netdev - Re: [PATCH net-next] net: bridge: add STP xstats

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Date:   Tue, 10 Dec 2019 22:15:26 +0200
From:   Nikolay Aleksandrov <nikolay@...ulusnetworks.com>
To:     Vivien Didelot <vivien.didelot@...il.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Roopa Prabhu <roopa@...ulusnetworks.com>,
        netdev@...r.kernel.org, bridge@...ts.linux-foundation.org,
        Stephen Hemminger <stephen@...workplumber.org>
Subject: Re: [PATCH net-next] net: bridge: add STP xstats

On 10/12/2019 22:10, Vivien Didelot wrote:
> Hi Nikolay,
> 
> On Tue, 10 Dec 2019 21:50:10 +0200, Nikolay Aleksandrov <nikolay@...ulusnetworks.com> wrote:
>>>> Why do you need percpu ? All of these seem to be incremented with the
>>>> bridge lock held. A few more comments below.
>>>
>>> All other xstats are incremented percpu, I simply followed the pattern.
>>>
>>
>> We have already a lock, we can use it and avoid the whole per-cpu memory handling.
>> It seems to be acquired in all cases where these counters need to be changed.
> 
> Since the other xstats counters are currently implemented this way, I prefer
> to keep the code as is, until we eventually change them all if percpu is in
> fact not needed anymore.
> 
> The new series is ready and I can submit it now if there's no objection.
> 
> 
> Thanks,
> 
> 	Vivien
> 

There is a reason other counters use per-cpu - they're incremented without any locking from fast-path.
The bridge STP code already has a lock which is acquired in all of these paths and we don't need
this overhead and the per-cpu memory allocations. Unless you can find a STP codepath which actually
needs per-cpu, I'd prefer you drop it.

Thank you,
 Nik