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* [PATCH 1/6] cifs: deal with the channel loading lag while picking channels
@ 2025-06-02 17:07 nspmangalore
  2025-06-02 17:07 ` [PATCH 2/6] cifs: reset connections for all channels when reconnect requested nspmangalore
                   ` (5 more replies)
  0 siblings, 6 replies; 7+ messages in thread
From: nspmangalore @ 2025-06-02 17:07 UTC (permalink / raw)
  To: linux-cifs, smfrench, bharathsm.hsk, meetakshisetiyaoss, pc
  Cc: Shyam Prasad N, stable

From: Shyam Prasad N <sprasad@microsoft.com>

Our current approach to select a channel for sending requests is this:
1. iterate all channels to find the min and max queue depth
2. if min and max are not the same, pick the channel with min depth
3. if min and max are same, round robin, as all channels are equally loaded

The problem with this approach is that there's a lag between selecting
a channel and sending the request (that increases the queue depth on the channel).
While these numbers will eventually catch up, there could be a skew in the
channel usage, depending on the application's I/O parallelism and the server's
speed of handling requests.

With sufficient parallelism, this lag can artificially increase the queue depth,
thereby impacting the performance negatively.

This change will change the step 1 above to start the iteration from the last
selected channel. This is to reduce the skew in channel usage even in the presence
of this lag.

Fixes: ea90708d3cf3 ("cifs: use the least loaded channel for sending requests")
Cc: <stable@vger.kernel.org>
Signed-off-by: Shyam Prasad N <sprasad@microsoft.com>
---
 fs/smb/client/transport.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/fs/smb/client/transport.c b/fs/smb/client/transport.c
index 266af17aa7d9..191783f553ce 100644
--- a/fs/smb/client/transport.c
+++ b/fs/smb/client/transport.c
@@ -1018,14 +1018,16 @@ struct TCP_Server_Info *cifs_pick_channel(struct cifs_ses *ses)
 	uint index = 0;
 	unsigned int min_in_flight = UINT_MAX, max_in_flight = 0;
 	struct TCP_Server_Info *server = NULL;
-	int i;
+	int i, start, cur;
 
 	if (!ses)
 		return NULL;
 
 	spin_lock(&ses->chan_lock);
+	start = atomic_inc_return(&ses->chan_seq);
 	for (i = 0; i < ses->chan_count; i++) {
-		server = ses->chans[i].server;
+		cur = (start + i) % ses->chan_count;
+		server = ses->chans[cur].server;
 		if (!server || server->terminate)
 			continue;
 
@@ -1042,17 +1044,15 @@ struct TCP_Server_Info *cifs_pick_channel(struct cifs_ses *ses)
 		 */
 		if (server->in_flight < min_in_flight) {
 			min_in_flight = server->in_flight;
-			index = i;
+			index = cur;
 		}
 		if (server->in_flight > max_in_flight)
 			max_in_flight = server->in_flight;
 	}
 
 	/* if all channels are equally loaded, fall back to round-robin */
-	if (min_in_flight == max_in_flight) {
-		index = (uint)atomic_inc_return(&ses->chan_seq);
-		index %= ses->chan_count;
-	}
+	if (min_in_flight == max_in_flight)
+		index = (uint)start % ses->chan_count;
 
 	server = ses->chans[index].server;
 	spin_unlock(&ses->chan_lock);
-- 
2.43.0


^ permalink raw reply related	[flat|nested] 7+ messages in thread
end of thread, other threads:[~2025-06-02 18:49 UTC | newest]

Thread overview: 7+ messages (download: mbox.gz follow: Atom feed
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2025-06-02 17:07 [PATCH 1/6] cifs: deal with the channel loading lag while picking channels nspmangalore
2025-06-02 17:07 ` [PATCH 2/6] cifs: reset connections for all channels when reconnect requested nspmangalore
2025-06-02 17:07 ` [PATCH 3/6] cifs: update dstaddr whenever channel iface is updated nspmangalore
2025-06-02 17:07 ` [PATCH 4/6] cifs: serialize other channels when query server interfaces is pending nspmangalore
2025-06-02 17:07 ` [PATCH 5/6] cifs: dns resolution is needed only for primary channel nspmangalore
2025-06-02 17:07 ` [PATCH 6/6] cifs: do not disable interface polling on failure nspmangalore
2025-06-02 18:47 ` [PATCH 1/6] cifs: deal with the channel loading lag while picking channels Steve French

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