Re: [net PATCH] fib_trie: Fix trie balancing issue if new node pushes down existing node

From: Alexander Duyck <redacted>
Date: Fri, 12 Dec 2014 07:55:02 -0800

On 12/11/2014 06:32 PM, David Miller wrote:

quoted

From: Alexander Duyck <redacted>
Date: Wed, 10 Dec 2014 21:49:22 -0800

quoted

This patch addresses an issue with the level compression of the fib_trie.
Specifically in the case of adding a new leaf that triggers a new node to
be added that takes the place of the old node.  The result is a trie where
the 1 child tnode is on one side and one leaf is on the other which gives
you a very deep trie.  Below is the script I used to generate a trie on
dummy0 with a 10.X.X.X family of addresses.

 ...

quoted

What this fix does is start the rebalance at the newly created tnode
instead of at the parent tnode.  This way if there is a gap between the
parent and the new node it doesn't prevent the new tnode from being
coalesced with any pre-existing nodes that may have been pushed into one
of the new nodes child branches.

Signed-off-by: Alexander Duyck <redacted>

One has to be mindful with this code that what it's doing now might
be intentional.  For example, it might be doing things this way
on purpose in order to minimize rebalancing during route flaps.

Barring anything like that, I think your change is fine.

I'm fairly certain that this isn't intentional.  If we replace a NULL
pointer in an existing tnode then we rebalance starting at that tnode,
it is only when there is no room in the trie and we have to add a new
tnode that the issue occurs where we rebalance at the parent and not the
tnode that the leaf was added to.

Ok, thanks for taking the time to explain this, I'm now convinced :)

Applied, thanks again.