Subject: Re: [PATCH nf-next 3/9] netfilter: conntrack: don't attempt to iterate over empty table

From: Pablo Neira Ayuso <[email-masked]>
Date: Tue, 3 May 2016 19:03:57 +0200

On Thu, Apr 28, 2016 at 07:13:42PM +0200, Florian Westphal wrote:
> Once we place all conntracks into same table iteration becomes more
> costly because the table contains conntracks that we are not interested
> in (belonging to other netns).
> 
> So don't bother scanning if the current namespace has no entries.
> 
> Signed-off-by: Florian Westphal <[email-masked]>
> ---
>  net/netfilter/nf_conntrack_core.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/net/netfilter/nf_conntrack_core.c b/net/netfilter/nf_conntrack_core.c
> index 29fa08b..f2e75a5 100644
> --- a/net/netfilter/nf_conntrack_core.c
> +++ b/net/netfilter/nf_conntrack_core.c
> @@ -1428,6 +1428,9 @@ void nf_ct_iterate_cleanup(struct net *net,
>  
>  	might_sleep();
>  
> +	if (atomic_read(&net->ct.count) == 0)
> +		return;

This optimization gets defeated with just one single conntrack (ie.
net->ct.count == 1), so I wonder if this is practical thing.

At the cost of consuming more memory per conntrack, we may consider
adding a per-net list so this iteration doesn't become a problem.

>  	while ((ct = get_next_corpse(net, iter, data, &bucket)) != NULL) {
>  		/* Time to push up daises... */
>  		if (del_timer(&ct->timeout))
> -- 
> 2.7.3
> 
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