[PATCH] cifs: try to pick channel with a minimum of credits

["Aurélien Aptel" <aaptel@suse.com>]

From: Aurelien Aptel <aaptel@suse.com>

Check channel credits to prevent the client from using a starved
channel that cannot send anything.

Special care must be taken in selecting the minimum value: when
channels are created they start off with a small amount that slowly
ramps up as the channel gets used. Thus a new channel might never be
picked if the min value is too small.

Signed-off-by: Aurelien Aptel <aaptel@suse.com>
---
 fs/cifs/transport.c | 57 ++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 49 insertions(+), 8 deletions(-)

diff --git a/fs/cifs/transport.c b/fs/cifs/transport.c
index e90a1d1380b0..7bb1584b3724 100644
--- a/fs/cifs/transport.c
+++ b/fs/cifs/transport.c
@@ -44,6 +44,14 @@
 /* Max number of iovectors we can use off the stack when sending requests. */
 #define CIFS_MAX_IOV_SIZE 8
 
+/*
+ * Min number of credits for a channel to be picked.
+ *
+ * Note that once a channel reaches this threshold it will never be
+ * picked again as no credits can be requested from it.
+ */
+#define CIFS_CHANNEL_MIN_CREDITS 3
+
 void
 cifs_wake_up_task(struct mid_q_entry *mid)
 {
@@ -1051,20 +1059,53 @@ cifs_cancelled_callback(struct mid_q_entry *mid)
 struct TCP_Server_Info *cifs_pick_channel(struct cifs_ses *ses)
 {
 	uint index = 0;
+	struct TCP_Server_Info *s;
 
 	if (!ses)
 		return NULL;
 
-	if (!ses->binding) {
-		/* round robin */
-		if (ses->chan_count > 1) {
-			index = (uint)atomic_inc_return(&ses->chan_seq);
-			index %= ses->chan_count;
-		}
-		return ses->chans[index].server;
-	} else {
+	if (ses->binding)
 		return cifs_ses_server(ses);
+
+	/*
+	 * Channels are created right after the session is made. The
+	 * count cannot change after that so it is not racy to check.
+	 */
+	if (ses->chan_count == 1)
+		return ses->chans[index].server;
+
+	/* round robin */
+	index = (uint)atomic_inc_return(&ses->chan_seq);
+	index %= ses->chan_count;
+	s = ses->chans[index].server;
+
+	/*
+	 * Checking server credits is racy, but getting a slightly
+	 * stale value should not be an issue here
+	 */
+	if (s->credits <= CIFS_CHANNEL_MIN_CREDITS) {
+		uint i;
+
+		cifs_dbg(VFS, "cannot pick conn_id=0x%llx not enough credits (%u)",
+			 s->conn_id,
+			 s->credits);
+
+		/*
+		 * Look at all other channels starting from the next
+		 * one and pick first possible channel.
+		 */
+		for (i = 1; i < ses->chan_count; i++) {
+			s = ses->chans[(index+i) % ses->chan_count].server;
+			if (s->credits > CIFS_CHANNEL_MIN_CREDITS)
+				return s;
+		}
 	}
+
+	/*
+	 * If none are possible, keep the initially picked one, but
+	 * later on it will block to wait for credits or fail.
+	 */
+	return ses->chans[index].server;
 }
 
 int
-- 
2.30.0