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 23:27:20 +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 23:24, Stephen Hemminger wrote:
> On Fri, 3 Feb 2017 19:34:19 +0100
> Nikolay Aleksandrov <nikolay@...ulusnetworks.com> wrote:
> 
>> 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.
>>
>>
> 
> USER_HZ is set by userspace ABI to 100 hz. HZ is configurable when kernel is built.
> 

Yes, the point I was trying to make is that we want to take the number of jiffies
we can skip by converting 1 clock_t to X jiffies because the user-space granularity
is clock_t and HZ can change, thus clock_t_to_jiffies(1) should give us the number
of updates we can skip for "used" and "updated".
By "both" I meant "used" and "updated" fields, not HZ and USER_HZ.