netdev - Re: neighbour table RCU

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Date:	Tue, 1 Sep 2009 14:24:06 -0700
From:	Stephen Hemminger <shemminger@...tta.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	netdev@...r.kernel.org
Subject: Re: neighbour table RCU

On Tue, 01 Sep 2009 18:13:40 +0200
Eric Dumazet <eric.dumazet@...il.com> wrote:

> Stephen Hemminger a écrit :
> > On Tue, 01 Sep 2009 08:50:17 +0200
> > Eric Dumazet <eric.dumazet@...il.com> wrote:
> > 
> >> Stephen Hemminger a écrit :
> >>> Looking at the neighbour table, it should be possible to get
> >>> rid of the two reader/writer locks.  The hash table lock is pretty
> >>> amenable to RCU, but the dynamic resizing makes it non-trivial.
> >>> Thinking of using a combination of RCU and sequence counts so that the
> >>> reader would just rescan if resize was in progress.
> >> I am not sure neigh_tbl_lock rwlock should be changed, I did not
> >> see any contention on it.
> >>
> >>> The reader/writer lock on the neighbour entry is more of a problem.
> >>> Probably would be simpler/faster to change it into a spinlock and
> >>> be done with it.
> >>>
> >>> The reader/writer lock is also used for the proxy list hash table,
> >>> but that can just be a simple spinlock.
> >>>
> >> This is probably is the only thing we want to do at this moment,
> >> halving atomic ops on neigh_resolve_output()
> >>
> >> But why neigh_resolve_output() was called so much in the bench
> >> is the question...
> >>
> > 
> > Every packet has to have an ARP resolution.
> > 
> 
> Sure, but I thought we had a cache ?
> 
> static inline int ip_finish_output2(struct sk_buff *skb)
> {
> ...
> 	if (dst->hh)
> 		return neigh_hh_output(dst->hh, skb);
> 	else if (dst->neighbour)
> 		return dst->neighbour->output(skb);  << should fill cache first time >>
> ...
> }
> 
> in my pktgen benches, I always hit same dst so should take the hh cache ?

I ping the remote host before starting pktgen, that way I figure
the ARP and cache is available.

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