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net: avoid RCU for NOCACHE dst

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There is no point using RCU for dst we allocate for a very short time
(used once).

Change dst_release() to take DST_NOCACHE into account, but also change
skb_dst_set_noref() to force a refcount increment for such dst.

This is a _huge_ gain, because we dont waste memory to store xx thousand
of dsts. Instead of queueing them to RCU, we can free them instantly.

CPU caches can stay hot, re-using same memory blocks to hold temporary
dsts.

Note : remove unneeded smp_mb__before_atomic_dec(); in dst_release(),
since atomic_dec_return() implies a full memory barrier.

Stress test, 160.000.000 udp frames sent, IP route cache disabled
(DDOS).

Before:

real    0m38.091s
user    0m13.189s
sys     7m53.018s

After:

real	0m29.946s
user	0m12.157s
sys	7m40.605s

For reference, if IP route cache was enabled :

real	0m32.030s
user	0m10.521s
sys	8m15.243s

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Eric Dumazet 2010-10-15 05:44:11 +00:00 • committed by David S. Miller
parent e6484930d7
commit 27b75c95f1
3 changed files with 33 additions and 19 deletions
Download patch file Download diff file Expand all files Collapse all files

9
net/ipv4/route.c
View file

@ -1105,9 +1105,9 @@ restart:
* Note that we do rt_free on this new route entry, so that
* once its refcount hits zero, we are still able to reap it
* (Thanks Alexey)
* Note also the rt_free uses call_rcu. We don't actually
* need rcu protection here, this is just our path to get
* on the route gc list.
* Note: To avoid expensive rcu stuff for this uncached dst,
* we set DST_NOCACHE so that dst_release() can free dst without
* waiting a grace period.
*/
rt->dst.flags |= DST_NOCACHE;
@ -1117,12 +1117,11 @@ restart:
if (net_ratelimit())
printk(KERN_WARNING
"Neighbour table failure & not caching routes.\n");
rt_drop(rt);
ip_rt_put(rt);
return err;
}
}
rt_free(rt);
goto skip_hashing;
}