Subject: Re: neighbour table RCU

From: Eric Dumazet <[email-masked]>
Date: Tue, 01 Sep 2009 18:14:37 +0200

Octavian Purdila a écrit :
> On Tuesday 01 September 2009 09:50:17 Eric Dumazet wrote:
>> Stephen Hemminger a écrit :
>>> Looking at the neighbour table, it should be possible to get
>>> rid of the two reader/writer locks.  The hash table lock is pretty
>>> amenable to RCU, but the dynamic resizing makes it non-trivial.
>>> Thinking of using a combination of RCU and sequence counts so that the
>>> reader would just rescan if resize was in progress.
>> I am not sure neigh_tbl_lock rwlock should be changed, I did not
>> see any contention on it.
>>
> 
> Speaking about neighbour optimizations, here is a RFC patch which makes the 
> tables double linked, for constant time deletion. It has given us a significant 
> performance improvement - in less then usual setups though, with lots of 
> neighbours.

How many "struct neigh_parms" do you have in your setups, and
what is the frequency of neigh_parms_release() calls ???

> 
> Would something like this be acceptable for upstream? (pardon the p4 diff dump 
> :) - but I think it will give a rough idea, if acceptable will clean it up and 
> properly submit it)

Seems straightforward

> 
> BTW, would switching to list_head be better?

Yes, definitly :)

> 
> Thanks,
> tavi
> 
> ==== //packages/linux-2.6.7/main/src/include/net/neighbour.h#2 (text) ====
> 
> @@ -56,6 +56,7 @@
>  struct neigh_parms
>  {
>         struct neigh_parms *next;
> +       struct neigh_parms **pprev;
>         int     (*neigh_setup)(struct neighbour *);
>         struct neigh_table *tbl;
>         int     entries;
> 
> ==== //packages/linux-2.6.7/main/src/net/core/neighbour.c#3 (text) ====
> 
> @@ -1127,8 +1127,10 @@
>                 }
>                 p->sysctl_table = NULL;
>                 write_lock_bh(&tbl->lock);
> -               p->next         = tbl->parms.next;
> +               if ((p->next = tbl->parms.next))
> +                       p->next->pprev = &p->next;
>                 tbl->parms.next = p;
> +               p->pprev = &tbl->parms.next;
>                 write_unlock_bh(&tbl->lock);
>         }
>         return p;
> @@ -1136,21 +1138,14 @@
>  
>  void neigh_parms_release(struct neigh_table *tbl, struct neigh_parms *parms)
>  {
> -       struct neigh_parms **p;
> -
>         if (!parms || parms == &tbl->parms)
>                 return;
>         write_lock_bh(&tbl->lock);
> -       for (p = &tbl->parms.next; *p; p = &(*p)->next) {
> -               if (*p == parms) {
> -                       *p = parms->next;
> -                       write_unlock_bh(&tbl->lock);
> -                       kfree(parms);
> -                       return;
> -               }
> -       }
> +       if ((*parms->pprev = parms->next))
> +               parms->next->pprev = parms->pprev;
>         write_unlock_bh(&tbl->lock);
> -       NEIGH_PRINTK1("neigh_parms_release: not found\n");
> +       kfree(parms);
> +       return;
>  }
> --