6.15-stable review patch. If anyone has any objections, please let me know.
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From: Shyam Prasad N sprasad@microsoft.com
commit 66d590b828b1fd9fa337047ae58fe1c4c6f43609 upstream.
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 Signed-off-by: Steve French stfrench@microsoft.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- fs/smb/client/transport.c | 14 +++++++------- 1 file changed, 7 insertions(+), 7 deletions(-)
--- a/fs/smb/client/transport.c +++ b/fs/smb/client/transport.c @@ -1018,14 +1018,16 @@ struct TCP_Server_Info *cifs_pick_channe 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_channe */ 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);