[dpdk-dev] [RFC] ethdev: support Tx queue free descriptor query

[Bruce Richardson]

On Mon, Jan 08, 2024 at 10:15:40PM +0100, Morten Brørup wrote:
> > From: Bruce Richardson [mailto:bruce.richardson at intel.com]
> > Sent: Monday, 8 January 2024 11.54
> > 
> > On Tue, Dec 19, 2023 at 10:59:48PM +0530, jerinj at marvell.com wrote:
> > > From: Jerin Jacob <jerinj at marvell.com>
> > >
> > > Introduce a new API to retrieve the number of available free
> > descriptors
> > > in a Tx queue. Applications can leverage this API in the fast path to
> > > inspect the Tx queue occupancy and take appropriate actions based on
> > the
> > > available free descriptors.
> > >
> > > A notable use case could be implementing Random Early Discard (RED)
> > > in software based on Tx queue occupancy.
> > >
> > > Signed-off-by: Jerin Jacob <jerinj at marvell.com>
> > > ---
> > 
> > Hi Jerin,
> > 
> > while I don't strongly object to this patch, I wonder if it encourages
> > sub-optimal code implementations. To determine the number of free
> > descriptors in a ring, the driver in many cases will need to do a scan
> > of
> > the descriptor ring to see how many are free/used. However, I suspect
> > that
> > in most cases we will see something like the following being done:
> > 
> >     count = rte_eth_rx_free_count();
> 
> Typo: rte_eth_rx_free_count() -> rte_eth_tx_free_count()
> 
> >     if (count > X) {
> >         /* Do something */
> >     }
> > 
> > For instances like this, scanning the entire ring is wasteful of
> > resources.
> > Instead it would be better to just check the descriptor at position X
> > explicitly. Going to the trouble of checking the exact descriptor count
> > is
> > unnecessary in this case.
> 
> Yes, it introduces such a risk.
> All DPDK examples simply call tx_burst() without checking free space first, so I think the probability (of the simple case) is low.
> And the probability for the case comparing to X could be mitigated by referring to rte_eth_tx_descriptor_status() in the function description.
> 
> > 
> > Out of interest, are you aware of a case where an app would need to
> > know
> > exactly the number of free descriptors, and where the result would not
> > be
> > just compared to one or more threshold values? Do we see cases where it
> > would be used in a computation, perhaps?
> 
> Yes: RED. When exceeding the minimum threshold, the probability of marking/dropping a packet increases linearly with the queue's fill level.
> E.g.:
> 0-900 packets in queue: don't drop,
> 901-1000 packets in queue: probability of dropping the packet is 1-100 % (e.g. 980 packets in queue = 80 % drop probability).
> 
Ok, thanks for the info. All good with me so.

/Bruce