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

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

> -----Original Message-----
> From: Richardson, Bruce
> Sent: Thursday, December 04, 2014 3:15 PM
> To: Ananyev, Konstantin
> Cc: Jean-Mickael Guerin; dev@dpdk.org
> Subject: Re: [dpdk-dev] [PATCH 2/2] ixgbe: don't override mbuf buffer length
> 
> 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.
> 

I understand that, but  why it can't be done in some other way?
Without allocating/freeing?
Let say, at mempool_create() store obj_init() and then add ability to call it again?
Anyway, it doesn't look to me like a critical problem, that requires an urgent patch for 1.8.

> > 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.

Why it doesn't?
I suppose compiler has all the knowledge of the mbuf structure layout at that point.
Or there is a some sort of bug in some version of the compiler?

> >
> > > -	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
> >