netdev - Re: neighbour table RCU

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Date:	Tue, 1 Sep 2009 18:55:34 +0300
From:	Octavian Purdila <opurdila@...acom.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Stephen Hemminger <shemminger@...tta.com>,
	Lucian Adrian Grijincu <lgrijincu@...acom.com>,
	netdev@...r.kernel.org
Subject: Re: neighbour table RCU

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.

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)

BTW, would switching to list_head be better?

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;
 }
--
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