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

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Date:   Tue, 1 Dec 2020 16:54:21 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Vladimir Oltean <olteanv@...il.com>
Cc:     Andrew Lunn <andrew@...n.ch>,
        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 Wed, 2 Dec 2020 02:28:51 +0200 Vladimir Oltean wrote:
> On Fri, Nov 27, 2020 at 10:36:42PM +0100, Andrew Lunn wrote:
> > > Either way, can we conclude that ndo_get_stats64 is not a replacement
> > > for ethtool -S, since the latter is blocking and, if implemented correctly,
> > > can return the counters at the time of the call (therefore making sure
> > > that anything that happened before the syscall has been accounted into
> > > the retrieved values), and the former isn't?  
> >
> > ethtool -S is the best source of consistent, up to date statistics we
> > have. It seems silly not to include everything the hardware offers
> > there.  
> 
> To add to this, it would seem odd to me if we took the decision to not
> expose MAC-level counters any longer in ethtool. Say the MAC has a counter
> named rx_dropped. If we are only exposing this counter in ndo_get_stats64,
> then we could hit the scenario where this counter keeps incrementing,
> but it is the network stack who increments it, and not the MAC.
> 
> dev_get_stats() currently does:
> 	storage->rx_dropped += (unsigned long)atomic_long_read(&dev->rx_dropped);
> 	storage->tx_dropped += (unsigned long)atomic_long_read(&dev->tx_dropped);
> 	storage->rx_nohandler += (unsigned long)atomic_long_read(&dev->rx_nohandler);
> 
> thereby clobbering the MAC-provided counter. We would not know if it is
> a MAC-level drop or not.

Fine granularity HW stats are fine, but the aggregate must be reported
in standard stats first.

The correct stat for MAC drops (AFAIU) is rx_missed.

This should act as a generic "device had to drop valid packets"
indication and ethtool -S should serve for manual debugging to find 
out which stage of pipeline / reason caused the drop.