linux-kernel - Re: [RFC PATCH net-next 3/6] virtio-net: small optimization on free_old_xmit

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Message-ID: <20141015104806.GK25776@redhat.com>
Date:	Wed, 15 Oct 2014 13:48:06 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	David Laight <David.Laight@...LAB.COM>
Cc:	Jason Wang <jasowang@...hat.com>,
	"rusty@...tcorp.com.au" <rusty@...tcorp.com.au>,
	"virtualization@...ts.linux-foundation.org" 
	<virtualization@...ts.linux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"eric.dumazet@...il.com" <eric.dumazet@...il.com>
Subject: Re: [RFC PATCH net-next 3/6] virtio-net: small optimization on
 free_old_xmit_skbs()

On Wed, Oct 15, 2014 at 09:49:01AM +0000, David Laight wrote:
> From: Of Michael S. Tsirkin
> > On Wed, Oct 15, 2014 at 03:25:27PM +0800, Jason Wang wrote:
> > > Accumulate the sent packets and sent bytes in local variables and perform a
> > > single u64_stats_update_begin/end() after.
> > >
> > > Cc: Rusty Russell <rusty@...tcorp.com.au>
> > > Cc: Michael S. Tsirkin <mst@...hat.com>
> > > Signed-off-by: Jason Wang <jasowang@...hat.com>
> > 
> > Not sure how much it's worth but since Eric suggested it ...
> 
> Probably depends on the actual cost of u64_stats_update_begin/end
> against the likely extra saving of the tx_bytes and tx_packets
> values onto the stack across the call to dev_kfree_skb_any().
> (Which depends on the number of caller saved registers.)

Yea, some benchmark results would be nice to see.


> > Acked-by: Michael S. Tsirkin <mst@...hat.com>
> > 
> > > ---
> > >  drivers/net/virtio_net.c |   12 ++++++++----
> > >  1 files changed, 8 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > > index 3d0ce44..a4d56b8 100644
> > > --- a/drivers/net/virtio_net.c
> > > +++ b/drivers/net/virtio_net.c
> > > @@ -833,17 +833,21 @@ static void free_old_xmit_skbs(struct send_queue *sq)
> > >  	unsigned int len;
> > >  	struct virtnet_info *vi = sq->vq->vdev->priv;
> > >  	struct virtnet_stats *stats = this_cpu_ptr(vi->stats);
> > > +	u64 tx_bytes = 0, tx_packets = 0;
> 
> tx_packets need only be 'unsigned int'.
> The same is almost certainly true of tx_bytes.
> 
> 	David
> 
> > >  	while ((skb = virtqueue_get_buf(sq->vq, &len)) != NULL) {
> > >  		pr_debug("Sent skb %p\n", skb);
> > >
> > > -		u64_stats_update_begin(&stats->tx_syncp);
> > > -		stats->tx_bytes += skb->len;
> > > -		stats->tx_packets++;
> > > -		u64_stats_update_end(&stats->tx_syncp);
> > > +		tx_bytes += skb->len;
> > > +		tx_packets++;
> > >
> > >  		dev_kfree_skb_any(skb);
> > >  	}
> > > +
> > > +	u64_stats_update_begin(&stats->tx_syncp);
> > > +	stats->tx_bytes += tx_bytes;
> > > +	stats->tx_packets =+ tx_packets;
> > > +	u64_stats_update_end(&stats->tx_syncp);
> > >  }
> > >
> > >  static int xmit_skb(struct send_queue *sq, struct sk_buff *skb)
> > > --
> > > 1.7.1
> > --
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> 
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