netdev - Re: [V2 RFC net-next PATCH 1/2] virtio_net: convert the statistics into array

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Date:	Wed, 06 Jun 2012 10:22:19 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jason Wang <jasowang@...hat.com>
Cc:	netdev@...r.kernel.org, rusty@...tcorp.com.au,
	virtualization@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org, mst@...hat.com
Subject: Re: [V2 RFC net-next PATCH 1/2] virtio_net: convert the statistics
 into array

On Wed, 2012-06-06 at 15:52 +0800, Jason Wang wrote:
> Currently, we store the statistics in the independent fields of virtnet_stats,
> this is not scalable when we want to add more counters. As suggested by Michael,
> this patch convert it to an array and use the enum as the index to access them.
> 
> Signed-off-by: Jason Wang <jasowang@...hat.com>
> ---
>  drivers/net/virtio_net.c |   30 +++++++++++++++++-------------
>  1 files changed, 17 insertions(+), 13 deletions(-)
> 

>  struct virtnet_stats {
>  	struct u64_stats_sync syncp;
> -	u64 tx_bytes;
> -	u64 tx_packets;
> -
> -	u64 rx_bytes;
> -	u64 rx_packets;
> +	u64 data[VIRTNET_NUM_STATS];
>  };
>  

Interesting, but I fear you'll have a lot of problems.

Current code is buggy, and you are adding more possible races.

We could have one cpu doing the :

       u64_stats_update_begin(&stats->syncp);
       stats->rx_bytes += skb->len;
       stats->rx_packets++;
       u64_stats_update_end(&stats->syncp);

And another one doing :

       u64_stats_update_begin(&stats->syncp);
       stats->tx_bytes += skb->len;
       stats->tx_packets++;
       u64_stats_update_end(&stats->syncp);
 
And one syncp sequence increment can be lost, since both cpus are
basically doing this at the same time :

    write_seqcount_begin(&syncp->seq);

I'll send a fix in a separate thread.



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