Re: [dpdk-dev] [PATCH RFC 1/2] net/i40e: allow bulk alloc for the max size desc ring

[Ilya Maximets <i.maximets@samsung.com>]

On 29.11.2016 15:50, Ananyev, Konstantin wrote:
> Hi Ilya,
> 
>> Ping.
>>
>> Best regards, Ilya Maximets.
>>
>> On 19.10.2016 17:07, Ilya Maximets wrote:
>>> The only reason why bulk alloc disabled for the rings with
>>> more than (I40E_MAX_RING_DESC - RTE_PMD_I40E_RX_MAX_BURST)
>>> descriptors is the possible out-of-bound access to the dma
>>> memory. But it's the artificial limit and can be easily
>>> avoided by allocating of RTE_PMD_I40E_RX_MAX_BURST more
>>> descriptors in memory. This will not interfere the HW and,
>>> as soon as all rings' memory zeroized, Rx functions will
>>> work correctly.
>>>
>>> This change allows to use vectorized Rx functions with
>>> 4096 descriptors in Rx ring which is important to achieve
>>> zero packet drop rate in high-load installations.
>>>
>>> Signed-off-by: Ilya Maximets <i.maximets@samsung.com>
>>> ---
>>>  drivers/net/i40e/i40e_rxtx.c | 24 +++++++++++++-----------
>>>  1 file changed, 13 insertions(+), 11 deletions(-)
>>>
>>> diff --git a/drivers/net/i40e/i40e_rxtx.c b/drivers/net/i40e/i40e_rxtx.c
>>> index 7ae7d9f..1f76691 100644
>>> --- a/drivers/net/i40e/i40e_rxtx.c
>>> +++ b/drivers/net/i40e/i40e_rxtx.c
>>> @@ -409,15 +409,6 @@ check_rx_burst_bulk_alloc_preconditions(__rte_unused struct i40e_rx_queue *rxq)
>>>  			     "rxq->rx_free_thresh=%d",
>>>  			     rxq->nb_rx_desc, rxq->rx_free_thresh);
>>>  		ret = -EINVAL;
>>> -	} else if (!(rxq->nb_rx_desc < (I40E_MAX_RING_DESC -
>>> -				RTE_PMD_I40E_RX_MAX_BURST))) {
>>> -		PMD_INIT_LOG(DEBUG, "Rx Burst Bulk Alloc Preconditions: "
>>> -			     "rxq->nb_rx_desc=%d, "
>>> -			     "I40E_MAX_RING_DESC=%d, "
>>> -			     "RTE_PMD_I40E_RX_MAX_BURST=%d",
>>> -			     rxq->nb_rx_desc, I40E_MAX_RING_DESC,
>>> -			     RTE_PMD_I40E_RX_MAX_BURST);
>>> -		ret = -EINVAL;
>>>  	}
>>>  #else
>>>  	ret = -EINVAL;
>>> @@ -1698,8 +1689,19 @@ i40e_dev_rx_queue_setup(struct rte_eth_dev *dev,
>>>  	rxq->rx_deferred_start = rx_conf->rx_deferred_start;
>>>
>>>  	/* Allocate the maximun number of RX ring hardware descriptor. */
>>> -	ring_size = sizeof(union i40e_rx_desc) * I40E_MAX_RING_DESC;
>>> -	ring_size = RTE_ALIGN(ring_size, I40E_DMA_MEM_ALIGN);
>>> +	len = I40E_MAX_RING_DESC;
>>> +
>>> +#ifdef RTE_LIBRTE_I40E_RX_ALLOW_BULK_ALLOC
>>> +	/**
>>> +	 * Allocating a little more memory because vectorized/bulk_alloc Rx
>>> +	 * functions doesn't check boundaries each time.
>>> +	 */
>>> +	len += RTE_PMD_I40E_RX_MAX_BURST;
>>> +#endif
>>> +
> 
> Looks good to me.
> One question, though do we really need '+#ifdef RTE_LIBRTE_I40E_RX_ALLOW_BULK_ALLOC' here?
> Why just not remove this ifdef here and always add allocate extra descriptors.
> Konstantin

I put it there because all other bulk_alloc related code in i40e is under this
ifdef. Just to keep the code in consistent state. Plus, it saves memory a bit.
I prefer to keep ifdef here, but maintainers are free to remove it while applying.

>>> +	ring_size = RTE_ALIGN(len * sizeof(union i40e_rx_desc),
>>> +			      I40E_DMA_MEM_ALIGN);
>>> +
>>>  	rz = rte_eth_dma_zone_reserve(dev, "rx_ring", queue_idx,
>>>  			      ring_size, I40E_RING_BASE_ALIGN, socket_id);
>>>  	if (!rz) {
>>>
> 
> 
>