[dpdk-dev] [PATCH v1 3/9] mempool: remove callback to get capabilities

[Andrew Rybchenko]

On 03/14/2018 07:53 PM, Burakov, Anatoly wrote:
> On 14-Mar-18 4:12 PM, Andrew Rybchenko wrote:
>> On 03/14/2018 05:40 PM, Burakov, Anatoly wrote:
>>> On 10-Mar-18 3:39 PM, Andrew Rybchenko wrote:
>>>> The callback was introduced to let generic code to know octeontx
>>>> mempool driver requirements to use single physically contiguous
>>>> memory chunk to store all objects and align object address to
>>>> total object size. Now these requirements are met using a new
>>>> callbacks to calculate required memory chunk size and to populate
>>>> objects using provided memory chunk.
>>>>
>>>> These capability flags are not used anywhere else.
>>>>
>>>> Restricting capabilities to flags is not generic and likely to
>>>> be insufficient to describe mempool driver features. If required
>>>> in the future, API which returns structured information may be
>>>> added.
>>>>
>>>> Signed-off-by: Andrew Rybchenko <arybchenko at solarflare.com>
>>>> ---
>>>
>>> Just a general comment - it is not enough to describe minimum 
>>> memchunk requirements. With memory hotplug patchset that's hopefully 
>>> getting merged in 18.05, memzones will no longer be guaranteed to be 
>>> IOVA-contiguous. So, if a driver requires its mempool to not only be 
>>> populated from a single memzone, but a single *physically 
>>> contiguous* memzone, going by only callbacks will not do, because 
>>> whether or not something should be a single memzone says nothing 
>>> about whether this memzone has to also be IOVA-contiguous.
>>>
>>> So i believe this needs to stay in one form or another.
>>>
>>> (also it would be nice to have a flag that a user could pass to 
>>> mempool_create that would force memzone reservation be 
>>> IOVA-contiguous, but that's a topic for another conversation. prime 
>>> user for this would be KNI.)
>>
>> I think that min_chunk_size should be treated as IOVA-contiguous.
>
> Why? It's perfectly reasonable to e.g. implement a software mempool 
> driver that would perform some optimizations due to all objects being 
> in the same VA-contiguous memzone, yet not be dependent on underlying 
> physical memory layout. These are two separate concerns IMO.

It looks like there is some misunderstanding here or I simply don't 
understand your point.
Above I mean that driver should be able to advertise its requirements on 
IOVA-contiguous regions.
If driver do not care about physical memory layout, no problem.

> > So, we
>> have 4 levels:
>>   - MEMPOOL_F_NO_PHYS_CONTIG (min_chunk_size == 0) -- 
>> IOVA-congtiguous is not required at all
>>   - no MEMPOOL_F_NO_PHYS_CONTIG (min_chunk_size == total_obj_size) -- 
>> object should be IOVA-contiguous
>>   - min_chunk_size > total_obj_size  -- group of objects should be 
>> IOVA-contiguous
>>   - min_chunk_size == <all-objects-size> -- all objects should be 
>> IOVA-contiguous
>
> I don't think this "automagic" decision on what should be 
> IOVA-contiguous or not is the way to go. It needlessly complicates 
> things, when all it takes is another flag passed to mempool allocator 
> somewhere.

No, it is not just one flag. We really need option (3) above: group of 
objects IOVA-contiguous in [1].
Of course, it is possible to use option (4) instead: everything 
IOVA-contigous, but I think it is bad - it may be very big and 
hard/impossible to allocate due to fragmentation.

> I'm not sure what is the best solution here. Perhaps another option 
> would be to let mempool drivers allocate their memory as well? I.e. 
> leave current behavior as default, as it's likely that it would be 
> suitable for nearly all use cases, but provide another option to 
> override memory allocation completely, so that e.g. octeontx could 
> just do a memzone_reserve_contig() without regard for default 
> allocation settings. I think this could be the cleanest solution.

For me it is hard to say. I don't know DPDK history good enough to say 
why there is a mempool API to populate objects on externally provided 
memory. If it may be removed, it is OK for me to do memory allocation 
inside rte_mempool or mempool drivers. Otherwise, if it is still allowed 
to allocate memory externally and pass it to mempool, it must be a way 
to express IOVA-contiguos requirements.

[1] https://dpdk.org/dev/patchwork/patch/34338/

>
>>
>> If so, how allocation should be implemented?
>>   1. if (min_chunk_size > min_page_size)
>>      a. try all contiguous
>>      b. if cannot, do by mem_chunk_size contiguous
>>   2. else allocate non-contiguous
>>
>> -- 
>> Andrew.
>
>