[PATCH 4/5 v7] IB/srp: Reduce number of BUSY conditions

[Bart Van Assche <bvanassche-HInyCGIudOg@public.gmane.org>]

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 (62) by reducing the number of
BUSY responses reported by ib_srp to the SCSI mid-layer.

Signed-off-by: Bart Van Assche <bvanassche-HInyCGIudOg@public.gmane.org>
Cc: Roland Dreier <rolandd-FYB4Gu1CFyUAvxtiuMwx3w@public.gmane.org>

---
 drivers/infiniband/ulp/srp/ib_srp.c |    9 +++++++--
 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 17e948d..2de894e 100644
--- a/drivers/infiniband/ulp/srp/ib_srp.c
+++ b/drivers/infiniband/ulp/srp/ib_srp.c
@@ -1345,8 +1345,13 @@ 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);
+			/*
+			 * Set can_queue such that we don't needlessly
+			 * 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);
-- 
1.6.4.2

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