[PATCH] netvsc: optimize stats counters performance

[Morten Brørup]

> From: Stephen Hemminger [mailto:stephen at networkplumber.org]
> Sent: Friday, 2 August 2024 19.33
> 
> On Fri, 2 Aug 2024 19:28:26 +0200
> Morten Brørup <mb at smartsharesystems.com> wrote:
> 
> > > >  	ea = rte_pktmbuf_mtod(m, const struct rte_ether_addr *);
> > > > -	if (rte_is_multicast_ether_addr(ea)) {
> > > > -		if (rte_is_broadcast_ether_addr(ea))
> > > > -			stats->broadcast++;
> > > > -		else
> > > > -			stats->multicast++;
> > > > -	}
> > > > +	RTE_BUILD_BUG_ON(offsetof(struct hn_stats, broadcast) !=
> > > > +			offsetof(struct hn_stats, multicast) +
> > > sizeof(uint64_t));
> > > > +	if (unlikely(rte_is_multicast_ether_addr(ea)))
> > > > +		(&stats->multicast)[rte_is_broadcast_ether_addr(ea)]++;
> > > >  }
> > >
> > > This makes the code a little harder to read.
> >
> > I agree it is somewhat convoluted.
> > It's a tradeoff... I preferred performance at the cost of making the code
> somewhat harder to read.
> > The RTE_BUILD_BUG_ON() also helps showing what is going on with the weird
> indexing.

Similar patches have been accepted by other drivers:
[virtio]: https://patchwork.dpdk.org/project/dpdk/patch/20240801160312.205281-1-mb@smartsharesystems.com/
[vhost-user]: https://patchwork.dpdk.org/project/dpdk/patch/20240802143259.269827-1-mb@smartsharesystems.com/

> 
> Optimizing for multicast packets is not worth bothering.

Optimizing for multicast/broadcast comes into play in multicast environments, and during network broadcast storms.
Although I don't know if any of those two scenarios are relevant for this specific driver.

> Keep the original code it is simpler.

Let's keep similar code similar across drivers.

@Long, @Wei, please Review/Ack, so the patch can be applied.