RE: [RFC PATCH net-next 3/6] virtio-net: small optimization on free_old_xmit_skbs()

[David Laight]

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@xxxxxxxxxxxxxxx>
> > Cc: Michael S. Tsirkin <mst@xxxxxxxxxx>
> > Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
> 
> 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.)

> Acked-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> 
> > ---
> >  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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