Subject: Re: [PATCH net-next V6 02/14] bridge: Add vlan filtering infrastructure

From: Vlad Yasevich <[email-masked]>
Date: Sun, 20 Jan 2013 12:59:22 -0500

On 01/17/2013 08:57 PM, Michał Mirosław wrote:
> 2013/1/16 Vlad Yasevich <[email-masked]>:
> [...]
>> --- /dev/null
>> +++ b/net/bridge/br_vlan.c
> [...]
>> +struct net_port_vlan *nbp_vlan_find(const struct net_port_vlans *v, u16 vid)
>> +{
>> +       struct net_port_vlan *pve;
>> +
>> +       /* Must be done either in rcu critical section or with RTNL held */
>> +       WARN_ON_ONCE(!rcu_read_lock_held() && !rtnl_is_locked());
>> +
>> +       list_for_each_entry_rcu(pve, &v->vlan_list, list) {
>> +               if (pve->vid == vid)
>> +                       return pve;
>> +       }
>> +
>> +       return NULL;
>> +}
>
> This looks expensive - it's O(n) with n = number of configured VLANs on a port.
> And this is called for every packet. The bridge already has a hash of VLAN
> structures found by br_vlan_find(). You could add a second bitmap there
> (eg. ingres_ports[]) and check port's bit instead of walking the list.
> You would use a bit more memory (64 bytes minus the removed list-head)
> per configured VLAN but save some cycles in hot path.
>

Technically wouldn't even need another bitmap as an existing membership 
bitmap would cover this case.  I did some profiling and the list is 
faster for 3 vlans per port.  Hash is faster for more then 3 vlans.

I can easily switch to hash if that is what others think.

-vlad
> Best Regards,
> Michał Mirosław