linux-kernel - Re: [PATCH 1/2] net: ipv4: route.c: add likely() statements

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Date:   Wed, 24 Mar 2021 14:19:32 +0200
From:   Leon Romanovsky <leon@...nel.org>
To:     Yejune Deng <yejune.deng@...il.com>
Cc:     David Miller <davem@...emloft.net>, yoshfuji@...ux-ipv6.org,
        dsahern@...nel.org, Jakub Kicinski <kuba@...nel.org>,
        netdev <netdev@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        yejune@...il.com
Subject: Re: [PATCH 1/2] net: ipv4: route.c: add likely() statements

On Wed, Mar 24, 2021 at 07:01:19PM +0800, Yejune Deng wrote:
> My reasons are as following: ipv4_confirm_neigh() belongs to
> ipv4_dst_ops that family is AF_INET, and ipv4_neigh_lookup() is also
> added likely() when rt->rt_gw_family is equal to AF_INET.

It is part of that cargo cult. Please support your claim with
performance numbers when this likely/unlikely will give any difference.

Thanks

> 
> On Wed, Mar 24, 2021 at 6:34 PM Leon Romanovsky <leon@...nel.org> wrote:
> >
> > On Wed, Mar 24, 2021 at 11:09:22AM +0800, Yejune Deng wrote:
> > > Add likely() statements in ipv4_confirm_neigh() for 'rt->rt_gw_family
> > > == AF_INET'.
> >
> > Why? Such macros are beneficial in only specific cases, most of the time,
> > likely/unlikely is cargo cult.
> >
> > >
> > > Signed-off-by: Yejune Deng <yejune.deng@...il.com>
> > > ---
> > >  net/ipv4/route.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/net/ipv4/route.c b/net/ipv4/route.c
> > > index fa68c2612252..5762d9bc671c 100644
> > > --- a/net/ipv4/route.c
> > > +++ b/net/ipv4/route.c
> > > @@ -440,7 +440,7 @@ static void ipv4_confirm_neigh(const struct dst_entry *dst, const void *daddr)
> > >       struct net_device *dev = dst->dev;
> > >       const __be32 *pkey = daddr;
> > >
> > > -     if (rt->rt_gw_family == AF_INET) {
> > > +     if (likely(rt->rt_gw_family == AF_INET)) {
> > >               pkey = (const __be32 *)&rt->rt_gw4;
> > >       } else if (rt->rt_gw_family == AF_INET6) {
> > >               return __ipv6_confirm_neigh_stub(dev, &rt->rt_gw6);
> > > --
> > > 2.29.0
> > >