cifs: deal with the channel loading lag while picking channels

From: Shyam Prasad N <[email protected]>

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 <[email protected]>

Signed-off-by Shyam Prasad N <[email protected]>

---
 fs/smb/client/transport.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

fs/smb/client/transport.c +7 -8

index 0dc80959ce48..e2fbf8b18eb2 100644
@@ -1015,14 +1015,16 @@ 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;
 

@@ -1039,17 +1041,15 @@ 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

tentatively merged into cifs-2.6.git for-next pending more reviews and testing

Quoted 73 lines · On Wed, Feb 12, 2025 at 1:35 AM <[email protected]> wrote:

From: Shyam Prasad N <[email protected]>

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 <[email protected]>

Signed-off-by Shyam Prasad N <[email protected]>

---
 fs/smb/client/transport.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

fs/smb/client/transport.c +7 -8

index 0dc80959ce48..e2fbf8b18eb2 100644
@@ -1015,14 +1015,16 @@ 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;
 

@@ -1039,17 +1041,15 @@ 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);
--

Quoted 2 lines

2.43.0

--
Thanks,

Steve