Re: [PATCH net-next v2] ipv6: ioam: Support for Queue depth data field

[Ido Schimmel <idosch@idosch.org>]

On Mon, Dec 27, 2021 at 03:06:42PM +0100, Justin Iurman wrote:
> On Dec 26, 2021, at 2:15 PM, Ido Schimmel idosch@idosch.org wrote:
> > On Sun, Dec 26, 2021 at 01:59:08PM +0100, Justin Iurman wrote:
> >> On Dec 26, 2021, at 1:40 PM, Ido Schimmel idosch@idosch.org wrote:
> >> > On Sun, Dec 26, 2021 at 12:47:51PM +0100, Justin Iurman wrote:
> >> >> On Dec 24, 2021, at 6:53 PM, Ido Schimmel idosch@idosch.org wrote:
> >> >> > Why 'qlen' is used and not 'backlog'? From the paragraph you quoted it
> >> >> > seems that queue depth needs to take into account the size of the
> >> >> > enqueued packets, not only their number.
> >> >> 
> >> >> The quoted paragraph contains the following sentence:
> >> >> 
> >> >>    "The queue depth is expressed as the current amount of memory
> >> >>     buffers used by the queue"
> >> >> 
> >> >> So my understanding is that we need their number, not their size.
> >> > 
> >> > It also says "a packet could consume one or more memory buffers,
> >> > depending on its size". If, for example, you define tc-red limit as 1M,
> >> > then it makes a lot of difference if the 1,000 packets you have in the
> >> > queue are 9,000 bytes in size or 64 bytes.
> >> 
> >> Agree. We probably could use 'backlog' instead, regarding this
> >> statement:
> >> 
> >>   "It should be noted that the semantics of some of the node data fields
> >>    that are defined below, such as the queue depth and buffer occupancy,
> >>    are implementation specific.  This approach is intended to allow IOAM
> >>    nodes with various different architectures."
> >> 
> >> It would indeed make more sense, based on your example. However, the
> >> limit (32 bits) could be reached faster using 'backlog' rather than
> >> 'qlen'. But I guess this tradeoff is the price to pay to be as close
> >> as possible to the spec.
> > 
> > At least in Linux 'backlog' is 32 bits so we are OK :)
> > We don't have such big buffers in hardware and I'm not sure what
> > insights an operator will get from a queue depth larger than 4GB...
> 
> Indeed :-)
> 
> > I just got an OOO auto-reply from my colleague so I'm not sure I will be
> > able to share his input before next week. Anyway, reporting 'backlog'
> > makes sense to me, FWIW.
> 
> Right. I read that Linus is planning to release a -rc8 so I think I can
> wait another week before posting -v3.

The answer I got from my colleagues is that they expect the field to
either encode bytes (what Mellanox/Nvidia is doing) or "cells", which is
an "allocation granularity of memory within the shared buffer" (see man
devlink-sb).