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

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Date:   Fri, 3 Feb 2017 10:28:25 -0800
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     Nikolay Aleksandrov <nikolay@...ulusnetworks.com>
Cc:     David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
        roopa@...ulusnetworks.com
Subject: Re: [PATCH RFC net-next 4/4] bridge: add ability to turn off fdb
 used updates

On Fri, 3 Feb 2017 09:30:37 +0100
Nikolay Aleksandrov <nikolay@...ulusnetworks.com> wrote:

> On 03/02/17 03:47, David Miller wrote:
> > From: Nikolay Aleksandrov <nikolay@...ulusnetworks.com>
> > Date: Tue, 31 Jan 2017 16:31:58 +0100
> >   
> >> @@ -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