netdev - Re: [PATCH net-next v2 2/3] net: dsa: add Arrow SpeedChips XRS700x driver

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Date:   Fri, 27 Nov 2020 13:13:46 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     Vladimir Oltean <olteanv@...il.com>,
        George McCollister <george.mccollister@...il.com>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        "David S . Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
        "open list:OPEN FIRMWARE AND..." <devicetree@...r.kernel.org>
Subject: Re: [PATCH net-next v2 2/3] net: dsa: add Arrow SpeedChips XRS700x
 driver

On Fri, 27 Nov 2020 21:47:14 +0100 Andrew Lunn wrote:
> > Is the periodic refresh really that awful? We're mostly talking error
> > counters here so every second or every few seconds should be perfectly
> > fine.  
> 
> Humm, i would prefer error counts to be more correct than anything
> else. When debugging issues, you generally don't care how many packets
> worked. It is how many failed you are interesting, and how that number
> of failures increases.

Right, but not sure I'd use the word "correct". Perhaps "immediately up
to date"?

High speed NICs usually go through a layer of firmware before they
access the stats, IOW even if we always synchronously ask for the stats
in the kernel - in practice a lot of NICs (most?) will return some form
of cached information.

> So long as these counters are still in ethtool -S, i guess it does not
> matter. That i do trust to be accurate, and probably consistent across
> the counters it returns.

Not in the NIC designs I'm familiar with.

But anyway - this only matters in some strict testing harness, right?
Normal users will look at a stats after they noticed issues (so minutes
/ hours later) or at the very best they'll look at a graph, which will
hardly require <1sec accuracy to when error occurred.