Re: [PATCH RFC net-next 4/4] bridge: add ability to turn off fdb used updates

On Fri, 3 Feb 2017 19:34:19 +0100
Nikolay Aleksandrov [off-list ref] wrote:

On 03/02/17 19:28, Stephen Hemminger wrote:

quoted

On Fri, 3 Feb 2017 09:30:37 +0100
Nikolay Aleksandrov [off-list ref] wrote:
  

quoted

On 03/02/17 03:47, David Miller wrote:  

quoted

From: Nikolay Aleksandrov <redacted>
Date: Tue, 31 Jan 2017 16:31:58 +0100
    

quoted

@@ -197,7 +197,8 @@ int br_handle_frame_finish(struct net *net, struct sock *sk, struct sk_buff *skb
 		if (dst->is_local)
 			return br_pass_frame_up(skb);
 
-		dst->used = jiffies;
+		if (br->used_enabled)
+			dst->used = jiffies;    

Have you tried:

	if (dst->used != jiffies)
		dst->used = jiffies;

If that isn't effective, you can tweak the test to decrease the
granularity of the value.  Basically, if dst->used is within
1 HZ of jiffies, don't do the write.

I suspect this might help a lot, and not require a new bridging
option.
    

Yes, I actually have a patch titled "used granularity". :-) I've tested with different
values and it does help but it either needs to be paired with another similar test for
the "updated" field (since they share a write-heavy cache line) or they need to be
in separate cache lines to avoid that dst's source port from causing the load HitM for
all who check the value.

I'll run some more tests and probably go this way for now.

Thanks,
 Nik
 

Since used doesn't need HZ granularity, it reports values in clock_t resolution so
storing (and doing cmp and set would mean that it would only be 100 HZ
  

Yes, exactly what I'm currently testing. Will post the new set soon.
Since HZ can be different a generic way to obtain the granularity for
both should be clock_t_to_jiffies(1) if I'm not missing something.

USER_HZ is set by userspace ABI to 100 hz. HZ is configurable when kernel is built.