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From: Kevin Traynor <ktraynor@redhat.com>
To: yuanhan.liu@linux.intel.com, maxime.coquelin@redhat.com
Cc: dev@dpdk.org, Kevin Traynor <ktraynor@redhat.com>, stable@dpdk.org
Subject: [dpdk-stable] [PATCH v2] vhost: fix virtio_net false sharing
Date: Thu, 23 Mar 2017 15:44:58 +0000	[thread overview]
Message-ID: <1490283898-23019-1-git-send-email-ktraynor@redhat.com> (raw)
In-Reply-To: <1489605049-18686-1-git-send-email-ktraynor@redhat.com>

The broadcast_rarp field in the virtio_net struct is checked in the
dequeue datapath regardless of whether descriptors are available or not.

As it is checked with cmpset leading to a write, false sharing on the
virtio_net struct can happen between enqueue and dequeue datapaths
regardless of whether a RARP is requested. In OVS, the issue can cause
a uni-directional performance drop of up to 15%.

Fix that by only performing the cmpset if a read of broadcast_rarp
indicates that the cmpset is likely to succeed.

Fixes: a66bcad32240 ("vhost: arrange struct fields for better cache sharing")
Cc: stable@dpdk.org

Signed-off-by: Kevin Traynor <ktraynor@redhat.com>
---

V2:
Change from fixing by moving broadcast_rarp location in virtio_net struct
to performing a read before cmpset.

 lib/librte_vhost/virtio_net.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/lib/librte_vhost/virtio_net.c b/lib/librte_vhost/virtio_net.c
index 337470d..d0a3b11 100644
--- a/lib/librte_vhost/virtio_net.c
+++ b/lib/librte_vhost/virtio_net.c
@@ -1057,7 +1057,19 @@ static inline bool __attribute__((always_inline))
 	 *
 	 * Check user_send_rarp() for more information.
+	 *
+	 * broadcast_rarp shares a cacheline in the virtio_net structure
+	 * with some fields that are accessed during enqueue and
+	 * rte_atomic16_cmpset() causes a write if using cmpxchg. This could
+	 * result in false sharing between enqueue and dequeue.
+	 *
+	 * Prevent unnecessary false sharing by reading broadcast_rarp first
+	 * and only performing cmpset if the read indicates it is likely to
+	 * be set.
 	 */
-	if (unlikely(rte_atomic16_cmpset((volatile uint16_t *)
-					 &dev->broadcast_rarp.cnt, 1, 0))) {
+
+	if (unlikely(rte_atomic16_read(&dev->broadcast_rarp) &&
+			rte_atomic16_cmpset((volatile uint16_t *)
+				&dev->broadcast_rarp.cnt, 1, 0))) {
+
 		rarp_mbuf = rte_pktmbuf_alloc(mbuf_pool);
 		if (rarp_mbuf == NULL) {
-- 
1.8.3.1

  parent reply	other threads:[~2017-03-23 15:45 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-03-15 19:10 [dpdk-stable] [PATCH] vhost: fix virtio_net cache sharing of broadcast_rarp Kevin Traynor
2017-03-16  6:21 ` Yuanhan Liu
2017-03-16 10:10   ` Kevin Traynor
2017-03-17  5:47     ` Yuanhan Liu
2017-03-17 10:01       ` Maxime Coquelin
2017-03-20 11:13         ` Kevin Traynor
2017-03-23 15:44 ` Kevin Traynor [this message]
2017-03-27  7:34   ` [dpdk-stable] [PATCH v2] vhost: fix virtio_net false sharing Maxime Coquelin
2017-03-27  8:33     ` Yuanhan Liu

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