Re: [PATCH net-next] ipv6: do lazy dst->__use update when per cpu dst is available

On Wed, 2017-09-27 at 11:03 -0700, Eric Dumazet wrote:

quoted

quoted

diff --git a/net/ipv6/route.c b/net/ipv6/route.c

Hi Paolo,

Eric and I discussed about this issue recently as well :).

What about the following change:

diff --git a/include/net/dst.h b/include/net/dst.h
index 93568bd0a352..33e1d86bcef6 100644
--- a/include/net/dst.h
+++ b/include/net/dst.h

@@ -258,14 +258,18 @@ static inline void dst_hold(struct dst_entry *dst)
 static inline void dst_use(struct dst_entry *dst, unsigned long time)
 {
        dst_hold(dst);
-       dst->__use++;
-       dst->lastuse = time;
+       if (dst->lastuse != time) {
+               dst->__use++;
+               dst->lastuse = time;
+       }
 }

 static inline void dst_use_noref(struct dst_entry *dst, unsigned long time)
 {
-       dst->__use++;
-       dst->lastuse = time;
+       if (dst->lastuse != time) {
+               dst->__use++;
+               dst->lastuse = time;
+       }
 }

 static inline struct dst_entry *dst_clone(struct dst_entry *dst)

diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index 26cc9f483b6d..e195f093add3 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c

@@ -1170,8 +1170,7 @@ struct rt6_info *ip6_pol_route(struct net *net,

struct fib6_table *table,

                struct rt6_info *pcpu_rt;

-               rt->dst.lastuse = jiffies;
-               rt->dst.__use++;
+               dst_use_noref(rt, jiffies);
                pcpu_rt = rt6_get_pcpu_route(rt);

                if (pcpu_rt) {


This way we always only update dst->__use and dst->lastuse at most
once per jiffy. And we don't really need to update pcpu and then do
the copy over from pcpu_rt to rt operation.

Another thing is that I don't really see any places making use of
dst->__use. So maybe we can also get rid of this dst->__use field?

Thanks.
Wei

Paolo, given we are very close to send Wei awesome work about IPv6
routing cache,
could we ask you to wait few days before doing the same work from your side ?

Ok, no problem - thanks instead. I'll wait for it. 

Main issue is the rwlock, and we are converting it to full RCU.

You are sending patches that are making our job very difficult IMO.

On my side I have only another small change in this area, I'll
eventually try to rebase it later, if still relevant.

Or I can share it now, if you are interested.

Cheers,

Paolo