Re: [dpdk-dev] [PATCH v2 1/5] mbuf: fix clone support when application uses private mbuf data

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

Hi Konstantin,

Thank you for your comments.

On 03/27/2015 01:24 AM, Ananyev, Konstantin wrote:
> Hi Olivier,
> 
>> -----Original Message-----
>> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Olivier Matz
>> Sent: Thursday, March 26, 2015 4:00 PM
>> To: dev@dpdk.org
>> Cc: zoltan.kiss@linaro.org
>> Subject: [dpdk-dev] [PATCH v2 1/5] mbuf: fix clone support when application uses private mbuf data
>>
>> Add a new private_size field in mbuf structure that should
>> be initialized at mbuf pool creation. This field contains the
>> size of the application private data in mbufs.
>>
>> Introduce new static inline functions rte_mbuf_from_indirect()
>> and rte_mbuf_to_baddr() to replace the existing macros, which
>> take the private size in account when attaching and detaching
>> mbufs.
>>
>> Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
>> ---
>>  app/test-pmd/testpmd.c     |  1 +
>>  examples/vhost/main.c      |  2 +-
>>  lib/librte_mbuf/rte_mbuf.c |  1 +
>>  lib/librte_mbuf/rte_mbuf.h | 44 ++++++++++++++++++++++++++++++++++----------
>>  4 files changed, 37 insertions(+), 11 deletions(-)
>>
>> diff --git a/app/test-pmd/testpmd.c b/app/test-pmd/testpmd.c
>> index 3057791..c5a195a 100644
>> --- a/app/test-pmd/testpmd.c
>> +++ b/app/test-pmd/testpmd.c
>> @@ -425,6 +425,7 @@ testpmd_mbuf_ctor(struct rte_mempool *mp,
>>  	mb->tx_offload   = 0;
>>  	mb->vlan_tci     = 0;
>>  	mb->hash.rss     = 0;
>> +	mb->priv_size    = 0;
>>  }
>>
>>  static void
>> diff --git a/examples/vhost/main.c b/examples/vhost/main.c
>> index c3fcb80..d542461 100644
>> --- a/examples/vhost/main.c
>> +++ b/examples/vhost/main.c
>> @@ -139,7 +139,7 @@
>>  /* Number of descriptors per cacheline. */
>>  #define DESC_PER_CACHELINE (RTE_CACHE_LINE_SIZE / sizeof(struct vring_desc))
>>
>> -#define MBUF_EXT_MEM(mb)   (RTE_MBUF_FROM_BADDR((mb)->buf_addr) != (mb))
>> +#define MBUF_EXT_MEM(mb)   (rte_mbuf_from_indirect(mb) != (mb))
>>
>>  /* mask of enabled ports */
>>  static uint32_t enabled_port_mask = 0;
>> diff --git a/lib/librte_mbuf/rte_mbuf.c b/lib/librte_mbuf/rte_mbuf.c
>> index 526b18d..e095999 100644
>> --- a/lib/librte_mbuf/rte_mbuf.c
>> +++ b/lib/librte_mbuf/rte_mbuf.c
>> @@ -125,6 +125,7 @@ rte_pktmbuf_init(struct rte_mempool *mp,
>>  	m->pool = mp;
>>  	m->nb_segs = 1;
>>  	m->port = 0xff;
>> +	m->priv_size = 0;
>>  }
>>
>>  /* do some sanity checks on a mbuf: panic if it fails */
>> diff --git a/lib/librte_mbuf/rte_mbuf.h b/lib/librte_mbuf/rte_mbuf.h
>> index 17ba791..45ac948 100644
>> --- a/lib/librte_mbuf/rte_mbuf.h
>> +++ b/lib/librte_mbuf/rte_mbuf.h
>> @@ -317,18 +317,42 @@ struct rte_mbuf {
>>  			/* uint64_t unused:8; */
>>  		};
>>  	};
>> +
>> +	uint16_t priv_size;       /**< size of the application private data */
>>  } __rte_cache_aligned;
>>
>>  /**
>> - * Given the buf_addr returns the pointer to corresponding mbuf.
>> + * Return the mbuf owning the data buffer address of an indirect mbuf.
>> + *
>> + * @param mi
>> + *   The pointer to the indirect mbuf.
>> + * @return
>> + *   The address of the direct mbuf corresponding to buffer_addr.
>>   */
>> -#define RTE_MBUF_FROM_BADDR(ba)     (((struct rte_mbuf *)(ba)) - 1)
>> +static inline struct rte_mbuf *
>> +rte_mbuf_from_indirect(struct rte_mbuf *mi)
>> +{
>> +       struct rte_mbuf *md;
>> +       md = (struct rte_mbuf *)((char *)mi->buf_addr - sizeof(*mi) -
>> +	       mi->priv_size);
>> +       return md;
>> +}
>>
>>  /**
>> - * Given the pointer to mbuf returns an address where it's  buf_addr
>> - * should point to.
>> + * Return the buffer address embedded in the given mbuf.
>> + *
>> + * @param md
>> + *   The pointer to the mbuf.
>> + * @return
>> + *   The address of the data buffer owned by the mbuf.
>>   */
>> -#define RTE_MBUF_TO_BADDR(mb)       (((struct rte_mbuf *)(mb)) + 1)
>> +static inline char *
>> +rte_mbuf_to_baddr(struct rte_mbuf *md)
>> +{
>> +       char *buffer_addr;
>> +       buffer_addr = (char *)md + sizeof(*md) + md->priv_size;
>> +       return buffer_addr;
>> +}
>>
> 
> I am a bit puzzled here, so for indirect mbuf, what value priv_size should hold?
> Is that priv_size of indirect mfuf, or priv_size of direct mbuf, that mbuf is attached to?
> If it is first one, then your changes in __rte_pktmbuf_prefree_seg() wouldn't work properly,
> If second one then  changes in rte_pktmbuf_detach() looks wrong to me.
> Unless, of course priv_size for all mbufs in the system should always be the same value.
> But I suppose, that's not what your intention was, otherwise we don't need priv_size inside mbuf at all -
> just a new macro in config file seems enough, plus it would be easier and faster.

Yes, my assumption was that the priv_size isi the same on all mbufs
of a pool, and probably in most cases all mbufs of the system.
To me, a config macro is not th best solution because it prevents the
application to choose the proper size at run-time, especially if the
dpdk is distributed in binary format.

That's why I decided to have priv_size in mbufs, although having it in
the mempool private area is possible but it adds an additional
indirection (see the cover letter).

> I think that to support ability to setup priv_size on a mempool basis,
> and reserve private space between struct rte_mbuf and rte_mbuf. buf_addr,
> we need to: 
> 
> 1. Store priv_size both inside the mempool and inside the mbuf.
> 
> 2. rte_pktmbuf_attach() should change the value of priv_size to the priv_size of the direct mbuf we are going to attach to:
> rte_pktmbuf_attach(struct rte_mbuf *mi, struct rte_mbuf *md) {... mi->priv_size = md->priv_size; ...}
> 
> 3. rte_pktmbuf_detach() should restore original value of mbuf's priv_size:
> rte_pktmbuf_detach(struct rte_mbuf *m) 
> {
>   ...
>    m->priv_size = rte_mempool_get_privsize(m->pool);
>    m->buf _addr= rte_mbuf_to_baddr(m);
>    ...
> }
> 
> Also I think we need to provide a way to specify priv_size for all mbufs of the mepool at init time:
> - either force people to specify it at rte_mempool_create() time (probably use init_arg for that),
> - or provide separate function that could be called straight after rte_mempool_create() , that
> would setup priv_size for the  pool and for all its mbufs.
> - or some sort of combination of these 2 approaches - introduce a wrapper function 
> (rte_mbuf_pool_create() or something) that would take priv_size as parameter, 
> create a new mempool and then setup priv_size.

Your solution looks good to me, and even if I'm not absolutely
convinced that there is a use case where a packet is attached to
another one with a different priv_size, it's stronger from an
API perspective to support this. Maybe it could happen in a
virtualization or secondary process context where there are 2
different mbuf pools.

The mbuf_priv_size could go in struct rte_pktmbuf_pool_private, but
it can already be deducted from what we have today without changing
anything:

priv_size = pool->elt_size - RTE_PKTMBUF_HEADROOM -
	pool_private->mbuf_data_room_size -
	sizeof(rte_mbuf)

Introducing rte_mbuf_pool_create() seems a good idea to me, it
would hide 'rte_pktmbuf_pool_private' from the user and force
to initialize all the required fields (mbuf_data_room_size only
today, and maybe mbuf_priv_size).

The API would be:

struct rte_mempool *
rte_mbuf_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)

I can give it a try and send a patch for this.

> Though, I still think that the better approach is to reserve private space before struct rte_mbuf, not after.
> In that case, user got his private space, and we keep buf_addr straight after  rte_mbuf, without any whole.
> So we don't need steps 2 and 3, above,
> plus we don't need rte_mbuf_to_baddr() and rte_mbuf_from_indirect() - 
> RTE_MBUF_TO_BADDR/ RTE_MBUF_FROM_BADDR would keep working correctly.
> In fact, with this scheme - we don't even need priv_size for mbuf management (attach/detach/free).
> 
> Wonder did you try that approach?

Yes, I though about this approach too. But I think it would require
more changes. When an application or a driver allocate a mbuf with
mempool_get(), it expects that the returned pointer is the rte_mbuf *.

With this solution, there are 2 options:
- no mempool modification, so each application/driver has to add
  priv_size bytes to the object to get the mbuf pointer. This does not
  look feasible.
- change the mempool to support private area before each object. I
  think mempool API is already quite complex, and I think it's not
  the role of the mempool library to provide such features.

In any case, I think it would require more modifications (in terms of
lines of code, but also in terms of "allocation logic"). At the end,
the patch I suggested does not break any paradygm and just add ~10 lines
of code.

Regards,
Olivier