netdev - Re: [RFC net-next 2/2] ipv6: ioam: Support for Buffer occupancy data field

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Date:   Tue, 21 Dec 2021 19:23:37 +0200
From:   Vladimir Oltean <olteanv@...il.com>
To:     Justin Iurman <justin.iurman@...ege.be>
Cc:     Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
        davem@...emloft.net, dsahern@...nel.org, yoshfuji@...ux-ipv6.org,
        linux-mm@...ck.org, cl@...ux.com, penberg@...nel.org,
        rientjes@...gle.com, iamjoonsoo kim <iamjoonsoo.kim@....com>,
        akpm@...ux-foundation.org, vbabka@...e.cz,
        Roopa Prabhu <roopa@...dia.com>,
        Nikolay Aleksandrov <nikolay@...dia.com>,
        Andrew Lunn <andrew@...n.ch>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Florian Westphal <fw@...len.de>,
        Paolo Abeni <pabeni@...hat.com>
Subject: Re: [RFC net-next 2/2] ipv6: ioam: Support for Buffer occupancy data
 field

On Tue, Dec 21, 2021 at 06:06:39PM +0100, Justin Iurman wrote:
> On Dec 10, 2021, at 1:38 AM, Jakub Kicinski kuba@...nel.org wrote:
> > [...]
> > I think we're on the same page, the main problem is I've not seen
> > anyone use the skbuff_head_cache occupancy as a signal in practice.
> > 
> > I'm adding a bunch of people to the CC list, hopefully someone has
> > an opinion one way or the other.
> 
> It looks like we won't have more opinions on that, unfortunately.
> 
> @Jakub - Should I submit it as a PATCH and see if we receive more
> feedback there?

I know nothing about OAM and therefore did not want to comment, but I
think the point raised about the metric you propose being irrelevant in
the context of offloaded data paths is quite important. The "devlink-sb"
proposal was dismissed very quickly on grounds of requiring sleepable
context, is that a deal breaker, and if it is, why? Not only offloaded
interfaces like switches/routers can report buffer occupancy. Plain NICs
also have buffer pools, DMA RX/TX rings, MAC FIFOs, etc, that could
indicate congestion or otherwise high load. Maybe slab information could
be relevant, for lack of a better option, on virtual interfaces, but if
they're physical, why limit ourselves on reporting that? The IETF draft
you present says "This field indicates the current status of the
occupancy of the common buffer pool used by a set of queues." It appears
to me that we could try to get a reporting that has better granularity
(per interface, per queue) than just something based on
skbuff_head_cache. What if someone will need that finer granularity in
the future.