netdev - Re: [PATCH net-next v2 1/1] driver: veth: Refine the statistics codes of veth driver

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Date:   Thu, 3 Nov 2016 22:38:28 +0800
From:   Gao Feng <fgao@...ai8.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Cong Wang <cwang@...pensource.com>,
        Vijay Pandurangan <vijayp@...ayp.ca>,
        Evan Jones <ej@...njones.ca>, pabeni@...hat.com,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next v2 1/1] driver: veth: Refine the statistics codes
 of veth driver

On Thu, Nov 3, 2016 at 10:31 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Thu, 2016-11-03 at 21:39 +0800, Gao Feng wrote:
>> Hi Eric,
>>
>> On Thu, Nov 3, 2016 at 9:30 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>> > On Thu, 2016-11-03 at 21:03 +0800, fgao@...ai8.com wrote:
>> >> From: Gao Feng <fgao@...ai8.com>
>> >>
>> >> The dropped count of veth is located in struct veth_priv, but other
>> >> statistics like packets and bytes are in another struct pcpu_vstats.
>> >> Now keep these three counters in the same struct.
>> >>
>> >> Signed-off-by: Gao Feng <fgao@...ai8.com>
>> >> ---
>> >>  v2: Use right "peer" instead of "dev";
>> >>  v1: Initial version
>> >
>> > May I ask : Why ?
>>
>> Just because I think statistics should be in the same struct.
>
> That is not a good reason then.

Because other net devices put the statistics together.
Take tun/tap as example, it is a virtual device, but its all
statistics are percpu including dropped.

Regards
Feng

>
>>
>> >
>> > We did that because there was no point making per-cpu requirements
>> > bigger, for a counter that is hardly ever updated.
>> >
>> > Do you have a real case where performance dropping packets in a driver
>> > is needed ?
>>
>> No, I haven't met the performance issue now.
>
> OK then kill this patch.
>
>>
>> >
>> > At some point we will have to stop dumb percpu explosion, when we have
>> > 128+ cores per host. Folding all these percpu counters is taking a lot
>> > of time too.
>> >
>> >
>> >
>> Ok, I get it. It is designed specially to put the dropped counter as
>> atomic counter, not percpu.
>> But I have one question that when put the counters as percpu, and when not?
>
> Because the regular fast path needs to be fast ?
>
> Try to _use_ veth without these percpu stats and be prepared to be
> shocked.
>
>
>