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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 15 Oct 2014 15:00:28 +0300

On Wed, Oct 15, 2014 at 10:51:32AM +0000, David Laight wrote:
> From: Michael S. Tsirkin
> > 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 <[email-masked]>
> > > > > Cc: Michael S. Tsirkin <[email-masked]>
> > > > > Signed-off-by: Jason Wang <[email-masked]>
> > > >
> > > > 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.
> 
> I there are likely to be multiple skb on the queue the fastest
> code would probably do one 'virtqueue_get_all()' that returned
> a linked list of buffers, then follow the list to get the stats,
> and follow it again to free the skb.
> 
> 	David

Interesting. Each time we tried playing with linked list in the past,
it simply destroyed performance.
Maybe this case is different but I have my doubts.

> > > > Acked-by: Michael S. Tsirkin <[email-masked]>
> > > >
> > > > > ---
> > > > >  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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>