netdev - RE: Clarification regarding IFLA_BRPORT_LEARNING_SYNC and aging of fdb entries learnt via br_fdb_external_learn

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Date:	Fri, 9 Jan 2015 16:15:58 +0000
From:	"Arad, Ronen" <ronen.arad@...el.com>
To:	Scott Feldman <sfeldma@...il.com>, Jiri Pirko <jiri@...nulli.us>
CC:	Siva Mannem <siva.mannem.lnx@...il.com>,
	Netdev <netdev@...r.kernel.org>
Subject: RE: Clarification regarding IFLA_BRPORT_LEARNING_SYNC and aging of
 fdb entries learnt via br_fdb_external_learn_add()



>-----Original Message-----
>From: netdev-owner@...r.kernel.org [mailto:netdev-owner@...r.kernel.org] On
>Behalf Of Scott Feldman
>Sent: Friday, January 09, 2015 3:47 AM
>To: Jiri Pirko
>Cc: Siva Mannem; Netdev
>Subject: Re: Clarification regarding IFLA_BRPORT_LEARNING_SYNC and aging of
>fdb entries learnt via br_fdb_external_learn_add()
>
>On Wed, Jan 7, 2015 at 4:53 AM, Jiri Pirko <jiri@...nulli.us> wrote:
>> Tue, Dec 30, 2014 at 07:20:21PM CET, siva.mannem.lnx@...il.com wrote:
>>>Hi,
>>>
>>>I am trying to understand the ongoing switch device offload effort and
>>>am following the discussions. I have a question regarding
>>>IFLA_BRPORT_LEARNING_SYNC flag and how aging happens when this flag is
>>>enabled on a port that is attached to a bridge that has vlan filtering
>>>enabled.
>>>
>>>If I understand correctly, when  IFLA_BRPORT_LEARNING_SYNC is set on a
>>>bridge port, fdb entries that are learnt externally(may be learnt by
>>>hardware and driver is notified) are synced to bridges fdb using
>>>br_fdb_external_learn_add(). The fdb
>>>entries(fdb->added_by_external_learn set to true) that are learnt via
>>>this method are also deleted by the aging logic after the aging time
>>>even though L2 data forwadring  happens in hardware.
>
>This is correct...
>
>>> Is there a way
>>>where aging can be disabled for these entries? and let the entries be
>>>removed only via br_fdb_external_learn_delete()? or am I missing
>>>something?
>>
>> Currently extenaly learned fdb entries are indeed removed during aging
>> cleanup. I believe that br_fdb_cleanup should check added_by_external_learn
>> and not remove that fdbs. What do you think Scott?
>
>Something like that would work, if we added another brport flag to
>control that.  With the current arrangement, using bridge for aging
>out entries, we want br_fdb_cleanup removing the external_learned
>fdbs, but if there was another brport flag we could fine tune that.
>Say new flag is IFLA_BRPORT_AGING_OFFLOAD or something like that.  I'm
>not sure how aging settings for the bridge are pushed down to offload
>hw, or if there is a different set for hw.
>
>But, isn't it nice to let Linux bridge control aging?  That way,
>bridge -s fdb dump shows nice statistics on fdb entries.  Hardware
>isn't involved in the aging processes (other than being told to remove
>an entry).  Simple hardware equals simple driver.  Linux remains
>control point.
>
It is indeed simpler. However, if the overhead of reading hit bits from the HW
and updating freshness of entries using br_fdb_external_learn_add() is too
expensive, it would force such platforms to disable learning sync altogether.
Therefore, I believe aging offload flag (could be sufficient at bridge level)
and external aging interval (possibly longer than the software aging interval)
will encourage drivers to use leaning sync.
>-scott
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