netdev - Re: [PATCH net-next 1/8] rtnetlink: Do not generate notifications for MTU events

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Date:   Sat, 8 Apr 2017 22:58:44 -0400
From:   David Ahern <dsa@...ulusnetworks.com>
To:     vyasevic@...hat.com, netdev@...r.kernel.org
Cc:     davem@...emloft.net
Subject: Re: [PATCH net-next 1/8] rtnetlink: Do not generate notifications for
 MTU events

On 4/8/17 9:36 PM, Vlad Yasevich wrote:
> On 04/07/2017 05:25 PM, David Ahern wrote:
>> Changing MTU on a link currently causes 3 messages to be sent to userspace:
>>
>> [LINK]11: dummy1: <BROADCAST,NOARP,UP,LOWER_UP> mtu 1490 qdisc noqueue state UNKNOWN group default event PRE_CHANGE_MTU
>>     link/ether f2:52:5c:6d:21:f3 brd ff:ff:ff:ff:ff:ff
>>
>> [LINK]11: dummy1: <BROADCAST,NOARP,UP,LOWER_UP> mtu 1500 qdisc noqueue state UNKNOWN group default event CHANGE_MTU
>>     link/ether f2:52:5c:6d:21:f3 brd ff:ff:ff:ff:ff:ff
>>
>> [LINK]11: dummy1: <BROADCAST,NOARP,UP,LOWER_UP> mtu 1500 qdisc noqueue state UNKNOWN group default
>>     link/ether f2:52:5c:6d:21:f3 brd ff:ff:ff:ff:ff:ff
>>
>> Remove the PRE_CHANGE_MTU and CHANGE_MTU messages.
> Actually, I have plans for the CHANGE_MTU event.  The last message is not an event. If you
> remove the event, it is much harder to track mtu changes.

it is a set of redundant messages. Roopa suggested using a bitmask for
the events changed by do_setlink which gets added to the last message
seen above. In doing so you only generate 1 message. The above causes
libnl (for example):

- message 1: delete dummy1 from its cache and re-create it
- message 2: delete dummy1 from its cache and re-create it
- message 3: delete dummy1 from its cache and re-create it

All you need is a single message at the end with an attribute that says
"MTU changed"