Re: [PATCH v4 3/7] ethdev: introduce Rx queue based fill threshold

[Thomas Monjalon <thomas@monjalon.net>]

It seems we share a common understanding
and we need to agree on a good wording
for the most meaningful API.
Questions inline below:

06/06/2022 19:15, Andrew Rybchenko:
> On 6/6/22 16:16, Spike Du wrote:
> > Hi Andrew,
> > 	Please see below for "fill threshold" concept, I'm ok with other comments about code.
> > 
> > Regards,
> > Spike.
> > 
> > 
> > From: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
> >> On 6/3/22 15:48, Spike Du wrote:
> >>> Fill threshold describes the fullness of a Rx queue. If the Rx queue
> >>> fullness is above the threshold, the device will trigger the event
> >>> RTE_ETH_EVENT_RX_FILL_THRESH.
> >>
> >> Sorry, I'm not sure that I understand. As far as I know the process to add
> >> more Rx buffers to Rx queue is called 'refill' in many drivers. So fill level is a
> >> number (or percentage) of free buffers in an Rx queue.
> >> If so, fill threashold should be a minimum fill level and below the level we
> >> should generate an event.
> >>
> >> However reading the first paragraph of the descrition it looks like you mean
> >> oposite thing - a number (or percentage) of ready Rx buffers with received
> >> packets.
> >>
> >> I think that the term "fill threshold" is suggested by me, but I did it with mine
> >> understanding of the added feature. Now I'm confused.
> >>
> >> Moreover, I don't understand how "fill threshold" could be in terms of ready
> >> Rx buffers. HW simply don't really know when ready Rx buffers are
> >> processed by SW. So, HW can't say for sure how many ready Rx buffers are
> >> pending. It could be calculated as Rx queue size minus number of free Rx
> >> buffers, but it is imprecise. First of all not all Rx descriptors could be used.
> >> Second, HW ring size could differ queue size specified in SW.
> >> Queue size specified in SW could just limit maximum nubmer of free Rx
> >> buffers provided by the driver.
> >>
> > 
> > Let me use other terms because "fill"/"refill" is also ambiguous to me.
> > In a RX ring, there are Rx buffers with received packets, you call it "ready Rx buffers", there is a RTE api rte_eth_rx_queue_count() to get the number,
> > It's also called "used descriptors" in the code.
> > Also there are Rx buffers provided by SW to allow HW "fill in" received packets, we can call it "usable Rx buffers" (here "usable" means usable for HW).
> 
> May be it is better to stick to Rx descriptor status terminology?
> Available - Rx descriptor available to HW to put received packet to
> Done - Rx descriptor with received packet reported to Sw
> Unavailable - other (e.g. gap which cannot be used or just processed
> Done, but not refilled (made available to HW).
> 
> > Let's define Rx queue "fullness":
> > 	Fullness = ready-Rx-buffers/Rxq-size
> 
> i.e. number of DONE descriptors divided by RxQ size
> 
> > On the opposite, we have "emptiness"
> > 	Emptiness = usable-Rx-buffers/Rxq-size
> 
> i.e. number of AVAIL descriptors divided by RxQ size
> Note, that AVAIL != RxQ-size - DONE
> 
> HW really knows number of available descriptors by its nature.
> It is a space between latest done and latest received on refill.
> 
> HW does not know which descriptors are DONE, since some which
> are DONE before could be already processed by SW, but not yet
> made available again.
> 
> 
> > Here "fill threshold" describes "fullness", it's not "refill" described in you above words. Because in your words, "refill" is the opposite, it's filling "usable/free Rx buffers", or "emptiness".
> > 
> > I can only briefly explain how mlx5 works to get LWM, because I'm not a Firmware guy.
> > Mlx5 Rx queue is basically RDMA queue. It has two indexes: producer index which increases when HW fills in packet, consumer index which increases when SW consumes the packet.
> > The queue size is known when it's created. The fullness is something like (producer_index - consumer_index) (I don't consider in wrap-around here).
> > So mlx5 has the way to get the fullness or emptiness in HW or FW.
> > Another detail is mlx5 uses the term "LWM"(limit watermark), which describes "emptiness". When usable-Rx-buffers is below LWM, we trigger an event.
> > But Thomas think "fullness" is easier to understand, so we use "fullness" in rte APIs and we'll translate it to LWM in mlx5 PMD.

I may be wrong :)

> HW simply does now know fullness and there can't generate any events
> based on it. It is a problem on Rx when there is now available
> descriptors.  I.e. emptiness.

So you think "empty_thresh" would be better?
Or "avail_thresh"?

> >>> Fill threshold is defined as a percentage of Rx queue size with valid
> >>> value of [0,99].
> >>> Setting fill threshold to 0 means disable it, which is the default.
> >>> Add fill threshold configuration and query driver callbacks in eth_dev_ops.
> >>> Add command line options to support fill_thresh per-rxq configure.
> >>> - Command syntax:
> >>>     set port <port_id> rxq <rxq_id> fill_thresh <fill_thresh_num>
> >>>
> >>> - Example commands:
> >>> To configure fill_thresh as 30% of rxq size on port 1 rxq 0:
> >>> testpmd> set port 1 rxq 0 fill_thresh 30
> >>>
> >>> To disable fill_thresh on port 1 rxq 0:
> >>> testpmd> set port 1 rxq 0 fill_thresh 0
> >>>
> >>> Signed-off-by: Spike Du <spiked@nvidia.com>