[dpdk-dev] [PATCH] nfp: handle packets with length 0 as usual ones
Ferruh Yigit
ferruh.yigit at intel.com
Mon Aug 21 15:25:38 CEST 2017
On 8/21/2017 2:08 PM, Alejandro Lucero wrote:
>
>
> On Mon, Aug 21, 2017 at 11:34 AM, Ferruh Yigit <ferruh.yigit at intel.com
> <mailto: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>
> > <mailto: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>
> > <mailto: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?
Yes, thank you.
>
>
>
> >
> > 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++;
> > > }
> > >
> >
> >
>
>
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