[Bridge] [PATCH net-next v2] bridge: vlan: allow to suppress local mac install for all vlans

[Nikolay Aleksandrov]

> On Aug 27, 2015, at 4:47 PM, Vlad Yasevich <vyasevic at redhat.com> wrote:
> 
> On 08/27/2015 05:02 PM, Nikolay Aleksandrov wrote:
>> 
>>> On Aug 26, 2015, at 9:57 PM, roopa <roopa at cumulusnetworks.com> wrote:
>>> 
>>> On 8/26/15, 4:33 AM, Nikolay Aleksandrov wrote:
>>>>> On Aug 25, 2015, at 11:06 PM, David Miller <davem at davemloft.net> wrote:
>>>>> 
>>>>> From: Nikolay Aleksandrov <nikolay at cumulusnetworks.com>
>>>>> Date: Tue, 25 Aug 2015 22:28:16 -0700
>>>>> 
>>>>>> Certainly, that should be done and I will look into it, but the
>>>>>> essence of this patch is a bit different. The problem here is not
>>>>>> the size of the fdb entries, it’s more the number of them - having
>>>>>> 96000 entries (even if they were 1 byte ones) is just way too much
>>>>>> especially when the fdb hash size is small and static. We could work
>>>>>> on making it dynamic though, but still these type of local entries
>>>>>> per vlan per port can easily be avoided with this option.
>>>>> 96000 bits can be stored in 12k.  Get where I'm going with this?
>>>>> 
>>>>> Look at the problem sideways.
>>>> Oh okay, I misunderstood your previous comment. I’ll look into that.
>>>> 
>>> I just wanted to add the other problems we have had with keeping these macs (mostly from userspace POV):
>>> - add/del netlink notification storms
>>> - and large netlink dumps
>>> 
>>> In addition to in-kernel optimizations, will be nice to have a solution that reduces the burden on userspace. That will need a newer netlink dump format for fdbs. Considering all the changes needed, Nikolays patch seems less intrusive.
>> 
>> Right, we need to take these into account as well. I’ll continue the discussion on this (or restart it) because
>> I looked into using a bitmap for the local entries only and while it fixes the scalability issue, it presents
>> a few new ones which are mostly related to the fact that these entries now exist only without a vlan
>> and if a new mac comes along which matches one of these but is in a vlan, the entry will get created
>> in br_fdb_update() unless we add a second lookup, but that will slow down the learning path.
>> Also this change requires an update of every fdb function that uses the vid as a key (every fdb function?!)
>> because now we can have the mac in two places instead of one which is a pretty big churn with lots
>> of conditionals all over the place and I don’t like it. Adding this complexity for the local addresses only
>> seems like an overkill, so I think to drop this issue for now.
> 
> I seem to recall Roopa and I and maybe a few others have discussing this a few
> years ago at plumbers, I can't remember the details any more.  All these local
> addresses add a ton of confusion.  Does anyone (Stephen?) remember what the
> original reason was for all these local addresses? I wonder if we can have
> a nob to disable all of them (not just per vlan)?  That might be cleaner and
> easier to swallow.
> 

Right, this would be the easiest way and if the others agree - I’ll post a patch for it so we can
have some way to resolve it today and even if we fix the scalability issue, this is still a valid case
that some people don’t want local fdbs installed automatically.
Any objections to this ?

>> This patch (that works around the initial problem) also has these issues.
>> Note that one way to take care of this in a more straight-forward way would be to have each entry
>> with some sort of a bitmap (like Vlad has tried earlier) and then we can combine the paths so most
>> of these issues disappear, but that will not be easy as was already commented earlier. I’ve looked
>> briefly into doing this with rhashtable so we can keep the memory footprint for each entry relatively
>> small but it still affects the performance and we can have thousands of resizes happening. 
>> 
> 
> So, one of the earlier approaches that I've tried (before rhashtable was
> in the kernel) was to have a hash of vlan ids each with a data structure
> pointing to a list of ports for a given vlan as well as a list of fdbs for
> a given vlan.  As far as scalability goes, that's really the best approach.
> It would also allow us to do packet accounting per vlan.  The only concern
> at the time was performance of ingress lookup.   I think rhashtables might
> help with this as well as ability to grow the footprint of the vlan hash
> table dynamically.
> 
> -vlad
> 
I’ll look into it but I’m guessing the learning will become a more complicated process with additional 
allocations and some hash handling.

>> On the notification side if we can fix that, we can actually delete the 96000 entries without creating a
>> huge notification storm and do a user-land workaround of the original issue, so I’ll look into that next.
>> 
>> Any comments or ideas are very welcome.
>> 
>> Thank you,
>> Nik