45735f74eb86 100644 --- a/net/sunrpc/xprtrdma/svc_rdma_sendto.c +++ b/net/sunrpc/xprtrdma/svc_rdma_sendto.c @@ -653,7 +653,7 @@ static int svc_rdma_xb_count_sges(const struct xdr_buf *xdr, * 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 + * @write_pcl: Write chunk list provided by client * @xdr: xdr_buf containing RPC message to transmit * * Returns: @@ -662,7 +662,7 @@ static int svc_rdma_xb_count_sges(const struct xdr_buf *xdr, */ static bool svc_rdma_pull_up_needed(const struct svcxprt_rdma *rdma, const struct svc_rdma_send_ctxt *sctxt, - const struct svc_rdma_recv_ctxt *rctxt, + const struct svc_rdma_pcl *write_pcl, const struct xdr_buf *xdr) { /* Resources needed for the transport header */ @@ -672,7 +672,7 @@ static bool svc_rdma_pull_up_needed(const struct svcxprt_rdma *rdma, }; int ret; - ret = pcl_process_nonpayloads(&rctxt->rc_write_pcl, xdr, + ret = pcl_process_nonpayloads(write_pcl, xdr, svc_rdma_xb_count_sges, &args); if (ret < 0) return false; @@ -728,7 +728,7 @@ static int svc_rdma_xb_linearize(const struct xdr_buf *xdr, * svc_rdma_pull_up_reply_msg - Copy Reply into a single buffer * @rdma: controlling transport * @sctxt: send_ctxt for the Send WR; xprt hdr is already prepared - * @rctxt: Write and Reply chunks provided by client + * @write_pcl: Write chunk list provided by client * @xdr: prepared xdr_buf containing RPC message * * The device is not capable of sending the reply directly. @@ -743,7 +743,7 @@ static int svc_rdma_xb_linearize(const struct xdr_buf *xdr, */ static int svc_rdma_pull_up_reply_msg(const struct svcxprt_rdma *rdma, struct svc_rdma_send_ctxt *sctxt, - const struct svc_rdma_recv_ctxt *rctxt, + const struct svc_rdma_pcl *write_pcl, const struct xdr_buf *xdr) { struct svc_rdma_pullup_data args = { @@ -751,7 +751,7 @@ static int svc_rdma_pull_up_reply_msg(const struct svcxprt_rdma *rdma, }; int ret; - ret = pcl_process_nonpayloads(&rctxt->rc_write_pcl, xdr, + ret = pcl_process_nonpayloads(write_pcl, xdr, svc_rdma_xb_linearize, &args); if (ret < 0) return ret; @@ -764,7 +764,8 @@ static int svc_rdma_pull_up_reply_msg(const struct svcxprt_rdma *rdma, /* svc_rdma_map_reply_msg - DMA map the buffer holding RPC message * @rdma: controlling transport * @sctxt: send_ctxt for the Send WR - * @rctxt: Write and Reply chunks provided by client + * @write_pcl: Write chunk list provided by client + * @reply_pcl: Reply chunk provided by client * @xdr: prepared xdr_buf containing RPC message * * Returns: @@ -776,7 +777,8 @@ static int svc_rdma_pull_up_reply_msg(const struct svcxprt_rdma *rdma, */ int svc_rdma_map_reply_msg(struct svcxprt_rdma *rdma, struct svc_rdma_send_ctxt *sctxt, - const struct svc_rdma_recv_ctxt *rctxt, + const struct svc_rdma_pcl *write_pcl, + const struct svc_rdma_pcl *reply_pcl, const struct xdr_buf *xdr) { struct svc_rdma_map_data args = { @@ -789,18 +791,18 @@ int svc_rdma_map_reply_msg(struct svcxprt_rdma *rdma, sctxt->sc_sges[0].length = sctxt->sc_hdrbuf.len; /* If there is a Reply chunk, nothing follows the transport - * header, and we're done here. + * header, so there is nothing to map. */ - if (!pcl_is_empty(&rctxt->rc_reply_pcl)) + if (!pcl_is_empty(reply_pcl)) return 0; /* For pull-up, svc_rdma_send() will sync the transport header. * No additional DMA mapping is necessary. */ - if (svc_rdma_pull_up_needed(rdma, sctxt, rctxt, xdr)) - return svc_rdma_pull_up_reply_msg(rdma, sctxt, rctxt, xdr); + if (svc_rdma_pull_up_needed(rdma, sctxt, write_pcl, xdr)) + return svc_rdma_pull_up_reply_msg(rdma, sctxt, write_pcl, xdr); - return pcl_process_nonpayloads(&rctxt->rc_write_pcl, xdr, + return pcl_process_nonpayloads(write_pcl, xdr, svc_rdma_xb_dma_map, &args); } @@ -848,7 +850,8 @@ static int svc_rdma_send_reply_msg(struct svcxprt_rdma *rdma, { int ret; - ret = svc_rdma_map_reply_msg(rdma, sctxt, rctxt, &rqstp->rq_res); + ret = svc_rdma_map_reply_msg(rdma, sctxt, &rctxt->rc_write_pcl, + &rctxt->rc_reply_pcl, &rqstp->rq_res); if (ret < 0) return ret; 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