Re: [dpdk-dev] [PATCH v3 1/5] mbuf: fix clone support when application uses private mbuf data

[Olivier MATZ <olivier.matz@6wind.com>]

Hi Konstantin,

On 04/08/2015 03:45 PM, Ananyev, Konstantin wrote:
> Hi Olivier,
>
>> -----Original Message-----
>> From: Olivier MATZ [mailto:olivier.matz@6wind.com]
>> Sent: Wednesday, April 08, 2015 10:44 AM
>> To: Ananyev, Konstantin; dev@dpdk.org
>> Cc: zoltan.kiss@linaro.org; Richardson, Bruce
>> Subject: Re: [PATCH v3 1/5] mbuf: fix clone support when application uses private mbuf data
>>
>> Hi Konstantin,
>>
>> On 04/07/2015 07:17 PM, Ananyev, Konstantin wrote:
>>>> Just to be sure we're on the same line:
>>>>
>>>> - before the patch series
>>>>
>>>>      - private area was working before that patch series if clones were not
>>>>        used. To use a private are, the user had to provide another
>>>>        function derived from pktmbuf_init() to change m->buf_addr and
>>>>        m->buf_len.
>>>>      - using both private area + clones was broken
>>>>
>>>> - after the patch series
>>>>
>>>>      - private area is working with or without clone. But yo use it,
>>>>        the user still has to provide another function to change
>>>>        m->buf_addr, m->buf_len *and m->priv_size*.
>>>>
>>>> The series just fixes the fact that "clones + priv" was not working.
>>>> It does not address the problem that providing a new pktmbuf_init()
>>>> function is required to use privata area. To fix this, I think it
>>>> could require a API evolution that should be part of another series.
>>>
>>> I don't think we need new pktmbuf_init().
>>> We just need to update it, so both pktmbuf_init() and detach() setup
>>> buf_addr, buf_len (and priv_size) to exactly the same values.
>>> If they don't do that, it means that you can't use attach/detach with
>>> mempools created with pktmbuf_init() any more.
>>>
>>> BTW, another thing that I just realised:
>>> examples/ipv4_multicast and examples/ip_fragmentation/ -
>>> both create a pool of mbufs with elem_size < 2K and don't populate mempool's private area -
>>> so mbp_priv->mbuf_data_room_size == 0, for them.
>>>
>>> So that code in detach():
>>>
>>>    +	mbp_priv = rte_mempool_get_priv(mp);
>>>    +	m->priv_size = mp->elt_size - RTE_PKTMBUF_HEADROOM -
>>>    +		mbp_priv->mbuf_data_room_size -
>>>    +		sizeof(struct rte_mbuf);
>>>
>>>
>>> Would break both these samples.
>>> I suppose we need to handle situation when mp->elt_size < RTE_PKTMBUF_HEADROOM + sizeof(struct rte_mbuf),
>>> (and probably also when mbuf_data_room_size == 0) correctly.
>>
>> Indeed. I think a mbuf pool (even with buf_len == 0 like in
>> ip_fragmentation example) should have a pool with a private area and
>> should call rte_pktmbuf_pool_init() to populate it. So
>> rte_pktmbuf_pool_init() has to be fixed first to use elt_size
>> and support the buf_len < RTE_PKTMBUF_HEADROOM, then we can
>> update frag/multicast examples.
>>
>> Unfortunately, we don't know the size of the mbuf private area
>> in rte_pktmbuf_pool_init() if the opaque arg (data_room_size)
>> is 0, which is the default. I think it should be replaced by a structure
>> containing data_room_size and mbuf_priv_size, but it would break
>> applications that are setting data_room_size.
>
> Yes, same thoughts here.
>
>> I don't see any good
>> solution to do that while keeping a backward compatibility for
>> rte_pktmbuf_pool_init(), but as the current API is not ideal,
>> I think it's worth changing it and add something in the release
>> note.
>
> If no one else has a better alternative than that, then I suppose it is good enough.
>
>>
>> We may also want to introduce a new helper as discussed previously:
>>
>> struct rte_mempool *
>> rte_pktmbuf_pool_create(const char *name, unsigned n, unsigned elt_size,
>> 	unsigned cache_size, size_t mbuf_priv_size,
>> 	rte_mempool_obj_ctor_t *obj_init, void *obj_init_arg,
>> 	int socket_id, unsigned flags)
>>
>> Any comment?
>
> Looks good to me.
> Should we also introduce rte_pktmbuf_pool_xmem_create()?

Hmmm as it's only used in one place, I'm not sure it's really
required. As the previous way to create mbuf pool using
rte_mempool_create() will still work, I think we could consider
it's a special case. If it becomes necessary, there's no problem
to add it later.

Olivier



> Konstantin
>
>>
>>
>>>
>>> Konstantin
>