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vhost: fix false sharing

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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>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Acked-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
This commit is contained in:
Kevin Traynor 2017-03-23 15:44:58 +00:00 committed by Yuanhan Liu
parent 52d6beb9d2
commit 4e9474141e
1 changed files with 14 additions and 2 deletions
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16
lib/librte_vhost/virtio_net.c
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@ -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,