Re: [Patch v2] net/mana: use rte_pktmbuf_alloc_bulk for allocating RX WQEs

[Ferruh Yigit <ferruh.yigit@amd.com>]

On 1/30/2024 4:43 PM, Stephen Hemminger wrote:
> On Tue, 30 Jan 2024 10:19:32 +0000
> Ferruh Yigit <ferruh.yigit@amd.com> wrote:
> 
>>> -mana_alloc_and_post_rx_wqes(struct mana_rxq *rxq)
>>> +mana_alloc_and_post_rx_wqes(struct mana_rxq *rxq, uint32_t count)
>>>  {
>>>  	int ret;
>>>  	uint32_t i;
>>> +	struct rte_mbuf **mbufs;
>>> +
>>> +	mbufs = rte_calloc_socket("mana_rx_mbufs", count, sizeof(struct rte_mbuf *),
>>> +				  0, rxq->mp->socket_id);
>>> +	if (!mbufs)
>>> +		return -ENOMEM;
>>>  
>>
>> 'mbufs' is temporarily storage for allocated mbuf pointers, why not
>> allocate if from stack instead, can be faster and easier to manage:
>> "struct rte_mbuf *mbufs[count]"
> 
> That would introduce a variable length array.
> VLA's should be removed, they are not supported on Windows and many
> security tools flag them. The problem is that it makes the code brittle
> if count gets huge.
> 
> But certainly regular calloc() or alloca() would work here.
>

Most of the existing bulk alloc already uses VLA but I can see the
problem it is not being supported by Windows.

As this mbuf pointer array is short lived within the function, and being
in the fast path, I think continuous alloc and free can be prevented,

one option can be to define a fixed size, big enough, array which
requires additional loop for the cases 'count' size is bigger than array
size,

or an array can be allocated by driver init in device specific data ,as
we know it will be required continuously in the datapath, and it can be
freed during device close()/uninit().

I think an fixed size array from stack is easier and can be preferred.