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 19:34:19 +0100
From:   Nikolay Aleksandrov <nikolay@...ulusnetworks.com>
To:     Stephen Hemminger <stephen@...workplumber.org>
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 03/02/17 19:28, Stephen Hemminger wrote:
> 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
> 

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.