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

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Message-ID: <20201116143544.036baf58@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Mon, 16 Nov 2020 14:35:44 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Vladimir Oltean <olteanv@...il.com>
Cc:     Oleksij Rempel <o.rempel@...gutronix.de>,
        Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...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 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.

> 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).

> And what about the statistics gathered
> today in software, could we return them maybe via something like ifstat
> --extended=cpu_hits?

Yup, exactly, that's what --extended=cpu_hits is for.