Re: [PATCH 2/3] power: defer lcore variable allocation

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

On 2024-12-12 08:57, David Marchand wrote:
> On Fri, Dec 6, 2024 at 12:29 PM Mattias Rönnblom <hofors@lysator.liu.se> wrote:
>> I would wrap all RTE_LCORE_VAR_LCORE() and RTE_LCORE_VAR().
>>
>> static struct pmd_core_cfg *
>> get_cfg_lcore(unsigned int lcore_id)
>> {
>>          assure_lcore_cfgs_alloced();
>>          return RTE_LCORE_VAR_LCORE(lcore_cfgs, lcore_id);
>> }
>>
>> static struct pmd_core_cfg *
>> get_cfg(void)
>> {
>>          get_cfg_lcore(rte_lcore_id());
>> }
>>
>> Add
>>
>> static void
>> assure_lcore_cfgs_alloced(unsigned int lcore_id)
>> {
>>          if (lcore_cfgs != NULL)
> 
> ==
> 

Oops.

>>                  lcore_cfgs_alloc();
>> }
>>
>> ..or maybe better merge assure_lcore_cfgs_alloced() and lcore_cfgs_alloc().
>>
>> Makes it a little harder to make mistakes.
> 
> clb_multiwait, clb_pause and clb_scale_freq callbacks can only be
> reached after a successful call to
> rte_power_ethdev_pmgmt_queue_enable.
> Triggering an allocation in them means we are hiding a (internal)
> programatic error as allocation and use of a lcore variable are
> clearly separated atm.
> If we keep the lcore var api as is, I would add an assert() (maybe
> under a debug build option) in RTE_LCORE_VAR macros themselves, as
> calling with a NULL handle means the initialisation path in some
> code/RTE_LCORE_VAR API use was incorrect.
> 

Sure, that would make sense. RTE_ASSERT(), that is. RTE_VERIFY() would 
be too expensive.

> 
> Or because you propose to add the same type of helpers in both this
> patch and the next, I am considering the other way: hide the
> allocation in the RTE_LCORE_VAR* macros.
> Checking for already allocated var in RTE_LCORE_VAR_ALLOC seems fine.
> But the "fast path" RTE_LCORE_VAR would have an unwanted branch in most cases.
> 

I would prefer to have the ALLOC() macro with semantics most people 
expect from a macro (or function) with that name, which is, I would 
argue, an unconditional allocation.

It would make sense to have another macro, which performs an allocation 
only if the handle is NULL.

RTE_LCORE_VAR_ASSURE_ALLOCATED(), or just RTE_LCORE_VAR_ASSURE() 
(although the latter sounds a little like an assertion, and not an 
allocation).

RTE_LCORE_VAR_LAZY_ALLOC()

I don't know. Something like that.

> 
>> A somewhat unrelated question: why is pmd_core_cfg cache-line aligned? I
>> don't think it should be.
> 
> Before the conversion to per lcore variable, it was probably useful
> (avoiding false sharing).
> With the conversion, indeed, it looks like a waste of space.
> It seems worth a separate fix.
> 
> 

You will include it, or should I submit a separate patch?