Subject: Re: [PATCH net-2.6.26] fib_trie: RCU optimizations

From: Stephen Hemminger <[email-masked]>
Date: Fri, 21 Mar 2008 10:31:56 -0700

On Fri, 21 Mar 2008 18:25:04 +0100
Eric Dumazet <[email-masked]> wrote:

> Paul E. McKenney a écrit :
> > On Fri, Mar 21, 2008 at 07:55:21AM -0700, Stephen Hemminger wrote:
> >   
> >> Small performance improvements.
> >>
> >> Eliminate unneeded barrier on deletion. The first pointer to update
> >> the head of the list is ordered by the second call to rcu_assign_pointer.
> >> See hlist_add_after_rcu or comparision.
> >>
> >> Move rcu_derference to the loop check (like hlist_for_each_rcu), and
> >> add a prefetch.
> >>     
> >
> > Acked-by: Paul E. McKenney <[email-masked]>
> >
> > Justification below.
> >
> >   
> >> Signed-off-by: Stephen Hemminger <[email-masked]>
> >>
> >> --- a/net/ipv4/route.c	2008-03-19 08:45:32.000000000 -0700
> >> +++ b/net/ipv4/route.c	2008-03-19 08:54:57.000000000 -0700
> >> @@ -977,8 +977,8 @@ restart:
> >>  			 * must be visible to another weakly ordered CPU before
> >>  			 * the insertion at the start of the hash chain.
> >>  			 */
> >> -			rcu_assign_pointer(rth->u.dst.rt_next,
> >> -					   rt_hash_table[hash].chain);
> >> +			rth->u.dst.rt_next = rt_hash_table[hash].chain;
> >> +
> >>     
> >
> > This is OK because it is finalizing a deletion.  If this were instead
> > an insertion, this would of course be grossly illegal and dangerous.
> >
> >   
> >>  			/*
> >>  			 * Since lookup is lockfree, the update writes
> >>  			 * must be ordered for consistency on SMP.
> >> @@ -2076,8 +2076,9 @@ int ip_route_input(struct sk_buff *skb, 
> >>  	hash = rt_hash(daddr, saddr, iif);
> >>
> >>  	rcu_read_lock();
> >> -	for (rth = rcu_dereference(rt_hash_table[hash].chain); rth;
> >> -	     rth = rcu_dereference(rth->u.dst.rt_next)) {
> >> +	for (rth = rt_hash_table[hash].chain; rcu_dereference(rth);
> >> +	     rth = rth->u.dst.rt_next) {
> >> +		prefetch(rth->u.dst.rt_next);
> >>  		if (rth->fl.fl4_dst == daddr &&
> >>  		    rth->fl.fl4_src == saddr &&
> >>  		    rth->fl.iif == iif &&
> >>     
> >
> > Works, though I would guess that increasingly aggressive compiler
> > optimization will eventually force us to change the list.h macros
> > to look like what you had to begin with...  Sigh!!!
> >
> >   
> 
> Hum... I missed the original patch , but this prefetch() is wrong.
> 
> On lookups, we dont want to prefetch the begining of "struct rtable" 
> entries.

That makes sense when hash is perfect, but under DoS scenario
the hash table will not match exactly, and the next pointer will
be needed.