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Date: Wed, 23 Sep 2015 14:00:43 -0700
From: Tom Herbert <tom@...bertland.com>
To: Peter Nørlund <pch@...bogen.com>
Cc: Linux Kernel Network Developers <netdev@...r.kernel.org>,
"David S. Miller" <davem@...emloft.net>,
Alexey Kuznetsov <kuznet@....inr.ac.ru>,
James Morris <jmorris@...ei.org>,
Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
Patrick McHardy <kaber@...sh.net>
Subject: Re: [PATCH v4 net-next 1/2] ipv4: L3 hash-based multipath
On Wed, Sep 23, 2015 at 12:49 PM, Peter Nørlund <pch@...bogen.com> wrote:
> Replaces the per-packet multipath with a hash-based multipath using
> source and destination address.
>
It's good that round robin is going away, but this still looks very
different with how multipath routing is done done in IPv6
(rt6_multipath_select and rt6_info_hash_nhsfn). For instance IPv4
hashes addresses, but IPv6 includes ports. How can we rectify this?
> Signed-off-by: Peter Nørlund <pch@...bogen.com>
> ---
> include/net/ip_fib.h | 14 ++++-
> net/ipv4/fib_semantics.c | 140 +++++++++++++++++++++++++---------------------
> net/ipv4/route.c | 16 ++++--
> 3 files changed, 98 insertions(+), 72 deletions(-)
>
> diff --git a/include/net/ip_fib.h b/include/net/ip_fib.h
> index a37d043..c454203 100644
> --- a/include/net/ip_fib.h
> +++ b/include/net/ip_fib.h
> @@ -79,7 +79,7 @@ struct fib_nh {
> unsigned char nh_scope;
> #ifdef CONFIG_IP_ROUTE_MULTIPATH
> int nh_weight;
> - int nh_power;
> + atomic_t nh_upper_bound;
> #endif
> #ifdef CONFIG_IP_ROUTE_CLASSID
> __u32 nh_tclassid;
> @@ -118,7 +118,7 @@ struct fib_info {
> #define fib_advmss fib_metrics[RTAX_ADVMSS-1]
> int fib_nhs;
> #ifdef CONFIG_IP_ROUTE_MULTIPATH
> - int fib_power;
> + int fib_weight;
> #endif
> struct rcu_head rcu;
> struct fib_nh fib_nh[0];
> @@ -312,7 +312,15 @@ int ip_fib_check_default(__be32 gw, struct net_device *dev);
> int fib_sync_down_dev(struct net_device *dev, unsigned long event);
> int fib_sync_down_addr(struct net *net, __be32 local);
> int fib_sync_up(struct net_device *dev, unsigned int nh_flags);
> -void fib_select_multipath(struct fib_result *res);
> +
> +extern u32 fib_multipath_secret __read_mostly;
> +
> +static inline int fib_multipath_hash(__be32 saddr, __be32 daddr)
> +{
> + return jhash_2words(saddr, daddr, fib_multipath_secret) >> 1;
Why the >> 1?
> +}
> +
> +void fib_select_multipath(struct fib_result *res, int hash);
>
> /* Exported by fib_trie.c */
> void fib_trie_init(void);
> diff --git a/net/ipv4/fib_semantics.c b/net/ipv4/fib_semantics.c
> index 064bd3c..0c49d2f 100644
> --- a/net/ipv4/fib_semantics.c
> +++ b/net/ipv4/fib_semantics.c
> @@ -57,8 +57,7 @@ static unsigned int fib_info_cnt;
> static struct hlist_head fib_info_devhash[DEVINDEX_HASHSIZE];
>
> #ifdef CONFIG_IP_ROUTE_MULTIPATH
> -
> -static DEFINE_SPINLOCK(fib_multipath_lock);
> +u32 fib_multipath_secret __read_mostly;
>
> #define for_nexthops(fi) { \
> int nhsel; const struct fib_nh *nh; \
> @@ -532,7 +531,67 @@ errout:
> return ret;
> }
>
> -#endif
> +static void fib_rebalance(struct fib_info *fi)
> +{
> + int total;
> + int w;
> + struct in_device *in_dev;
> +
> + if (fi->fib_nhs < 2)
> + return;
> +
> + total = 0;
> + for_nexthops(fi) {
> + if (nh->nh_flags & RTNH_F_DEAD)
> + continue;
> +
> + in_dev = __in_dev_get_rcu(nh->nh_dev);
> +
> + if (in_dev &&
> + IN_DEV_IGNORE_ROUTES_WITH_LINKDOWN(in_dev) &&
> + nh->nh_flags & RTNH_F_LINKDOWN)
> + continue;
> +
> + total += nh->nh_weight;
> + } endfor_nexthops(fi);
> +
> + w = 0;
> + change_nexthops(fi) {
> + int upper_bound;
> +
> + in_dev = __in_dev_get_rcu(nexthop_nh->nh_dev);
> +
> + if (nexthop_nh->nh_flags & RTNH_F_DEAD) {
> + upper_bound = -1;
> + } else if (in_dev &&
> + IN_DEV_IGNORE_ROUTES_WITH_LINKDOWN(in_dev) &&
> + nexthop_nh->nh_flags & RTNH_F_LINKDOWN) {
> + upper_bound = -1;
> + } else {
> + w += nexthop_nh->nh_weight;
> + upper_bound = DIV_ROUND_CLOSEST(2147483648LL * w,
> + total) - 1;
> + }
> +
Is this complexity (aside from recomputing the key) needed because you
want consistent hashing for the anycast case?
> + atomic_set(&nexthop_nh->nh_upper_bound, upper_bound);
> + } endfor_nexthops(fi);
> +
> + net_get_random_once(&fib_multipath_secret,
> + sizeof(fib_multipath_secret));
> +}
> +
> +static inline void fib_add_weight(struct fib_info *fi,
> + const struct fib_nh *nh)
> +{
> + fi->fib_weight += nh->nh_weight;
> +}
> +
> +#else /* CONFIG_IP_ROUTE_MULTIPATH */
> +
> +#define fib_rebalance(fi) do { } while (0)
> +#define fib_add_weight(fi, nh) do { } while (0)
> +
> +#endif /* CONFIG_IP_ROUTE_MULTIPATH */
>
> static int fib_encap_match(struct net *net, u16 encap_type,
> struct nlattr *encap,
> @@ -1094,8 +1153,11 @@ struct fib_info *fib_create_info(struct fib_config *cfg)
>
> change_nexthops(fi) {
> fib_info_update_nh_saddr(net, nexthop_nh);
> + fib_add_weight(fi, nexthop_nh);
> } endfor_nexthops(fi)
>
> + fib_rebalance(fi);
> +
> link_it:
> ofi = fib_find_info(fi);
> if (ofi) {
> @@ -1317,12 +1379,6 @@ int fib_sync_down_dev(struct net_device *dev, unsigned long event)
> nexthop_nh->nh_flags |= RTNH_F_LINKDOWN;
> break;
> }
> -#ifdef CONFIG_IP_ROUTE_MULTIPATH
> - spin_lock_bh(&fib_multipath_lock);
> - fi->fib_power -= nexthop_nh->nh_power;
> - nexthop_nh->nh_power = 0;
> - spin_unlock_bh(&fib_multipath_lock);
> -#endif
> dead++;
> }
> #ifdef CONFIG_IP_ROUTE_MULTIPATH
> @@ -1345,6 +1401,8 @@ int fib_sync_down_dev(struct net_device *dev, unsigned long event)
> }
> ret++;
> }
> +
> + fib_rebalance(fi);
> }
>
> return ret;
> @@ -1467,20 +1525,15 @@ int fib_sync_up(struct net_device *dev, unsigned int nh_flags)
> !__in_dev_get_rtnl(dev))
> continue;
> alive++;
> -#ifdef CONFIG_IP_ROUTE_MULTIPATH
> - spin_lock_bh(&fib_multipath_lock);
> - nexthop_nh->nh_power = 0;
> - nexthop_nh->nh_flags &= ~nh_flags;
> - spin_unlock_bh(&fib_multipath_lock);
> -#else
> nexthop_nh->nh_flags &= ~nh_flags;
> -#endif
> } endfor_nexthops(fi)
>
> if (alive > 0) {
> fi->fib_flags &= ~nh_flags;
> ret++;
> }
> +
> + fib_rebalance(fi);
> }
>
> return ret;
> @@ -1488,62 +1541,19 @@ int fib_sync_up(struct net_device *dev, unsigned int nh_flags)
>
> #ifdef CONFIG_IP_ROUTE_MULTIPATH
>
> -/*
> - * The algorithm is suboptimal, but it provides really
> - * fair weighted route distribution.
> - */
> -void fib_select_multipath(struct fib_result *res)
> +void fib_select_multipath(struct fib_result *res, int hash)
> {
> struct fib_info *fi = res->fi;
> - struct in_device *in_dev;
> - int w;
> -
> - spin_lock_bh(&fib_multipath_lock);
> - if (fi->fib_power <= 0) {
> - int power = 0;
> - change_nexthops(fi) {
> - in_dev = __in_dev_get_rcu(nexthop_nh->nh_dev);
> - if (nexthop_nh->nh_flags & RTNH_F_DEAD)
> - continue;
> - if (in_dev &&
> - IN_DEV_IGNORE_ROUTES_WITH_LINKDOWN(in_dev) &&
> - nexthop_nh->nh_flags & RTNH_F_LINKDOWN)
> - continue;
> - power += nexthop_nh->nh_weight;
> - nexthop_nh->nh_power = nexthop_nh->nh_weight;
> - } endfor_nexthops(fi);
> - fi->fib_power = power;
> - if (power <= 0) {
> - spin_unlock_bh(&fib_multipath_lock);
> - /* Race condition: route has just become dead. */
> - res->nh_sel = 0;
> - return;
> - }
> - }
> -
>
> - /* w should be random number [0..fi->fib_power-1],
> - * it is pretty bad approximation.
> - */
> -
> - w = jiffies % fi->fib_power;
> + for_nexthops(fi) {
> + if (hash > atomic_read(&nh->nh_upper_bound))
> + continue;
>
> - change_nexthops(fi) {
> - if (!(nexthop_nh->nh_flags & RTNH_F_DEAD) &&
> - nexthop_nh->nh_power) {
> - w -= nexthop_nh->nh_power;
> - if (w <= 0) {
> - nexthop_nh->nh_power--;
> - fi->fib_power--;
> - res->nh_sel = nhsel;
> - spin_unlock_bh(&fib_multipath_lock);
> - return;
> - }
> - }
> + res->nh_sel = nhsel;
> + return;
> } endfor_nexthops(fi);
>
> /* Race condition: route has just become dead. */
> res->nh_sel = 0;
> - spin_unlock_bh(&fib_multipath_lock);
> }
> #endif
> diff --git a/net/ipv4/route.c b/net/ipv4/route.c
> index 80f7c5b..3dc83b6 100644
> --- a/net/ipv4/route.c
> +++ b/net/ipv4/route.c
> @@ -1659,8 +1659,12 @@ static int ip_mkroute_input(struct sk_buff *skb,
> __be32 daddr, __be32 saddr, u32 tos)
> {
> #ifdef CONFIG_IP_ROUTE_MULTIPATH
> - if (res->fi && res->fi->fib_nhs > 1)
> - fib_select_multipath(res);
> + if (res->fi && res->fi->fib_nhs > 1) {
> + int h;
> +
> + h = fib_multipath_hash(saddr, daddr);
> + fib_select_multipath(res, h);
> + }
> #endif
>
> /* create a routing cache entry */
> @@ -2190,8 +2194,12 @@ struct rtable *__ip_route_output_key(struct net *net, struct flowi4 *fl4)
> }
>
> #ifdef CONFIG_IP_ROUTE_MULTIPATH
> - if (res.fi->fib_nhs > 1 && fl4->flowi4_oif == 0)
> - fib_select_multipath(&res);
> + if (res.fi->fib_nhs > 1 && fl4->flowi4_oif == 0) {
> + int h;
> +
> + h = fib_multipath_hash(fl4->saddr, fl4->daddr);
> + fib_select_multipath(&res, h);
> + }
> else
> #endif
> if (!res.prefixlen &&
> --
> 1.7.10.4
>
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