Subject: Re: [RFC] [ver3 PATCH 3/6] virtio_net: virtio_net driver changes

From: Sasha Levin <[email-masked]>
Date: Fri, 18 Nov 2011 08:24:41 +0200

On Fri, 2011-11-18 at 01:08 +0000, Ben Hutchings wrote:
> On Fri, 2011-11-11 at 18:34 +0530, Krishna Kumar wrote:
> > Changes for multiqueue virtio_net driver.
> [...]
> > @@ -677,25 +730,35 @@ static struct rtnl_link_stats64 *virtnet
> >  {
> >  	struct virtnet_info *vi = netdev_priv(dev);
> >  	int cpu;
> > -	unsigned int start;
> >  
> >  	for_each_possible_cpu(cpu) {
> > -		struct virtnet_stats __percpu *stats
> > -			= per_cpu_ptr(vi->stats, cpu);
> > -		u64 tpackets, tbytes, rpackets, rbytes;
> > -
> > -		do {
> > -			start = u64_stats_fetch_begin(&stats->syncp);
> > -			tpackets = stats->tx_packets;
> > -			tbytes   = stats->tx_bytes;
> > -			rpackets = stats->rx_packets;
> > -			rbytes   = stats->rx_bytes;
> > -		} while (u64_stats_fetch_retry(&stats->syncp, start));
> > -
> > -		tot->rx_packets += rpackets;
> > -		tot->tx_packets += tpackets;
> > -		tot->rx_bytes   += rbytes;
> > -		tot->tx_bytes   += tbytes;
> > +		int qpair;
> > +
> > +		for (qpair = 0; qpair < vi->num_queue_pairs; qpair++) {
> > +			struct virtnet_send_stats __percpu *tx_stat;
> > +			struct virtnet_recv_stats __percpu *rx_stat;
> 
> While you're at it, you can drop the per-CPU stats and make them only
> per-queue.  There is unlikely to be any benefit in maintaining them
> per-CPU while receive and transmit processing is serialised per-queue.

It allows you to update stats without a lock.

Whats the benefit of having them per queue?

-- 

Sasha.