netdev - Re: [PATCH v1 net-next] net: dsa: qca: ar9331: add ethtool stats support

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Date:   Mon, 16 Nov 2020 15:30:39 -0800
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Vladimir Oltean <olteanv@...il.com>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     Oleksij Rempel <o.rempel@...gutronix.de>,
        Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Russell King <linux@...linux.org.uk>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mips@...r.kernel.org
Subject: Re: [PATCH v1 net-next] net: dsa: qca: ar9331: add ethtool stats
 support

On 11/16/20 3:27 PM, Vladimir Oltean wrote:
> On Mon, Nov 16, 2020 at 03:13:47PM -0800, Jakub Kicinski wrote:
>> On Tue, 17 Nov 2020 01:00:53 +0200 Vladimir Oltean wrote:
>>> On Mon, Nov 16, 2020 at 02:35:44PM -0800, Jakub Kicinski wrote:
>>>> On Tue, 17 Nov 2020 00:21:46 +0200 Vladimir Oltean wrote:
>>>>> On Mon, Nov 16, 2020 at 01:34:53PM -0800, Jakub Kicinski wrote:
>>>>>> You must expose relevant statistics via the normal get_stats64 NDO
>>>>>> before you start dumping free form stuff in ethtool -S.
>>>>>
>>>>> Completely agree on the point, Jakub, but to be honest we don't give him
>>>>> that possibility within the DSA framework today, see .ndo_get_stats64 in
>>>>> net/dsa/slave.c which returns the generic dev_get_tstats64 implementation,
>>>>> and not something that hooks into the hardware counters, or into the
>>>>> driver at all, for that matter.
>>>>
>>>> Simple matter of coding, right? I don't see a problem.
>>>>
>>>> Also I only mentioned .ndo_get_stats64, but now we also have stats in
>>>> ethtool->get_pause_stats.
>>>
>>> Yes, sure we can do that. The pause stats and packet counter ops would
>>> need to be exposed to the drivers by DSA first, though. Not sure if this
>>> is something you expect Oleksij to do or if we could pick that up separately
>>> afterwards.
>>
>> Well, I feel like unless we draw the line nobody will have
>> the incentive to do the work.
>>
>> I don't mind if it's Oleksij or anyone else doing the plumbing work,
>> but the task itself seems rather trivial.
> 
> So then I'll let Oleksij show his availability.
> 
>>>>> But it's good that you raise the point, I was thinking too that we
>>>>> should do better in terms of keeping the software counters in sync with
>>>>> the hardware. But what would be a good reference for keeping statistics
>>>>> on an offloaded interface? Is it ok to just populate the netdev counters
>>>>> based on the hardware statistics?
>>>>
>>>> IIRC the stats on the interface should be a sum of forwarded in software
>>>> and in hardware. Which in practice means interface HW stats are okay,
>>>> given eventually both forwarding types end up in the HW interface
>>>> (/MAC block).
>>>
>>> A sum? Wouldn't that count the packets sent/received by the stack twice?
>>
>> Note that I said _forwarded_. Frames are either forwarded by the HW or
>> SW (former never hit the CPU, while the latter do hit the CPU or
>> originate from it).
> 
> Ah, you were just thinking out loud, I really did not understand what
> you meant by the separation between "forwarded in software" and
> "forwarded in hardware".
> Yes, the hardware typically only gives us MAC-level counters anyway.
> Another way to look at it is that the number of packets forwarded in
> hardware from a given port are equal to the total number of RX packets
> on that MAC minus the packets seen by the CPU coming from that port.
> So all in all, it's the MAC-level counters we should expose in
> .ndo_get_stats64, I'm glad you agree. As for the error packets, I
> suppose that would be a driver-specific aggregate.
> 
> What about RMON/RFC2819 style etherStatsPkts65to127Octets? We have a
> number of switches supporting that style of counters, including the one
> that Oleksij is adding support for, apparently (but not all switches
> though). I suppose your M.O. is that anything standardizable is welcome
> to be standardized via rtnetlink?
> 
> Andrew, Florian, any opinions here?
> 

Settling on RMON/RFC2819 statistics would work for me, and hopefully is
not too hard to get the various drivers converted to. With respect to
Oleksij's patch, I would tend to accept it so we actually have more
visibility into what standardized counters are available across switch
drivers.
-- 
Florian