[dpdk-dev] [PATCH] nfp: handle packets with length 0 as usual ones

[Alejandro Lucero]

On Mon, Aug 21, 2017 at 11:34 AM, Ferruh Yigit <ferruh.yigit at intel.com>
wrote:

> On 8/18/2017 5:23 PM, Alejandro Lucero wrote:
> >
> >
> > On Fri, Aug 18, 2017 at 4:10 PM, Ferruh Yigit <ferruh.yigit at intel.com
> > <mailto:ferruh.yigit at intel.com>> wrote:
> >
> >     On 8/11/2017 11:05 AM, Alejandro Lucero wrote:
> >     > A DPDK app could, whatever the reason, send packets with size 0.
> >     > The PMD is not sending those packets, which does make sense,
> >     > but the problem is the mbuf is not released either. That leads
> >     > to mbufs not being available, because the app trusts the
> >     > PMD will do it.
> >     >
> >     > Although this is a problem related to app wrong behaviour, we
> >     > should harden the PMD in this regard. Not sending a packet with
> >     > size 0 could be problematic, needing special handling inside the
> >     > PMD xmit function. It could be a burst of those packets, which can
> >     > be easily handled, but it could also be a single packet in a burst,
> >     > what is harder to handle.
> >     >
> >     > It would be simpler to just send that kind of packets, which will
> >     > likely be dropped by the hw at some point. The main problem is how
> >     > the fw/hw handles the DMA, because a dma read to a hypothetical 0x0
> >     > address could trigger an IOMMU error. It turns out, it is safe to
> >     > send a descriptor with packet size 0 to the hardware: the DMA never
> >     > happens, from the PCIe point of view.
> >     >
> >     > Signed-off-by: Alejandro Lucero <alejandro.lucero at netronome.com
> >     <mailto:alejandro.lucero at netronome.com>>
> >     > ---
> >     >  drivers/net/nfp/nfp_net.c | 17 ++++++++++++-----
> >     >  1 file changed, 12 insertions(+), 5 deletions(-)
> >     >
> >     > diff --git a/drivers/net/nfp/nfp_net.c b/drivers/net/nfp/nfp_net.c
> >     > index 92b03c4..679a91b 100644
> >     > --- a/drivers/net/nfp/nfp_net.c
> >     > +++ b/drivers/net/nfp/nfp_net.c
> >     > @@ -2094,7 +2094,7 @@ uint32_t nfp_net_txq_full(struct nfp_net_txq
> >     *txq)
> >     >                */
> >     >               pkt_size = pkt->pkt_len;
> >     >
> >     > -             while (pkt_size) {
> >     > +             while (pkt) {
> >     >                       /* Copying TSO, VLAN and cksum info */
> >     >                       *txds = txd;
> >     >
> >     > @@ -2126,17 +2126,24 @@ uint32_t nfp_net_txq_full(struct
> >     nfp_net_txq *txq)
> >     >                               txq->wr_p = 0;
> >     >
> >     >                       pkt_size -= dma_size;
> >     > -                     if (!pkt_size) {
> >     > +                     if (!pkt_size)
> >     >                               /* End of packet */
> >     >                               txds->offset_eop |= PCIE_DESC_TX_EOP;
> >     > -                     } else {
> >     > +                     else
> >     >                               txds->offset_eop &=
> >     PCIE_DESC_TX_OFFSET_MASK;
> >     > -                             pkt = pkt->next;
> >     > -                     }
> >     > +
> >     > +                     pkt = pkt->next;
> >     >                       /* Referencing next free TX descriptor */
> >     >                       txds = &txq->txds[txq->wr_p];
> >     >                       lmbuf = &txq->txbufs[txq->wr_p].mbuf;
> >     >                       issued_descs++;
> >     > +
> >     > +                     /* Double-checking if we have to use chained
> >     mbuf.
> >     > +                      * It seems there are some apps which could
> >     wrongly
> >     > +                      * have zeroed mbufs chained leading to send
> >     null
> >     > +                      * descriptors to the hw. */
> >     > +                     if (!pkt_size)
> >     > +                             break;
> >
> >     For the case chained mbufs with all are zero size [1], won't this
> cause
> >     next mbufs not freed because rte_pktmbuf_free_seg(*lmbuf) used?
> >
> >
> > Good point. Being honest, we had the problem with mbufs and size 0, and
> > this last check
> > was not initially there. But we saw performance being low after the
> > change, and the only thing
> > which could explain it was this sort of chained mbufs. There was not
> > mbuf allocation problem at
> > all. It was like more (null) packets being sent to the hardware now.
> > This last check solved the
> > performance problem.
>
> I assume performance problem is with the chained mbufs with 0 size, I
> believe this should be fixed in application, not in PMD level.
>
> And if application is sending chained mbufs with 0 size, with above code
> it will eventually be out off mbufs, since they are not freed, and same
> problem will occur that this patch is trying to avoid, but perhaps in
> longer run.
>
>
This is definitely an app problem and maybe that last check should be
avoided and to process that chained mbuf, whatever is it coming from, if
"pkt = pkt->next" is not null.

Are you OK of I send another version without that last if clause?



> >
> > Once I have said that, I have to admit my explanation implies some
> > serious problem when
> > handling mbufs, and something the app is doing really badly, so I could
> > understand someone
> > saying this is hidden a serious problem and should not be there.
> >
> >     [1]
> >     As you mentioned in the commit log, this not correct thing to do, but
> >     since patch is trying to harden PMD for this wrong application
> >     behavior..
> >
> >
> > If you consider this last check should not be there, I'll be glad to
> > remove it.
> >
> >
> >
> >     >               }
> >     >               i++;
> >     >       }
> >     >
> >
> >
>
>