From: Vitaliy Makkoveev <mvs@openbsd.org>
Subject: Re: unlock nd6 hiint
To: Alexander Bluhm <bluhm@openbsd.org>
Cc: tech@openbsd.org
Date: Thu, 30 Jan 2025 18:57:46 +0300

On Thu, Jan 30, 2025 at 10:19:02PM +0100, Alexander Bluhm wrote:
> On Sun, Jan 26, 2025 at 06:32:23PM +0100, Alexander Bluhm wrote:
> > Hi,
> > 
> > TCP input is calling nd6_nud_hint() so I want to get rid of the
> > exclusiv net lock.  Better use nd6 mutex in combination with shared
> > net lock.
> 
> I did run this diff though my performance tests.  With IPv6 TCP I
> see 1.5% decrease in throughput.  This is more or less what happens
> due to a per packet mutex in the hot path.  As I see no other way
> to run tcp_input() in parallel.  ND6 complexity requires to read
> and update ln_state by multiple CPU.  We can always optmize later
> and make ND6 locking smarter.
> 
> > ok?

ok mvs

> 
> anyone?
> 
> > bluhm
> > 
> > Index: netinet6/nd6.c
> > ===================================================================
> > RCS file: /data/mirror/openbsd/cvs/src/sys/netinet6/nd6.c,v
> > diff -u -p -r1.283 nd6.c
> > --- netinet6/nd6.c	4 Sep 2024 07:54:52 -0000	1.283
> > +++ netinet6/nd6.c	26 Jan 2025 00:11:47 -0000
> > @@ -709,21 +709,25 @@ nd6_nud_hint(struct rtentry *rt)
> >  	struct llinfo_nd6 *ln;
> >  	struct ifnet *ifp;
> >  
> > -	NET_ASSERT_LOCKED_EXCLUSIVE();
> > -
> > -	ifp = if_get(rt->rt_ifidx);
> > -	if (ifp == NULL)
> > -		return;
> > +	NET_ASSERT_LOCKED();
> >  
> >  	if ((rt->rt_flags & RTF_GATEWAY) != 0 ||
> >  	    (rt->rt_flags & RTF_LLINFO) == 0 ||
> > -	    rt->rt_llinfo == NULL || rt->rt_gateway == NULL ||
> > +	    rt->rt_gateway == NULL ||
> >  	    rt->rt_gateway->sa_family != AF_LINK) {
> >  		/* This is not a host route. */
> > -		goto out;
> > +		return;
> >  	}
> >  
> > +	ifp = if_get(rt->rt_ifidx);
> > +	if (ifp == NULL)
> > +		return;
> > +
> > +	mtx_enter(&nd6_mtx);
> > +
> >  	ln = (struct llinfo_nd6 *)rt->rt_llinfo;
> > +	if (ln == NULL)
> > +		goto out;
> >  	if (ln->ln_state < ND6_LLINFO_REACHABLE)
> >  		goto out;
> >  
> > @@ -739,6 +743,7 @@ nd6_nud_hint(struct rtentry *rt)
> >  	if (!ND6_LLINFO_PERMANENT(ln))
> >  		nd6_llinfo_settimer(ln, ifp->if_nd->reachable);
> >  out:
> > +	mtx_leave(&nd6_mtx);
> >  	if_put(ifp);
> >  }
> >  
>