Re: [dpdk-dev] [PATCH 2/2] ixgbe: don't override mbuf buffer length

[Bruce Richardson <bruce.richardson@intel.com>]

On Thu, Dec 04, 2014 at 02:50:11PM +0000, Ananyev, Konstantin wrote:
> Hi,
> 
> > -----Original Message-----
> > From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Jean-Mickael Guerin
> > Sent: Thursday, December 04, 2014 2:26 PM
> > To: dev@dpdk.org
> > Subject: [dpdk-dev] [PATCH 2/2] ixgbe: don't override mbuf buffer length
> > 
> > The template mbuf_initializer is hard coded with a buflen which
> > might have been set differently by the application at the time of
> > mbuf pool creation.
> > 
> > Switch to a mbuf allocation, to fetch the correct default values.
> > There is no performance impact because this is not a data-plane API.
> > 
> > Signed-off-by: Jean-Mickael Guerin <jean-mickael.guerin@6wind.com>
> > Acked-by: David Marchand <david.marchand@6wind.com>
> > Fixes: 0ff3324da2 ("ixgbe: rework vector pmd following mbuf changes")
> > ---
> >  lib/librte_pmd_ixgbe/ixgbe_rxtx_vec.c | 19 ++++++++++++-------
> >  1 file changed, 12 insertions(+), 7 deletions(-)
> > 
> > diff --git a/lib/librte_pmd_ixgbe/ixgbe_rxtx_vec.c b/lib/librte_pmd_ixgbe/ixgbe_rxtx_vec.c
> > index c1b5a78..f7b02f5 100644
> > --- a/lib/librte_pmd_ixgbe/ixgbe_rxtx_vec.c
> > +++ b/lib/librte_pmd_ixgbe/ixgbe_rxtx_vec.c
> > @@ -732,17 +732,22 @@ static struct ixgbe_txq_ops vec_txq_ops = {
> >  int
> >  ixgbe_rxq_vec_setup(struct igb_rx_queue *rxq)
> >  {
> > -	struct rte_mbuf mb_def = { .buf_addr = 0 }; /* zeroed mbuf */
> > +	struct rte_mbuf *mb_def;
> > 
> > -	mb_def.nb_segs = 1;
> > -	mb_def.data_off = RTE_PKTMBUF_HEADROOM;
> > -	mb_def.buf_len = rxq->mb_pool->elt_size - sizeof(struct rte_mbuf);
> > -	mb_def.port = rxq->port_id;
> > -	rte_mbuf_refcnt_set(&mb_def, 1);
> > +	mb_def = rte_pktmbuf_alloc(rxq->mb_pool);
> 
> Could you explain to me, what is an advantage of using dynamic allocation vs local struct here?
> I don't see any.

It means that we get an mbuf that is initialized as done by the initialization
function passed to the mempool_create call. The static variable method assumes
that we configure the mbuf using default setting for buf_len etc.

> Plus if rte_pktmbuf_alloc() would fail, we'll leave our rx queue not configured properly.
> As I can see ixgbe_dev_rx_queue_setup() doesn't check the return value of >  ixgbe_rxq_vec_setup()
> (as it is just not supposed to fail).
> So ixgbe_dev_rx_queue_setup() will return OK for unconfigured RX queue.

Good catch, that's something that should perhaps be looked at in a V2 patch, I
think.

> 
> > +	if (mb_def == NULL) {
> > +		PMD_INIT_LOG(ERR, "ixgbe_rxq_vec_setup: could not allocate one mbuf");
> > +		return -1;
> > +	}
> > +	/* nb_segs, refcnt, data_off and buf_len are already set */
> > +	mb_def->port = rxq->port_id;
> > 
> >  	/* prevent compiler reordering: rearm_data covers previous fields */
> >  	rte_compiler_barrier();
> 
> I don't think we need it here.

I think we might, as the compiler doesn't know that the rearm data overlaps 
with the previously set fields, so may reorder the reads and writes thinking
they are independent.
> 
> > -	rxq->mbuf_initializer = *((uint64_t *)&mb_def.rearm_data);
> > +	rxq->mbuf_initializer = *((uint64_t *)&mb_def->rearm_data);
> > +
> > +	rte_pktmbuf_free(mb_def);
> > +
> >  	return 0;
> >  }
> > 
> > --
> > 2.1.3
> 
> Somy vote -  NACK for the whole series.
> Konstantin
>