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

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Message-ID: <20211207075037.6cda8832@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Tue, 7 Dec 2021 07:50:37 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Justin Iurman <justin.iurman@...ege.be>
Cc:     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
Subject: Re: [RFC net-next 2/2] ipv6: ioam: Support for Buffer occupancy
 data field

On Tue, 7 Dec 2021 12:54:04 +0100 (CET) Justin Iurman wrote:
> >> The function kmem_cache_size is used to retrieve the size of a slab
> >> object. Note that it returns the "object_size" field, not the "size"
> >> field. If needed, a new function (e.g., kmem_cache_full_size) could be
> >> added to return the "size" field. To match the definition from the
> >> draft, the number of bytes is computed as follows:
> >> 
> >> slabinfo.active_objs * size
> > 
> > Implementing the standard is one thing but how useful is this
> > in practice?  
> 
> IMHO, very useful. To be honest, if I were to implement only a few data
> fields, these two would be both included. Take the example of CLT [1]
> where the queue length data field is used to detect low-level issues
> from inside a L5-7 distributed tracing tool. And this is just one
> example among many others. The queue length data field is very specific
> to TX queues, but we could also use the buffer occupancy data field to
> detect more global loads on a node. Actually, the goal for operators
> running their IOAM domain is to quickly detect a problem along a path
> and react accordingly (human or automatic action). For example, if you
> monitor TX queues along a path and detect an increasing queue on a
> router, you could choose to, e.g.,  rebalance its queues. With the
> buffer occupancy, you could detect high-loaded nodes in general and,
> e.g., rebalance traffic to another branch. Again, this is just one
> example among others. Apart from more accurate ECMPs, you could for
> instance deploy a smart (micro)service selection based on different
> metrics, etc.
> 
>   [1] https://github.com/Advanced-Observability/cross-layer-telemetry

Ack, my question was more about whether the metric as implemented
provides the best signal. Since the slab cache scales dynamically
(AFAIU) it's not really a big deal if it's full as long as there's
memory available on the system.