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

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Message-ID: <20211221121306.487799cb@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Tue, 21 Dec 2021 12:13:06 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Justin Iurman <justin.iurman@...ege.be>
Cc:     Vladimir Oltean <olteanv@...il.com>, 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, 21 Dec 2021 19:23:37 +0200 Vladimir Oltean wrote:
> 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.

Indeed.

In my experience finding meaningful metrics is heard, the chances that
something that seems useful on the surface actually provides meaningful
signal in deployments is a lot lower than one may expect. And the
commit message reads as if the objective was checking a box in the
implemented IOAM metrics, rather exporting relevant information. 

We can do a roll call on people CCed but I read their silence as nobody
thinks this metric is useful. Is there any experimental data you can
point to which proves the signal strength?