netdev - [net][PATCH 2/5] Revert "RDS: IB: split the mr registration and invalidation path"

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Date:   Tue,  9 Jul 2019 22:32:41 -0700
From:   Santosh Shilimkar <santosh.shilimkar@...cle.com>
To:     netdev@...r.kernel.org, davem@...emloft.net
Cc:     santosh.shilimkar@...cle.com
Subject: [net][PATCH 2/5] Revert "RDS: IB: split the mr registration and invalidation path"

From: Gerd Rausch <gerd.rausch@...cle.com>

This reverts commit 56012459310a1dbcc55c2dbf5500a9f7571402cb.

RDS kept spinning inside function "rds_ib_post_reg_frmr", waiting for
"i_fastreg_wrs" to become incremented:
         while (atomic_dec_return(&ibmr->ic->i_fastreg_wrs) <= 0) {
                 atomic_inc(&ibmr->ic->i_fastreg_wrs);
                 cpu_relax();
         }

Looking at the original commit:

commit 56012459310a ("RDS: IB: split the mr registration and
invalidation path")

In there, the "rds_ib_mr_cqe_handler" was changed in the following
way:

 void rds_ib_mr_cqe_handler(struct
 rds_ib_connection *ic,
 struct ib_wc *wc)
        if (frmr->fr_inv) {
                  frmr->fr_state = FRMR_IS_FREE;
                  frmr->fr_inv = false;
                atomic_inc(&ic->i_fastreg_wrs);
        } else {
                atomic_inc(&ic->i_fastunreg_wrs);
        }

It looks like it's got it exactly backwards:

Function "rds_ib_post_reg_frmr" keeps track of the outstanding
requests via "i_fastreg_wrs".

Function "rds_ib_post_inv" keeps track of the outstanding requests
via "i_fastunreg_wrs" (post original commit). It also sets:
         frmr->fr_inv = true;

However the completion handler "rds_ib_mr_cqe_handler" adjusts
"i_fastreg_wrs" when "fr_inv" had been true, and adjusts
"i_fastunreg_wrs" otherwise.

The original commit was done in the name of performance:
to remove the performance bottleneck

No performance benefit could be observed with a fixed-up version
of the original commit measured between two Oracle X7 servers,
both equipped with Mellanox Connect-X5 HCAs.

The prudent course of action is to revert this commit.

Signed-off-by: Gerd Rausch <gerd.rausch@...cle.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@...cle.com>
---
 net/rds/ib.h      |  4 +---
 net/rds/ib_cm.c   |  9 ++-------
 net/rds/ib_frmr.c | 11 +++++------
 3 files changed, 8 insertions(+), 16 deletions(-)

diff --git a/net/rds/ib.h b/net/rds/ib.h
index 67a715b..66c03c7 100644
--- a/net/rds/ib.h
+++ b/net/rds/ib.h
@@ -15,8 +15,7 @@
 
 #define RDS_IB_DEFAULT_RECV_WR		1024
 #define RDS_IB_DEFAULT_SEND_WR		256
-#define RDS_IB_DEFAULT_FR_WR		256
-#define RDS_IB_DEFAULT_FR_INV_WR	256
+#define RDS_IB_DEFAULT_FR_WR		512
 
 #define RDS_IB_DEFAULT_RETRY_COUNT	1
 
@@ -157,7 +156,6 @@ struct rds_ib_connection {
 
 	/* To control the number of wrs from fastreg */
 	atomic_t		i_fastreg_wrs;
-	atomic_t		i_fastunreg_wrs;
 
 	/* interrupt handling */
 	struct tasklet_struct	i_send_tasklet;
diff --git a/net/rds/ib_cm.c b/net/rds/ib_cm.c
index 66c6eb5..8891822 100644
--- a/net/rds/ib_cm.c
+++ b/net/rds/ib_cm.c
@@ -460,10 +460,7 @@ static int rds_ib_setup_qp(struct rds_connection *conn)
 	 * completion queue and send queue. This extra space is used for FRMR
 	 * registration and invalidation work requests
 	 */
-	fr_queue_space = rds_ibdev->use_fastreg ?
-			 (RDS_IB_DEFAULT_FR_WR + 1) +
-			 (RDS_IB_DEFAULT_FR_INV_WR + 1)
-			 : 0;
+	fr_queue_space = (rds_ibdev->use_fastreg ? RDS_IB_DEFAULT_FR_WR : 0);
 
 	/* add the conn now so that connection establishment has the dev */
 	rds_ib_add_conn(rds_ibdev, conn);
@@ -530,7 +527,6 @@ static int rds_ib_setup_qp(struct rds_connection *conn)
 	attr.send_cq = ic->i_send_cq;
 	attr.recv_cq = ic->i_recv_cq;
 	atomic_set(&ic->i_fastreg_wrs, RDS_IB_DEFAULT_FR_WR);
-	atomic_set(&ic->i_fastunreg_wrs, RDS_IB_DEFAULT_FR_INV_WR);
 
 	/*
 	 * XXX this can fail if max_*_wr is too large?  Are we supposed
@@ -1009,8 +1005,7 @@ void rds_ib_conn_path_shutdown(struct rds_conn_path *cp)
 		wait_event(rds_ib_ring_empty_wait,
 			   rds_ib_ring_empty(&ic->i_recv_ring) &&
 			   (atomic_read(&ic->i_signaled_sends) == 0) &&
-			   (atomic_read(&ic->i_fastreg_wrs) == RDS_IB_DEFAULT_FR_WR) &&
-			   (atomic_read(&ic->i_fastunreg_wrs) == RDS_IB_DEFAULT_FR_INV_WR));
+			   (atomic_read(&ic->i_fastreg_wrs) == RDS_IB_DEFAULT_FR_WR));
 		tasklet_kill(&ic->i_send_tasklet);
 		tasklet_kill(&ic->i_recv_tasklet);
 
diff --git a/net/rds/ib_frmr.c b/net/rds/ib_frmr.c
index 688dcd6..32ae26e 100644
--- a/net/rds/ib_frmr.c
+++ b/net/rds/ib_frmr.c
@@ -239,8 +239,8 @@ static int rds_ib_post_inv(struct rds_ib_mr *ibmr)
 	if (frmr->fr_state != FRMR_IS_INUSE)
 		goto out;
 
-	while (atomic_dec_return(&ibmr->ic->i_fastunreg_wrs) <= 0) {
-		atomic_inc(&ibmr->ic->i_fastunreg_wrs);
+	while (atomic_dec_return(&ibmr->ic->i_fastreg_wrs) <= 0) {
+		atomic_inc(&ibmr->ic->i_fastreg_wrs);
 		cpu_relax();
 	}
 
@@ -257,7 +257,7 @@ static int rds_ib_post_inv(struct rds_ib_mr *ibmr)
 	if (unlikely(ret)) {
 		frmr->fr_state = FRMR_IS_STALE;
 		frmr->fr_inv = false;
-		atomic_inc(&ibmr->ic->i_fastunreg_wrs);
+		atomic_inc(&ibmr->ic->i_fastreg_wrs);
 		pr_err("RDS/IB: %s returned error(%d)\n", __func__, ret);
 		goto out;
 	}
@@ -285,10 +285,9 @@ void rds_ib_mr_cqe_handler(struct rds_ib_connection *ic, struct ib_wc *wc)
 	if (frmr->fr_inv) {
 		frmr->fr_state = FRMR_IS_FREE;
 		frmr->fr_inv = false;
-		atomic_inc(&ic->i_fastreg_wrs);
-	} else {
-		atomic_inc(&ic->i_fastunreg_wrs);
 	}
+
+	atomic_inc(&ic->i_fastreg_wrs);
 }
 
 void rds_ib_unreg_frmr(struct list_head *list, unsigned int *nfreed,
-- 
1.9.1