[dpdk-stable] patch 'vhost: fix false sharing' has been queued to LTS release 16.11.2
Yuanhan Liu
yuanhan.liu at linux.intel.com
Fri Apr 7 10:11:56 CEST 2017
Hi,
FYI, your patch has been queued to LTS release 16.11.2
Note it hasn't been pushed to http://dpdk.org/browse/dpdk-stable
yet. It will be pushed if I get no objections before 04/11/17.
So please shout if anyone has objections.
Thanks.
--yliu
---
>From 071a0f1441a087edc4337bdc2ea9f4e038099973 Mon Sep 17 00:00:00 2001
From: Kevin Traynor <ktraynor at redhat.com>
Date: Thu, 23 Mar 2017 15:44:58 +0000
Subject: [PATCH] vhost: fix false sharing
[ upstream commit 4e9474141e1115a3089896848f92b25b53cddfd9 ]
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")
Signed-off-by: Kevin Traynor <ktraynor at redhat.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin at redhat.com>
Acked-by: Yuanhan Liu <yuanhan.liu at linux.intel.com>
---
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
@@ -1056,9 +1056,21 @@ rte_vhost_dequeue_burst(int vid, uint16_t queue_id,
* array, to looks like that guest actually send such packet.
*
* 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) {
RTE_LOG(ERR, VHOST_DATA,
--
1.9.0
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