netdev - Re: [PATCH net-next 2/2] net: enetc: count the tc-taprio window drops

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Date:   Wed, 2 Jun 2021 20:59:37 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
        Claudiu Manoil <claudiu.manoil@....com>,
        Michael Walle <michael@...le.cc>, Po Liu <po.liu@....com>,
        Vinicius Costa Gomes <vinicius.gomes@...el.com>,
        Vladimir Oltean <vladimir.oltean@....com>
Subject: Re: [PATCH net-next 2/2] net: enetc: count the tc-taprio window drops

On Wed, Jun 02, 2021 at 10:19:20AM -0700, Jakub Kicinski wrote:
> On Wed,  2 Jun 2021 15:21:14 +0300 Vladimir Oltean wrote:
> > From: Po Liu <Po.Liu@....com>
> > 
> > The enetc scheduler for IEEE 802.1Qbv has 2 options (depending on
> > PTGCR[TG_DROP_DISABLE]) when we attempt to send an oversized packet
> > which will never fit in its allotted time slot for its traffic class:
> > either block the entire port due to head-of-line blocking, or drop the
> 
> the entire port or the entire queue?

I don't remember, I need to re-test.

> > packet and set a bit in the writeback format of the transmit buffer
> > descriptor, allowing other packets to be sent.
> > 
> > We obviously choose the second option in the driver, but we do not
> > detect the drop condition, so from the perspective of the network stack,
> > the packet is sent and no error counter is incremented.
> > 
> > This change checks the writeback of the TX BD when tc-taprio is enabled,
> > and increments a specific ethtool statistics counter and a generic
> > "tx_dropped" counter in ndo_get_stats64.
> 
> Any chance we should also report that back to the qdisc to have 
> a standard way of querying from user space? Qdisc offload supports
> stats in general, it shouldn't be an issue, and the stat seems generic
> enough, no?

You're thinking of something along the lines of tc_codel_xstats?
How do you propose I pass this on to the taprio qdisc? Just call a
function in enetc that is exported by net/sched/sch_taprio.c?
If the skb is bound to a socket, I'm thinking it might be more useful to
report a struct sock_extended_err similar to the SO_EE_TXTIME_MISSED
stuff for tc-etf, what do you think?