[PATCH 1/3] SUNRPC: Replace division by multiplication in calculation of queue length

[PATCH 1/3] SUNRPC: Replace division by multiplication in calculation of queue length

[Trond Myklebust]

When checking whether or not a particular xprt queue length is shorter
than the average queue length for all xprts, prefer to use multiplication
rather than division for performance reasons.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
---
 net/sunrpc/xprtmultipath.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/net/sunrpc/xprtmultipath.c b/net/sunrpc/xprtmultipath.c
index 9d66ce53355d..5df4e7adedf0 100644
--- a/net/sunrpc/xprtmultipath.c
+++ b/net/sunrpc/xprtmultipath.c
@@ -322,7 +322,6 @@ struct rpc_xprt *xprt_iter_next_entry_roundrobin(struct rpc_xprt_iter *xpi)
 	struct rpc_xprt *xprt;
 	unsigned long xprt_queuelen;
 	unsigned long xps_queuelen;
-	unsigned long xps_avglen;
 
 	do {
 		xprt = xprt_iter_next_entry_multiple(xpi,
@@ -333,8 +332,8 @@ struct rpc_xprt *xprt_iter_next_entry_roundrobin(struct rpc_xprt_iter *xpi)
 		if (xprt_queuelen <= 2)
 			break;
 		xps_queuelen = atomic_long_read(&xps->xps_queuelen);
-		xps_avglen = DIV_ROUND_UP(xps_queuelen, xps->xps_nactive);
-	} while (xprt_queuelen > xps_avglen);
+		/* Exit loop if xprt_queuelen <= average queue length */
+	} while (xprt_queuelen * READ_ONCE(xps->xps_nactive) > xps_queuelen);
 	return xprt;
 }
 
-- 
2.21.0

[PATCH 2/3] SUNRPC: Skip zero-refcount transports

[Trond Myklebust]

When looking for the next transport to use for an RPC call, skip those
that are in the process of being destroyed and that have a zero refcount.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
---
 net/sunrpc/xprtmultipath.c | 20 +++++++++++++++++++-
 1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/net/sunrpc/xprtmultipath.c b/net/sunrpc/xprtmultipath.c
index 5df4e7adedf0..c12778e1235e 100644
--- a/net/sunrpc/xprtmultipath.c
+++ b/net/sunrpc/xprtmultipath.c
@@ -193,10 +193,22 @@ void xprt_iter_default_rewind(struct rpc_xprt_iter *xpi)
 	WRITE_ONCE(xpi->xpi_cursor, NULL);
 }
 
+static
+bool xprt_is_active(const struct rpc_xprt *xprt)
+{
+	return kref_read(&xprt->kref) != 0;
+}
+
 static
 struct rpc_xprt *xprt_switch_find_first_entry(struct list_head *head)
 {
-	return list_first_or_null_rcu(head, struct rpc_xprt, xprt_switch);
+	struct rpc_xprt *pos;
+
+	list_for_each_entry_rcu(pos, head, xprt_switch) {
+		if (xprt_is_active(pos))
+			return pos;
+	}
+	return NULL;
 }
 
 static
@@ -214,9 +226,12 @@ struct rpc_xprt *xprt_switch_find_current_entry(struct list_head *head,
 		const struct rpc_xprt *cur)
 {
 	struct rpc_xprt *pos;
+	bool found = false;
 
 	list_for_each_entry_rcu(pos, head, xprt_switch) {
 		if (cur == pos)
+			found = true;
+		if (found && xprt_is_active(pos))
 			return pos;
 	}
 	return NULL;
@@ -261,9 +276,12 @@ struct rpc_xprt *xprt_switch_find_next_entry(struct list_head *head,
 		const struct rpc_xprt *cur)
 {
 	struct rpc_xprt *pos, *prev = NULL;
+	bool found = false;
 
 	list_for_each_entry_rcu(pos, head, xprt_switch) {
 		if (cur == prev)
+			found = true;
+		if (found && xprt_is_active(pos))
 			return pos;
 		prev = pos;
 	}
-- 
2.21.0

[PATCH 3/3] SUNRPC: Fix a hang in xprt_switch_set_next_cursor()

[Trond Myklebust]

Apparently we do need the memory barrier.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
---
 net/sunrpc/xprtmultipath.c | 12 ++++--------
 1 file changed, 4 insertions(+), 8 deletions(-)

diff --git a/net/sunrpc/xprtmultipath.c b/net/sunrpc/xprtmultipath.c
index c12778e1235e..852da66cdaec 100644
--- a/net/sunrpc/xprtmultipath.c
+++ b/net/sunrpc/xprtmultipath.c
@@ -293,18 +293,14 @@ struct rpc_xprt *xprt_switch_set_next_cursor(struct list_head *head,
 		struct rpc_xprt **cursor,
 		xprt_switch_find_xprt_t find_next)
 {
-	struct rpc_xprt *cur, *pos, *old;
+	struct rpc_xprt *pos, *old;
 
-	cur = READ_ONCE(*cursor);
-	for (;;) {
-		old = cur;
+	old = READ_ONCE(*cursor);
+	do {
 		pos = find_next(head, old);
 		if (pos == NULL)
 			break;
-		cur = cmpxchg_relaxed(cursor, old, pos);
-		if (cur == old)
-			break;
-	}
+	} while (!try_cmpxchg(cursor, &old, pos));
 	return pos;
 }
 
-- 
2.21.0