Re: [dpdk-stable] [PATCH] vhost: fix virtio_net cache sharing of broadcast_rarp

[Yuanhan Liu <yuanhan.liu@linux.intel.com>]

On Thu, Mar 16, 2017 at 10:10:05AM +0000, Kevin Traynor wrote:
> On 03/16/2017 06:21 AM, Yuanhan Liu wrote:
> > On Wed, Mar 15, 2017 at 07:10:49PM +0000, Kevin Traynor wrote:
> >> The virtio_net structure is used in both enqueue and dequeue datapaths.
> >> broadcast_rarp is checked with cmpset in the dequeue datapath regardless
> >> of whether descriptors are available or not.
> >>
> >> It is observed in some cases where dequeue and enqueue are performed by
> >> different cores and no packets are available on the dequeue datapath
> >> (i.e. uni-directional traffic), the frequent checking of broadcast_rarp
> >> in dequeue causes performance degradation for the enqueue datapath.
> >>
> >> In OVS the issue can cause a uni-directional performance drop of up to 15%.
> >>
> >> Fix that by moving broadcast_rarp to a different cache line in
> >> virtio_net struct.
> > 
> > Thanks, but I'm a bit confused. The drop looks like being caused by
> > cache false sharing, but I don't see anything would lead to a false
> > sharing. I mean, there is no write in the same cache line where the
> > broadcast_rarp belongs. Or, the "volatile" type is the culprit here?
> > 
> 
> Yes, the cmpset code uses cmpxchg and that performs a write regardless
> of the result - it either writes the new value or back the old value.

Oh, right, I missed this part!

> > Talking about that, I had actually considered to turn "broadcast_rarp"
> > to a simple "int" or "uint16_t" type, to make it more light weight.
> > The reason I used atomic type is to exactly send one broadcast RARP
> > packet once SEND_RARP request is recieved. Otherwise, we may send more
> > than one RARP packet when MQ is invovled. But I think we don't have
> > to be that accurate: it's tolerable when more RARP are sent. I saw 4
> > SEND_RARP requests (aka 4 RARP packets) in the last time I tried
> > vhost-user live migration after all. I don't quite remember why
> > it was 4 though.
> > 
> > That said, I think it also would resolve the performance issue if you
> > change "rte_atomic16_t" to "uint16_t", without moving the place?
> > 
> 
> Yes, that should work fine, with the side effect you mentioned of
> possibly some more rarps - no big deal.
> 
> I tested another solution also - as it is unlikely we would need to send
> the broadcast_rarp, you can first read and only do the cmpset if it is
> likely to succeed. This resolved the issue too.
> 
> --- a/lib/librte_vhost/virtio_net.c
> +++ b/lib/librte_vhost/virtio_net.c
> @@ -1057,7 +1057,8 @@ static inline bool __attribute__((always_inline))
>          *
>          * Check user_send_rarp() for more information.
>          */
> -       if (unlikely(rte_atomic16_cmpset((volatile uint16_t *)
> +       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) {

I'm okay with this one. It's simple and clean enough, that it could
be picked to a stable release. Later, I'd like to send another patch
to turn it to "uint16_t". Since it changes the behaviour a bit, it
is not a good candidate for stable release.

BTW, would you please include the root cause (false sharing) into
your commit log?

	--yliu
> 
> I choose changing the struct because the 'read && cmpset' code is
> non-obvious and someone might not think to do that in the future. I did
> a PVP test with testpmd and didn't see any degradation with the struct
> change, so I thought it can be a good solution.
> 
> I tested the struct change with several combinations of DPDK
> 16.11.1/17.02/master combined with OVS 2.6/2.7/master. If you prefer one
> of the other solutions, let me know and I'll perform some additional
> testing.
> 
> Kevin.
> 
> > 	--yliu
> >