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ipv4: Optimize flow initialization in output route lookup. 

We burn a lot of useless cycles, cpu store buffer traffic, and
memory operations memset()'ing the on-stack flow used to perform
output route lookups in __ip_route_output_key().

Only the first half of the flow object members even matter for
output route lookups in this context, specifically:

FIB rules matching cares about:

	dst, src, tos, iif, oif, mark

FIB trie lookup cares about:

	dst

FIB semantic match cares about:

	tos, scope, oif

Therefore only initialize these specific members and elide the
memset entirely.

On Niagara2 this kills about ~300 cycles from the output route
lookup path.

Likely, we can take things further, since all callers of output
route lookups essentially throw away the on-stack flow they use.
So they don't care if we use it as a scratch-pad to compute the
final flow key.

Signed-off-by: David S. Miller <davem@davemloft.net>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
This commit is contained in:
David S. Miller 2011-03-04 21:24:47 -08:00
parent 65e8354ec1
commit 44713b67db
1 changed files with 10 additions and 8 deletions
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18
net/ipv4/route.c
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@ -2431,14 +2431,7 @@ static struct rtable *ip_route_output_slow(struct net *net,
const struct flowi *oldflp)
{
u32 tos = RT_FL_TOS(oldflp);
struct flowi fl = { .fl4_dst = oldflp->fl4_dst,
.fl4_src = oldflp->fl4_src,
.fl4_tos = tos & IPTOS_RT_MASK,
.fl4_scope = ((tos & RTO_ONLINK) ?
RT_SCOPE_LINK : RT_SCOPE_UNIVERSE),
.mark = oldflp->mark,
.iif = net->loopback_dev->ifindex,
.oif = oldflp->oif };
struct flowi fl;
struct fib_result res;
unsigned int flags = 0;
struct net_device *dev_out = NULL;
@ -2449,6 +2442,15 @@ static struct rtable *ip_route_output_slow(struct net *net,
res.r = NULL;
#endif
fl.oif = oldflp->oif;
fl.iif = net->loopback_dev->ifindex;
fl.mark = oldflp->mark;
fl.fl4_dst = oldflp->fl4_dst;
fl.fl4_src = oldflp->fl4_src;
fl.fl4_tos = tos & IPTOS_RT_MASK;
fl.fl4_scope = ((tos & RTO_ONLINK) ?
RT_SCOPE_LINK : RT_SCOPE_UNIVERSE);
rcu_read_lock();
if (oldflp->fl4_src) {
rth = ERR_PTR(-EINVAL);