netdev - Re: [PATCH net-next v2 2/8] docs: net: include the new ethtool pause stats in the stats doc

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Message-ID: <20200914125210.3b230a32@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Mon, 14 Sep 2020 12:52:10 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Saeed Mahameed <saeedm@...dia.com>
Cc:     "davem@...emloft.net" <davem@...emloft.net>,
        "mkubecek@...e.cz" <mkubecek@...e.cz>,
        Tariq Toukan <tariqt@...dia.com>,
        "michael.chan@...adcom.com" <michael.chan@...adcom.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "alexander.duyck@...il.com" <alexander.duyck@...il.com>,
        "andrew@...n.ch" <andrew@...n.ch>
Subject: Re: [PATCH net-next v2 2/8] docs: net: include the new ethtool
 pause stats in the stats doc

On Mon, 14 Sep 2020 19:33:08 +0000 Saeed Mahameed wrote:
> > +Protocol-specific statistics
> > +----------------------------
> > +
> > +Some of the interfaces used for configuring devices are also able
> > +to report related statistics. For example ethtool interface used
> > +to configure pause frames can report corresponding hardware
> > counters::
> > +
> > +  $ ethtool --include-statistics -a eth0
> > +  Pause parameters for eth0:
> > +  Autonegotiate:	on
> > +  RX:			on
> > +  TX:			on
> > +  Statistics:
> > +    tx_pause_frames: 1
> > +    rx_pause_frames: 1
> > +  
> 
> this will require to access the HW twice per stats request to read both
> stats and current parameters, maybe this is not a big deal, but sharp
> accuracy can be important for some performance enthusiasts.
> 
> Do we need an API that only reports statistics without the current
> parameters ?

That crossed my mind. IDK how real this concern is if we have ethtool
-S which dumps half of the universe and nobody ever done anything
about it..

Once we start adding more interfaces (as I said elsewhere I plan to add
FEC counters) we'll also have to do multiple calls for multiple types
of stats. But I think that's fine as a starting point. We can extend
RTM_GETSTATS to provide an efficient interface when needed.

As you may recall a couple years back I posted a set with "hierarchical
stats" which as an attempt to solve all the problems at once. 
I concluded that it's a wrong approach. We should start with the simple
and obvious stuff. We can build complexity later.