netdev - Re: [PATCH 1/2] net: Allow to create links with given ifindex

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Date:	Thu, 02 Aug 2012 12:28:30 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Pavel Emelyanov <xemul@...allels.com>,
	Linux Netdev List <netdev@...r.kernel.org>,
	David Miller <davem@...emloft.net>
Subject: Re: [PATCH 1/2] net: Allow to create links with given ifindex

On Tue, 2012-07-31 at 04:58 -0700, Eric W. Biederman wrote:

> Making lo the particularly interesting case.

BTW, I noticed in my benchmarks, that once I remove the contention on
dst refcnt (using a percpu cache of dsts), I have a strange performance
cost accessing net->loopback_dev->ifindex in ip_route_output_key.

Strange because I see no false sharing on this ifindex location for
loopback device.

So we probably can save some cycles adding a net->loopback_ifindex
to remove one dereference.

If ifindex are per network space, I guess we'll need to change
arp_hashfn() or else we'll use some slots more than others.

diff --git a/include/net/arp.h b/include/net/arp.h
index 7f7df93..37aac58 100644
--- a/include/net/arp.h
+++ b/include/net/arp.h
@@ -10,7 +10,7 @@ extern struct neigh_table arp_tbl;

 static inline u32 arp_hashfn(u32 key, const struct net_device *dev, u32 hash_rnd)
 {
-	u32 val = key ^ dev->ifindex;
+	u32 val = key ^ (u32)(unsigned long)dev;

 	return val * hash_rnd;
 }

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