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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: Simon Horman <horms@xxxxxxxxxxxx>, Sergei Shtylyov <sergei.shtylyov@xxxxxxxxxxxxxxxxxx>, lvs-devel@xxxxxxxxxxxxxxx, Wensong Zhang <wensong@xxxxxxxxxxxx>, netdev@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx
From: Julian Anastasov <ja@xxxxxx>
Date: Thu, 26 Sep 2013 08:30:14 +0300 (EEST)
        Hello,

On Wed, 25 Sep 2013, 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>
> 
> --
> diff --git a/net/netfilter/ipvs/ip_vs_sh.c b/net/netfilter/ipvs/ip_vs_sh.c
> index 3588fae..3d5ab7c 100644
> --- a/net/netfilter/ipvs/ip_vs_sh.c
> +++ b/net/netfilter/ipvs/ip_vs_sh.c
> @@ -115,27 +115,47 @@ ip_vs_sh_get(struct ip_vs_service *svc, struct 
> ip_vs_sh_state *s,
>  }
>  
>  
> -/* As ip_vs_sh_get, but with fallback if selected server is unavailable */
> +/* As ip_vs_sh_get, but with fallback if selected server is unavailable
> + *
> + * The fallback strategy loops around the table starting from a "random"
> + * point (in fact, it is chosen to be the original hash value to make the
> + * algorithm deterministic) to find a new server.
> + */
>  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)",
> +     /* first try the dest it's supposed to go to */
> +     ihash = ip_vs_sh_hashkey(svc->af, addr, port, 0);
> +     dest = rcu_dereference(s->buckets[ihash].dest);
> +     if (!dest)
> +             return NULL;

        Can we reduce the indentation here, eg:

        if (!is_unavailable(dest))
                return dest;
        IP_VS_DBG_BUF(6, "SH: selected unavailable server "
        ...
        for ()...
        ...
        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));
> +             /* if the original dest is unavailable, loop around the table
> +              * starting from ihash to find a new dest
> +              */
> +             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);

        Every result of rcu_dereference should be checked
for NULL (no dests anymore):
                        if (!dest)
                                break;

        Then make sure there is correct indentation
for IP_VS_DBG_BUF parameters.

> +                     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;

Regards

--
Julian Anastasov <ja@xxxxxx>
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