RE: [PATCH v4] lib: set/get max memzone segments

[Ophir Munk <ophirmu@nvidia.com>]

> -----Original Message-----
> Subject: Re: [PATCH v4] lib: set/get max memzone segments
> 
> On 5/24/2023 11:25 PM, Ophir Munk wrote:
> > Currently, the max memzones count constat (RTE_MAX_MEMZONE) is used
> to
> > decide how many memzones a DPDK application can have. This value could
> > technically be changed by manually editing `rte_config.h` before
> > compilation, but if DPDK is already compiled, that option is not useful.
> > There are certain use cases that would benefit from making this value
> > configurable.
> >
> > This commit addresses the issue by adding a new API to set the max
> > number of memzones before EAL initialization (while using the old
> > constant as default value), as well as an API to get current maximum
> > number of memzones.
> >
> > Signed-off-by: Ophir Munk <ophirmu@nvidia.com>
> > Acked-by: Morten Brørup <mb@smartsharesystems.com>
> > ---
> 
> > +
> > +int
> > +rte_memzone_max_set(size_t max)
> > +{
> > +	struct rte_mem_config *mcfg;
> > +
> > +	if (eal_get_internal_configuration()->init_complete > 0)
> > +		return -1;
> > +
> > +	mcfg = rte_eal_get_configuration()->mem_config;
> > +	if (!mcfg)
> > +		return -1;
> > +
> > +	mcfg->max_memzone = max;
> > +
> > +	return 0;
> > +}
> 
> Would this even work? 

Yes. It's working. 
I successfully ran the following test:

int max = rte_memzone_max_get(); 
printf("Max memzone before eal_init: %d\n", max);      // Printing the default max value of 2560
rte_memzone_max_set(1000);
max = rte_memzone_max_get();
printf("Max memzone before eal_init after set to 1000: %d\n", max);    // Printing the new max value of 1000
rte_eal_init(argc, argv); 
rte_memzone_max_set(2000); // Here we fail with -1 since we set after eal init
max = rte_memzone_max_get();
printf("Max memzone after eal_init and after set to 2000: %d\n", max);   // Printing the correct max value of 1000

> AFAIR mem_config is only available some time during
> EAL init, not before (mem_config pointer will be NULL at that point).
> 

Please note that DPDK supports early memory config (lib/eal/common/eal_common_config.c):

/* early configuration structure, when memory config is not mmapped */             
static struct rte_mem_config early_mem_config;  

So max memzone is saved in the early memory and later this memory becomes mapped (shared).

Having said that - I think the current patch is correct.
Please confirm.

> I suggest the following flow:
> 
> set():
> 
> if init_complete => return -1

Also if mem_config is NULL => return -1

> else => set local static value
> 

> get():
> 
> if init_complete => return memzones.count else => return local static value (set
> to our default)
> 
> That way we don't actually need the memconfig, and multiprocess will work
> because memzones.count is shared between primary and secondary anyway.
> 
> --
> Thanks,
> Anatoly