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Re: [PATCH] ipvs: improved SH fallback strategy

To: Alexander Frolkin <avf@xxxxxxxxxxxxxx>
Subject: Re: [PATCH] ipvs: improved SH fallback strategy
Cc: Sergei Shtylyov <sergei.shtylyov@xxxxxxxxxxxxxxxxxx>, Julian Anastasov <ja@xxxxxx>, lvs-devel@xxxxxxxxxxxxxxx, Wensong Zhang <wensong@xxxxxxxxxxxx>, netdev@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx
From: Simon Horman <horms@xxxxxxxxxxxx>
Date: Wed, 25 Sep 2013 09:30:33 +0900
On Tue, Sep 24, 2013 at 10:32:38AM +0100, Alexander Frolkin wrote:
> Improve the SH fallback realserver selection strategy.
>  
> With sh and sh-fallback, if a realserver is down, this attempts to
> distribute the traffic that would have gone to that server evenly
> among the remaining servers.
>  
> Signed-off-by: Alexander Frolkin <avf@xxxxxxxxxxxxxx>

Hi Alexander,

could you add some comments to the code or at least a description of the
algorithm to the above the function.  The intent of original code may not
have been obvious to the eye but this version certainly isn't obvious to
mine.

> --
> diff --git a/net/netfilter/ipvs/ip_vs_sh.c b/net/netfilter/ipvs/ip_vs_sh.c
> index 3588fae..0db7d01 100644
> --- a/net/netfilter/ipvs/ip_vs_sh.c
> +++ b/net/netfilter/ipvs/ip_vs_sh.c
> @@ -120,22 +120,33 @@ static inline struct ip_vs_dest *
>  ip_vs_sh_get_fallback(struct ip_vs_service *svc, struct ip_vs_sh_state *s,
>                     const union nf_inet_addr *addr, __be16 port)
>  {
> -     unsigned int offset;
> -     unsigned int hash;
> +     unsigned int offset, roffset;
> +     unsigned int hash, ihash;
>       struct ip_vs_dest *dest;
>  
> -     for (offset = 0; offset < IP_VS_SH_TAB_SIZE; offset++) {
> -             hash = ip_vs_sh_hashkey(svc->af, addr, port, offset);
> -             dest = rcu_dereference(s->buckets[hash].dest);
> -             if (!dest)
> -                     break;
> -             if (is_unavailable(dest))
> -                     IP_VS_DBG_BUF(6, "SH: selected unavailable server "
> -                                   "%s:%d (offset %d)",
> +     ihash = ip_vs_sh_hashkey(svc->af, addr, port, 0);
> +     dest = rcu_dereference(s->buckets[ihash].dest);
> +     if (!dest)
> +             return NULL;
> +     if (is_unavailable(dest)) {
> +             IP_VS_DBG_BUF(6, "SH: selected unavailable server "
> +                   "%s:%d, reselecting",
> +                   IP_VS_DBG_ADDR(svc->af, &dest->addr),
> +                   ntohs(dest->port));
> +             for (offset = 0; offset < IP_VS_SH_TAB_SIZE; offset++) {
> +                     roffset = (offset + ihash) % IP_VS_SH_TAB_SIZE;
> +                     hash = ip_vs_sh_hashkey(svc->af, addr, port, roffset);
> +                     dest = rcu_dereference(s->buckets[hash].dest);
> +                     if (is_unavailable(dest))
> +                             IP_VS_DBG_BUF(6, "SH: selected unavailable "
> +                                   "server %s:%d (offset %d), reselecting",
>                                     IP_VS_DBG_ADDR(svc->af, &dest->addr),
> -                                   ntohs(dest->port), offset);
> -             else
> -                     return dest;
> +                                   ntohs(dest->port), roffset);
> +                     else
> +                             return dest;
> +             }
> +     } else {
> +             return dest;
>       }
>  
>       return NULL;
> 
> --
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