netdev - Re: [PATCH net-next v3 1/2] rtnetlink: add new RTM

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Date:	Sat, 16 Apr 2016 09:49:10 +0200
From:	Thomas Graf <tgraf@...g.ch>
To:	Roopa Prabhu <roopa@...ulusnetworks.com>
Cc:	netdev@...r.kernel.org, jhs@...atatu.com, davem@...emloft.net
Subject: Re: [PATCH net-next v3 1/2] rtnetlink: add new RTM_GETSTATS message
 to query stats

On 04/15/16 at 08:28pm, Roopa Prabhu wrote:
> +static u16 rtnl_stats_calcit(struct sk_buff *skb, struct nlmsghdr *nlh)
> +{
> +	struct net *net = sock_net(skb->sk);
> +	struct net_device *dev;
> +	u16 min_ifinfo_dump_size = 0;
> +	struct if_stats_msg *ifsm;
> +	u32 filter_mask;
> +
> +	ifsm = nlmsg_data(nlh);
> +	filter_mask = ifsm->filter_mask;
> +
> +	/* traverse the list of net devices and compute the minimum
> +	 * buffer size based upon the filter mask.
> +	 */
> +	list_for_each_entry(dev, &net->dev_base_head, dev_list) {
> +		min_ifinfo_dump_size = max_t(u16, min_ifinfo_dump_size,
> +					     if_nlmsg_stats_size(dev,
> +								 filter_mask));
> +	}

Iterating over all net_devices in the namespace is quite an expensive
operation and it would now be done twice.

I understand that this code is taken over from rtnl_calcit() but there
the cost is at least only paid if ext_filter_mask is actually set and
the user opts into additional statistics.

I wonder if we can reduce the cost for the stats interface as its
purpose is to be minimal cost. I suggest we only add the loop once we
have an extension which actually depends on it. We can then try and
figure out to not require it.