IB/srp: Reduce number of BUSY conditions

As proposed by the SRP (draft) standard, ib_srp reserves one ring
element for SRP_TSK_MGMT requests. This patch makes sure that the SCSI
mid-layer never tries to queue more than (SRP request limit) - 1 SCSI
commands to ib_srp. This improves performance for targets whose request
limit is less than or equal to SRP_NORMAL_REQ_SQ_SIZE by reducing the
number of BUSY responses reported by ib_srp to the SCSI mid-layer.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: David Dillow <dillowda@ornl.gov>
Signed-off-by: Roland Dreier <rolandd@cisco.com>

1 files changed, 7 insertions, 2 deletions

diff --git a/drivers/infiniband/ulp/srp/ib_srp.c b/drivers/infiniband/ulp/srp/ib_srp.c
index d4c08d648137..4df827591cad 100644
--- a/drivers/infiniband/ulp/srp/ib_srp.c
+++ b/drivers/infiniband/ulp/srp/ib_srp.c
@@ -1339,8 +1339,13 @@ static int srp_cm_handler(struct ib_cm_id *cm_id, struct ib_cm_event *event)
                         target->max_ti_iu_len = be32_to_cpu(rsp->max_ti_iu_len);
                         target->req_lim       = be32_to_cpu(rsp->req_lim_delta);
 
-                        target->scsi_host->can_queue = min(target->req_lim,
-                                                           target->scsi_host->can_queue);
+                        /*
+                         * Reserve credits for task management so we don't
+                         * bounce requests back to the SCSI mid-layer.
+                         */
+                        target->scsi_host->can_queue
+                                = min(target->req_lim - SRP_TSK_MGMT_SQ_SIZE,
+                                      target->scsi_host->can_queue);
                 } else {
                         shost_printk(KERN_WARNING, target->scsi_host,
                                     PFX "Unhandled RSP opcode %#x\n", opcode);