[PATCH v2 2/5] SUNRPC: Optimize xdr_reserve_space()

[Chuck Lever <chuck.lever@xxxxxxxxxx>]

Transitioning between encode buffers is quite infrequent. It happens
about 1 time in 400 calls to xdr_reserve_space(), measured on NFSD
with a typical build/test workload.

Force the compiler to remove that code from xdr_reserve_space(),
which is a hot path on both the server and the client. This change
reduces the size of xdr_reserve_space() from 10 cache lines to 2
when compiled with -Os.

Signed-off-by: Chuck Lever <chuck.lever@xxxxxxxxxx>
---
 include/linux/sunrpc/xdr.h |   16 +++++++++++++++-
 net/sunrpc/xdr.c           |   17 ++++++++++-------
 2 files changed, 25 insertions(+), 8 deletions(-)

diff --git a/include/linux/sunrpc/xdr.h b/include/linux/sunrpc/xdr.h
index 4417f667c757..5860f32e3958 100644
--- a/include/linux/sunrpc/xdr.h
+++ b/include/linux/sunrpc/xdr.h
@@ -243,7 +243,7 @@ extern void xdr_init_encode(struct xdr_stream *xdr, struct xdr_buf *buf,
 extern __be32 *xdr_reserve_space(struct xdr_stream *xdr, size_t nbytes);
 extern int xdr_reserve_space_vec(struct xdr_stream *xdr, struct kvec *vec,
 		size_t nbytes);
-extern void xdr_commit_encode(struct xdr_stream *xdr);
+extern void __xdr_commit_encode(struct xdr_stream *xdr);
 extern void xdr_truncate_encode(struct xdr_stream *xdr, size_t len);
 extern int xdr_restrict_buflen(struct xdr_stream *xdr, int newbuflen);
 extern void xdr_write_pages(struct xdr_stream *xdr, struct page **pages,
@@ -306,6 +306,20 @@ xdr_reset_scratch_buffer(struct xdr_stream *xdr)
 	xdr_set_scratch_buffer(xdr, NULL, 0);
 }
 
+/**
+ * xdr_commit_encode - Ensure all data is written to xdr->buf
+ * @xdr: pointer to xdr_stream
+ *
+ * Handle encoding across page boundaries by giving the caller a
+ * temporary location to write to, then later copying the data into
+ * place. __xdr_commit_encode() does that copying.
+ */
+static inline void xdr_commit_encode(struct xdr_stream *xdr)
+{
+	if (unlikely(xdr->scratch.iov_len))
+		__xdr_commit_encode(xdr);
+}
+
 /**
  * xdr_stream_remaining - Return the number of bytes remaining in the stream
  * @xdr: pointer to struct xdr_stream
diff --git a/net/sunrpc/xdr.c b/net/sunrpc/xdr.c
index b57cf9df4de8..1ad8b4ef14de 100644
--- a/net/sunrpc/xdr.c
+++ b/net/sunrpc/xdr.c
@@ -919,7 +919,7 @@ void xdr_init_encode(struct xdr_stream *xdr, struct xdr_buf *buf, __be32 *p,
 EXPORT_SYMBOL_GPL(xdr_init_encode);
 
 /**
- * xdr_commit_encode - Ensure all data is written to buffer
+ * __xdr_commit_encode - Ensure all data is written to buffer
  * @xdr: pointer to xdr_stream
  *
  * We handle encoding across page boundaries by giving the caller a
@@ -931,22 +931,25 @@ EXPORT_SYMBOL_GPL(xdr_init_encode);
  * required at the end of encoding, or any other time when the xdr_buf
  * data might be read.
  */
-inline void xdr_commit_encode(struct xdr_stream *xdr)
+void __xdr_commit_encode(struct xdr_stream *xdr)
 {
 	int shift = xdr->scratch.iov_len;
 	void *page;
 
-	if (shift == 0)
-		return;
 	page = page_address(*xdr->page_ptr);
 	memcpy(xdr->scratch.iov_base, page, shift);
 	memmove(page, page + shift, (void *)xdr->p - page);
 	xdr_reset_scratch_buffer(xdr);
 }
-EXPORT_SYMBOL_GPL(xdr_commit_encode);
+EXPORT_SYMBOL_GPL(__xdr_commit_encode);
 
-static __be32 *xdr_get_next_encode_buffer(struct xdr_stream *xdr,
-		size_t nbytes)
+/*
+ * The buffer space to be reserved crosses the boundary between
+ * xdr->buf->head and xdr->buf->pages, or between two pages
+ * in xdr->buf->pages.
+ */
+static noinline __be32 *xdr_get_next_encode_buffer(struct xdr_stream *xdr,
+						   size_t nbytes)
 {
 	__be32 *p;
 	int space_left;