netdev - Re: [RFC] [ver3 PATCH 3/6] virtio_net: virtio

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Date:	Fri, 18 Nov 2011 15:40:39 +0000
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Sasha Levin <levinsasha928@...il.com>
CC:	Krishna Kumar <krkumar2@...ibm.com>, <rusty@...tcorp.com.au>,
	<mst@...hat.com>, <netdev@...r.kernel.org>, <kvm@...r.kernel.org>,
	<davem@...emloft.net>, <virtualization@...ts.linux-foundation.org>
Subject: Re: [RFC] [ver3 PATCH 3/6] virtio_net: virtio_net driver changes

On Fri, 2011-11-18 at 08:24 +0200, Sasha Levin wrote:
> 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.

But you'll already be holding a lock related to the queue.

> Whats the benefit of having them per queue?

It should save some memory (and a little time when summing stats, though
that's unlikely to matter much).

The important thing is that splitting up stats per-CPU *and* per-queue
is a waste.

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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