[dpdk-dev] [PATCH v5 2/7] ethdev: allocate max space for internal queue array

[Ananyev, Konstantin]

> > At queue configure stage always allocate space for maximum possible
> > number (RTE_MAX_QUEUES_PER_PORT) of queue pointers.
> > That will allow 'fast' inline functions (eth_rx_burst, etc.) to refer
> > pointer to internal queue data without extra checking of current number
> > of configured queues.
> > That would help in future to hide rte_eth_dev and related structures.
> > It means that from now on, each ethdev port will always consume:
> > ((2*sizeof(uintptr_t))* RTE_MAX_QUEUES_PER_PORT)
> > bytes of memory for its queue pointers.
> > With RTE_MAX_QUEUES_PER_PORT==1024 (default value) it is 16KB per port.
> >
> > Signed-off-by: Konstantin Ananyev <konstantin.ananyev at intel.com>
> > ---
> >  lib/ethdev/rte_ethdev.c | 36 +++++++++---------------------------
> >  1 file changed, 9 insertions(+), 27 deletions(-)
> >
> > diff --git a/lib/ethdev/rte_ethdev.c b/lib/ethdev/rte_ethdev.c
> > index ed37f8871b..c8abda6dd7 100644
> > --- a/lib/ethdev/rte_ethdev.c
> > +++ b/lib/ethdev/rte_ethdev.c
> > @@ -897,7 +897,8 @@ eth_dev_rx_queue_config(struct rte_eth_dev *dev, uint16_t nb_queues)
> >
> >  	if (dev->data->rx_queues == NULL && nb_queues != 0) { /* first time configuration */
> >  		dev->data->rx_queues = rte_zmalloc("ethdev->rx_queues",
> > -				sizeof(dev->data->rx_queues[0]) * nb_queues,
> > +				sizeof(dev->data->rx_queues[0]) *
> > +				RTE_MAX_QUEUES_PER_PORT,
> >  				RTE_CACHE_LINE_SIZE);
> 
> Looking at it I have few questions:
> 1. Why is nb_queues == 0 case kept as an exception? Yes,
>    strictly speaking it is not the problem of the patch,
>    DPDK will still segfault (non-debug build) if I
>    allocate Tx queues only but call rte_eth_rx_burst().

eth_dev_rx_queue_config(.., nb_queues=0) is used in few places to clean-up things.

>    After reading the patch description I thought that
>    we're trying to address it.

We do, though I can't see how we can address it in this patch.
Though it is a good idea - I think I can add extra check in eth_dev_fp_ops_setup()
or around and setup RX function pointers only when dev->data->rx_queues != NULL.
Same for TX.

> 2. Why do we need to allocate memory dynamically?
>    Can we just make rx_queues an array of appropriate size?

Pavan already asked same question.
My answer to him:
Yep we can, and yes it will simplify this peace of code.
The main reason I decided no to do this change now -
it will change layout of the_eth_dev_data structure.
In this series I tried to mininize(/avoid) changes in rte_eth_dev and rte_eth_dev_data,
as much as possible to avoid any unforeseen performance and functional impacts.
If we'll manage to make rte_eth_dev and rte_eth_dev_data private we can in future
consider that one and other changes in rte_eth_dev and rte_eth_dev_data layouts
without worrying about ABI breakage

>    May be wasting 512K unconditionally is too much.
> 3. If wasting 512K is too much, I'd consider to move
>    allocation to eth_dev_get(). If

Don't understand where 512KB came from.
each ethdev port will always consume:
((2*sizeof(uintptr_t))* RTE_MAX_QUEUES_PER_PORT)
bytes of memory for its queue pointers.
With RTE_MAX_QUEUES_PER_PORT==1024 (default value) it is 16KB per port.
 
> >  		if (dev->data->rx_queues == NULL) {
> >  			dev->data->nb_rx_queues = 0;
> > @@ -908,21 +909,11 @@ eth_dev_rx_queue_config(struct rte_eth_dev *dev, uint16_t nb_queues)
> >
> >  		rxq = dev->data->rx_queues;
> >
> > -		for (i = nb_queues; i < old_nb_queues; i++)
> > +		for (i = nb_queues; i < old_nb_queues; i++) {
> >  			(*dev->dev_ops->rx_queue_release)(rxq[i]);
> > -		rxq = rte_realloc(rxq, sizeof(rxq[0]) * nb_queues,
> > -				RTE_CACHE_LINE_SIZE);
> > -		if (rxq == NULL)
> > -			return -(ENOMEM);
> > -		if (nb_queues > old_nb_queues) {
> > -			uint16_t new_qs = nb_queues - old_nb_queues;
> > -
> > -			memset(rxq + old_nb_queues, 0,
> > -				sizeof(rxq[0]) * new_qs);
> > +			rxq[i] = NULL;
> 
> It looks like the patch should be rebased on top of
> next-net main because of queue release patches.
> 
> [snip]