[RFC PATCH 09/15] svcrdma: Release write chunk resources without re-queuing

[Chuck Lever <cel@kernel.org>]

From: Chuck Lever <chuck.lever@oracle.com>

Each RDMA Send completion triggers a cascade of work items on the
svcrdma_wq unbound workqueue:

  ib_cq_poll_work (on ib_comp_wq, per-CPU)
    -> svc_rdma_send_ctxt_put -> queue_work    [work item 1]
      -> svc_rdma_write_info_free -> queue_work [work item 2]

Every transition through queue_work contends on the unbound
pool's spinlock. Profiling an 8KB NFSv3 read/write workload
over RDMA shows about 4% of total CPU cycles spent on this
lock, with the cascading re-queue of write_info release
contributing roughly 1%.

The initial queue_work in svc_rdma_send_ctxt_put is needed to
move release work off the CQ completion context (which runs on
a per-CPU bound workqueue). However, once executing on
svcrdma_wq, there is no need to re-queue for each write_info
structure. svc_rdma_reply_chunk_release already calls
svc_rdma_cc_release inline, confirming these operations are
safe in workqueue and nfsd thread context alike.

Release write chunk resources inline in
svc_rdma_write_info_free, removing the intermediate
svc_rdma_write_info_free_async work item and the wi_work
field from struct svc_rdma_write_info.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
---
 include/linux/sunrpc/svc_rdma.h   |  1 -
 net/sunrpc/xprtrdma/svc_rdma_rw.c | 13 ++-----------
 2 files changed, 2 insertions(+), 12 deletions(-)

diff --git a/include/linux/sunrpc/svc_rdma.h b/include/linux/sunrpc/svc_rdma.h
index e6cb52285818..9691238df47f 100644
--- a/include/linux/sunrpc/svc_rdma.h
+++ b/include/linux/sunrpc/svc_rdma.h
@@ -232,7 +232,6 @@ struct svc_rdma_write_info {
 	unsigned int		wi_next_off;
 
 	struct svc_rdma_chunk_ctxt	wi_cc;
-	struct work_struct	wi_work;
 };
 
 struct svc_rdma_send_ctxt {
diff --git a/net/sunrpc/xprtrdma/svc_rdma_rw.c b/net/sunrpc/xprtrdma/svc_rdma_rw.c
index b0bbebbecb3e..e62abdbf84f8 100644
--- a/net/sunrpc/xprtrdma/svc_rdma_rw.c
+++ b/net/sunrpc/xprtrdma/svc_rdma_rw.c
@@ -215,19 +215,10 @@ svc_rdma_write_info_alloc(struct svcxprt_rdma *rdma,
 	return info;
 }
 
-static void svc_rdma_write_info_free_async(struct work_struct *work)
-{
-	struct svc_rdma_write_info *info;
-
-	info = container_of(work, struct svc_rdma_write_info, wi_work);
-	svc_rdma_cc_release(info->wi_rdma, &info->wi_cc, DMA_TO_DEVICE);
-	kfree(info);
-}
-
 static void svc_rdma_write_info_free(struct svc_rdma_write_info *info)
 {
-	INIT_WORK(&info->wi_work, svc_rdma_write_info_free_async);
-	queue_work(svcrdma_wq, &info->wi_work);
+	svc_rdma_cc_release(info->wi_rdma, &info->wi_cc, DMA_TO_DEVICE);
+	kfree(info);
 }
 
 /**
-- 
2.52.0