netdev - Re: [PATCH net-next 4/4] ipv6: Add support for non-equal-cost multipath

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Date:   Wed, 10 Jan 2018 13:47:38 +0200
From:   Ido Schimmel <idosch@...sch.org>
To:     David Ahern <dsahern@...il.com>
Cc:     Ido Schimmel <idosch@...lanox.com>, netdev@...r.kernel.org,
        davem@...emloft.net, roopa@...ulusnetworks.com,
        nikolay@...ulusnetworks.com, pch@...bogen.com, jkbs@...hat.com,
        yoshfuji@...ux-ipv6.org, mlxsw@...lanox.com
Subject: Re: [PATCH net-next 4/4] ipv6: Add support for non-equal-cost
 multipath

Hi David,

On Tue, Jan 09, 2018 at 08:48:37PM -0700, David Ahern wrote:
> On 1/9/18 7:40 AM, Ido Schimmel wrote:
> > The use of hash-threshold instead of modulo-N makes it trivial to add
> > support for non-equal-cost multipath.
> > 
> > Instead of dividing the multipath hash function's output space equally
> > between the nexthops, each nexthop is assigned a region size which is
> > proportional to its weight.
> > 
> > Signed-off-by: Ido Schimmel <idosch@...lanox.com>
> > ---
> >  include/net/ip6_fib.h |  1 +
> >  net/ipv6/route.c      | 11 +++++++----
> >  2 files changed, 8 insertions(+), 4 deletions(-)
> > 
> > diff --git a/include/net/ip6_fib.h b/include/net/ip6_fib.h
> > index 97cd05d87780..34ec321d6a03 100644
> > --- a/include/net/ip6_fib.h
> > +++ b/include/net/ip6_fib.h
> > @@ -171,6 +171,7 @@ struct rt6_info {
> >  	u32				rt6i_metric;
> >  	u32				rt6i_pmtu;
> >  	/* more non-fragment space at head required */
> > +	int				rt6i_nh_weight;
> >  	unsigned short			rt6i_nfheader_len;
> >  	u8				rt6i_protocol;
> >  	u8				exception_bucket_flushed:1,
> 
> Since dst is cacheline aligned there is a hole after rt6i_nh_flags. In
> patch 1 you put rt6i_nh_upper_bound in that hole. Putting the weight
> there too keeps those variables together as well as using the open space.

Before patch 1 there's a hole of 4 bytes after rt6i_nh_flags which I use
for rt6i_nh_upper_bound. If I put rt6i_nh_weight there as well, then I
create a 60 bytes hole because the dst needs to be cached aligned.

Since rt6i_nh_weight isn't used in fast-path, I just put it at the end.