RE: [PATCH] net/null: Add fast mbuf release TX offload

["Morten Brørup" <mb@smartsharesystems.com>]

> From: Varghese, Vipin [mailto:Vipin.Varghese@amd.com]
> Sent: Friday, 27 June 2025 14.07
> 
> [Public]
> 
> Hi Morten,
> 
> We have tested the effect of the patch using func-latency and PPs via
> testpmd.
> Please find our observations below
> 
>  - DPDK tag: 25.07-rc1
>  - compiler: gcc 14.2
>  - platform: AMD EPYC 8534P 64core 2.3GHz
>  - app cmd:
>  -- One port: ` sudo build/app/dpdk-testpmd -l 15,16  --vdev=net_null1 -
> -no-pci  -- --nb-cores=1 --nb-ports=1 --txq=1 --rxq=1 --txd=2048 --
> rxd=2048  -a --forward-mode=io --stats-period=1`
>  -- Two port: ` sudo build/app/dpdk-testpmd -l 15,16,17  --
> vdev=net_null1 --vdev=net_null2 --no-pci  -- --nb-cores=2 --nb-ports=2 -
> -txq=1 --rxq=1 --txd=2048 --rxd=2048  -a --forward-mode=io --stats-
> period=1`
> 
> Result 1 port:
>  - Before patch: TX MPPs 117.61, RX-PPs 117.67, Func-latency TX: 1918ns,
> Func-latency free-bulk: 2667ns
>  - After patch: TX MPPs 117.55, RX-PPs 117.54, Func-latency TX: 1921ns,
> Func-latency free-bulk: 2660ns
> 
> Result 2 port:
>  - Before patch: TX MPPs 117.61, RX-PPs 117.67, Func-latency TX: 1942ns,
> Func-latency free-bulk: 2557ns
>  - After patch: TX MPPs 117.54, RX-PPs 117.54, Func-latency TX: 1946ns,
> Func-latency free-bulk: 2740ns
> 
> Perf Top: diff before vs after shows 13.84% vs 13.79%
> 
> Reviewed-by: Thiyagarjan P <Thiyagarajan.P@amd.com>
> Tested-by: Vipin Varghese <Vipin.Varghese@amd.com>

Thank you for reviewing and testing.

> 
> Clarification request: `with fast-mbuf-free on single port we see free-
> bulk reduction by -7ns. But null_tx increase by +3ns. TX PPs reduction
> by 0.07 Mpps. Is this anomaly of null_net PMD?`

I have finally found the bug in my patch:
It announces device-level capability for FAST_FREE, but ignores device-level FAST_FREE configuration, and uses queue-level FAST_FREE configuration instead.

Due to this bug, your testing probably shows the performance of the non-FAST_FREE code path.
The added comparison for FAST_FREE (code path not taken) might explain the null_tx +3ns increase.

I will send a v2 patch.

> 
> > >
> > > On Tue, 24 Jun 2025 18:14:16 +0000
> > > Morten Brørup <mb@smartsharesystems.com> wrote:
> > >
> > > > Added fast mbuf release, re-using the existing mbuf pool pointer
> in
> > > > the queue structure.
> > > >
> > > > Signed-off-by: Morten Brørup <mb@smartsharesystems.com>
> > >
> > > Makes sense.
> > >
> > > > ---
> > > >  drivers/net/null/rte_eth_null.c | 30 +++++++++++++++++++++++++++-
> --
> > > >  1 file changed, 27 insertions(+), 3 deletions(-)
> > > >
> > > > diff --git a/drivers/net/null/rte_eth_null.c
> > > b/drivers/net/null/rte_eth_null.c
> > > > index 8a9b74a03b..12c0d8d1ff 100644
> > > > --- a/drivers/net/null/rte_eth_null.c
> > > > +++ b/drivers/net/null/rte_eth_null.c
> > > > @@ -34,6 +34,17 @@ struct pmd_internals;  struct null_queue {
> > > >     struct pmd_internals *internals;
> > > >
> > > > +   /**
> > > > +    * For RX queue:
> > > > +    *  Mempool to allocate mbufs from.
> > > > +    *
> > > > +    * For TX queue:
> > > > +    *  Mempool to free mbufs to, if fast release of mbufs is
> enabled.
> > > > +    *  UINTPTR_MAX if the mempool for fast release of mbufs has
> not
> > > yet been detected.
> > > > +    *  NULL if fast release of mbufs is not enabled.
> > > > +    *
> > > > +    *  @see RTE_ETH_TX_OFFLOAD_MBUF_FAST_FREE
> > > > +    */
> > > >     struct rte_mempool *mb_pool;
> > >
> > > Do all drivers to it this way?
> >
> > No, I think most drivers have separate structures for rx and tx
> queues. This driver
> > doesn't so I'm reusing the existing mempool pointer.
> > Also, they don't cache the mempool pointer, but look at mbuf[0].pool
> at every burst;
> > so their tx queue structure doesn't have a mempool pointer field.
> > And they check an offload flag (either the bit in the raw offload
> field, or a shadow
> > variable for the relevant offload flag), instead of checking the
> mempool pointer.
> >
> > Other drivers can be improved, and I have submitted an optimization
> patch for the
> > i40e driver with some of the things I do in this patch:
> > https://inbox.dpdk.org/dev/20250624061238.89259-1-
> > mb@smartsharesystems.com/
> >
> > > Is it documented in ethdev?
> >
> > The RTE_ETH_TX_OFFLOAD_MBUF_FAST_FREE flag is documented.
> > How to implement it in the drivers is not.
> >
> > -Morten