From: "Michael J. Ruhl" <michael.j.ruhl@intel.com>
Date: Wed, 28 Nov 2018 06:44:36 -0800
Subject: IB/hfi1: Fix a latency issue for small messages
Patch-mainline: v4.20-rc7
Git-commit: 90b2620e6a8aa08c40cc78d61603e0acd853c33a
References: bsc#1114685 FATE#325854

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>
Acked-by: Thomas Bogendoerfer <tbogendoerfer@suse.de>
---
 drivers/infiniband/hw/hfi1/qp.c |    7 +++++++
 1 file changed, 7 insertions(+)

--- 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, st
 	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;
 }