netdev - Re: [PATCH 3/6] IPV4 : use xor rather than multiple ands for route compare

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Message-Id: <20080410.180148.120248426.yoshfuji@linux-ipv6.org>
Date:	Thu, 10 Apr 2008 18:01:48 +0900 (JST)
From:	YOSHIFUJI Hideaki / 吉藤英明 
	<yoshfuji@...ux-ipv6.org>
To:	davem@...emloft.net
Cc:	shemminger@...tta.com, dada1@...mosbay.com, netdev@...r.kernel.org,
	yoshfuji@...ux-ipv6.org
Subject: Re: [PATCH 3/6] IPV4 : use xor rather than multiple ands for route
 compare

In article <20080410.015118.103465510.davem@...emloft.net> (at Thu, 10 Apr 2008 01:51:18 -0700 (PDT)), David Miller <davem@...emloft.net> says:

> From: Stephen Hemminger <shemminger@...tta.com>
> Date: Tue, 1 Apr 2008 13:08:42 -0700
> 
> > The flow fields are all together, and the other parameters are local variables
> > in registers so that compare should be in one cache line.
> > 
> > --- a/net/ipv4/route.c	2008-03-31 17:12:30.000000000 -0700
> > +++ b/net/ipv4/route.c	2008-04-01 13:05:46.000000000 -0700
> > @@ -2079,12 +2079,12 @@ int ip_route_input(struct sk_buff *skb, 
> >  	rcu_read_lock();
> >  	for (rth = rcu_dereference(rt_hash_table[hash].chain); rth;
> >  	     rth = rcu_dereference(rth->u.dst.rt_next)) {
> > -		if (rth->fl.fl4_dst == daddr &&
> > -		    rth->fl.fl4_src == saddr &&
> > -		    rth->fl.iif == iif &&
> > -		    rth->fl.oif == 0 &&
> > +		if (((rth->fl.fl4_dst ^ daddr) |
> > +		     (rth->fl.fl4_src ^ saddr) |
> > +		     (rth->fl.iif ^ iif) |
> > +		     rth->fl.oif |
> > +		     (rth->fl.fl4_tos ^ tos)) == 0 &&
> >  		    rth->fl.mark == skb->mark &&
> > -		    rth->fl.fl4_tos == tos &&
> >  		    net_eq(dev_net(rth->u.dst.dev), net) &&
> >  		    rth->rt_genid == atomic_read(&rt_genid)) {
> >  			dst_use(&rth->u.dst, jiffies);
> 
> Eric, any objections to this version?

I'm not Eric, but well, I'm now doubting if this is really good.
If the comparision chain is long and it is unlikely to pass all the tests,
it would be better to cut the line.
If we use "or", we need to run through the test, in ayn case.

--yoshfuji
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