inetpeer: seqlock optimization
David noticed : ------------------ 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. ----------------- One solution is to use a seqlock instead of a spinlock to protect struct inet_peer_base. After a failed avl tree lookup, we can easily detect if a writer did some changes during our lookup. Taking the lock and redo the lookup is only necessary in this case. Note: Add one private rcu_deref_locked() macro to place in one spot the access to spinlock included in seqlock. Signed-off-by:Eric Dumazet <eric.dumazet@gmail.com> Signed-off-by:
David S. Miller <davem@davemloft.net>
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