IB/hfi1: Fix a latency issue for small messages

A recent performance enhancement introduced a latency issue in the
HFI message path.  The new algorithm removed a forced call send for
PIO messages and added a forced schedule event for messages larger
than the MTU.

For PIO, the schedule path can introduce thrashing that can
significantly impact the throughput for small messages.

If a message size is within the PIO threshold, always take the send
path.

Fixes: 0b79b27748cb ("IB/{hfi1, qib, rdmavt}: Schedule multi RC/UC packets instead of posting")
Reviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>

diff --git a/drivers/infiniband/hw/hfi1/qp.c b/drivers/infiniband/hw/hfi1/qp.c
index 6f3bc4d..1a01624 100644 (file)
--- a/drivers/infiniband/hw/hfi1/qp.c
+++ b/drivers/infiniband/hw/hfi1/qp.c
@@ -340,6 +340,13 @@ int hfi1_setup_wqe(struct rvt_qp *qp, struct rvt_swqe *wqe, bool *call_send)
        default:
                break;
        }
+
+       /*
+        * System latency between send and schedule is large enough that
+        * forcing call_send to true for piothreshold packets is necessary.
+        */
+       if (wqe->length <= piothreshold)
+               *call_send = true;
        return 0;
 }