Re: [PATCH 1/4] latencystats: use alloca instead of vla trivial

["Mattias Rönnblom" <hofors@lysator.liu.se>]

On 2024-04-06 17:28, Morten Brørup wrote:
>> From: Tyler Retzlaff [mailto:roretzla@linux.microsoft.com]
>> Sent: Thursday, 4 April 2024 19.15
>>
>> RFC sample illustrating simple conversion of VLA to alloca().
>>
>> Signed-off-by: Tyler Retzlaff <roretzla@linux.microsoft.com>
>> ---
> 
> [...]
> 
>> --- a/lib/latencystats/rte_latencystats.c
>> +++ b/lib/latencystats/rte_latencystats.c
>> @@ -159,7 +159,7 @@ struct latency_stats_nameoff {
>>   {
>>   	unsigned int i, cnt = 0;
>>   	uint64_t now;
>> -	float latency[nb_pkts];
>> +	float *latency = alloca(sizeof(float) * nb_pkts);
> 
> In cases where we are processing packet bursts, I would prefer introducing a global #define RTE_MAX_PKT_BURST_SIZE, indicating the max packet burst size supported by libraries and drivers.

First question: what is meant by a "packet" here? An mbuf? A 
network-layer PDU? Something that in some way relates to zero or more 
packets, like an rte_event? Or just any object that are sent or receive 
of some DPDK API in batches or bursts?

Second question: is RTE_MAX_PKT_BURST_SIZE meant as an upper bound, so 
no API can consumer or produce a burst larger than this, it does all 
APIs literally have to support that burst size.

Third question: why not just keep VLAs?

> For reference, rte_config.h already has #define RTE_GRAPH_BURST_SIZE 256.
> 
> Such a common define should also be used by functions such as rte_pktmbuf_free_bulk(); although it also supports segmented packets, so it must still be able to handle more mbufs.
> https://elixir.bootlin.com/dpdk/v24.03/source/lib/mbuf/rte_mbuf.c#L486
>