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Date: Wed, 02 Mar 2011 20:45:45 -0800 (PST)
From: David Miller <davem@...emloft.net>
To: eric.dumazet@...il.com
CC: netdev@...r.kernel.org
Subject: inetpeer with create==0
Eric, I was profiling the non-routing-cache case and something that stuck
out is the case of calling inet_getpeer() with create==0.
If an entry is not found, we have to redo the lookup under a spinlock
to make certain that a concurrent writer rebalancing the tree does
not "hide" an existing entry from us.
This makes the case of a create==0 lookup for a not-present entry
really expensive. It is on the order of 600 cpu cycles on my
Niagara2.
I added a hack to not do the relookup under the lock when create==0
and it now costs less than 300 cycles.
This is now a pretty common operation with the way we handle COW'd
metrics, so I think it's definitely worth optimizing.
I looked at the generic radix tree implementation, and it supports
full RCU lookups in parallel with insert/delete. It handles the race
case without the relookup under lock because it creates fixed paths
to "slots" where nodes live using shifts and masks. So if a path
to a slot ever existed, it will always exist.
Take a look at lib/radix-tree.c and include/linux/radix-tree.h if
you are curious.
I think we should do something similar for inetpeer. Currently we
cannot just use the existing generic radix-tree code because it only
supports indexes as large as "unsigned long" and we need to handle
128-bit ipv6 addresses.
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