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

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

> >>> 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@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.
> 
> No, as far as I know. For Tx only application (e.g. traffic generator)
> it is 100% legal to configure with tx_queues=X, rx_queues=0.
> The same is for Rx only application (e.g. packet capture).

Yes, that is valid config for sure.
I just pointed that simply ignoring 'nb_queues' value and
always allocating space for max possible queues, i.e:

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 */
+ if (dev->data->rx_queues == NULL) {
wouldn't work, as right now nb_queues == 0 has extra special meaning -
do final cleanup and free dev->data->rx_queues.
But re-reading the text below, it seems that I misunderstood you
and it probably wasn't your intention anyway.

> 
> >
> >>     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.
> 
> You don't need to care about these pointers, if these arrays are
> always allocated. See (3) below.
> 
> >
> >> 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
> 
> Thanks a lot. Makes sense.
> 
> >>     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.
> 
> 32 port * 1024 queues * 2 types * 8 pointer size
> if we allocate as in (2) above.
> 
> > 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.
> 
> IMHO it will be a bit nicer if queue pointers arrays are allocated
> on device get if size is fixed. It is just a suggestion. If you
> disagree, feel free to drop it.

You mean - allocate these arrays somewhere at rte_eth_dev_allocate() path?
That sounds like an interesting idea, but seems too drastic to me at that stage.

> 
> >>>   		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]