netdev - Re: [PATCH intel-net] i40e: fix receiving of single packets in xsk zero-copy mode

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Date:   Fri, 19 Mar 2021 13:14:50 +0100
From:   Magnus Karlsson <magnus.karlsson@...il.com>
To:     Maciej Fijalkowski <maciej.fijalkowski@...el.com>
Cc:     "Karlsson, Magnus" <magnus.karlsson@...el.com>,
        intel-wired-lan <intel-wired-lan@...ts.osuosl.org>,
        anthony.l.nguyen@...el.com,
        Network Development <netdev@...r.kernel.org>,
        Björn Töpel <bjorn.topel@...il.com>,
        Sreedevi Joshi <sreedevi.joshi@...el.com>
Subject: Re: [PATCH intel-net] i40e: fix receiving of single packets in xsk
 zero-copy mode

On Fri, Mar 19, 2021 at 12:10 PM Maciej Fijalkowski
<maciej.fijalkowski@...el.com> wrote:
>
> On Fri, Mar 19, 2021 at 10:44:10AM +0100, Magnus Karlsson wrote:
> > From: Magnus Karlsson <magnus.karlsson@...el.com>
> >
> > Fix so that single packets are received immediately instead of in
> > batches of 8. If you sent 1 pss to a system, you received 8 packets
>
> pps?

Arghh, yes it should be pps, i.e. packets per second. I wonder what pss is?

> > every 8 seconds instead of 1 packet every second. The problem behind
> > this was that the work_done reporting from the Tx part of the driver
> > was broken. The work_done reporting in i40e controls not only the
> > reporting back to the napi logic but also the setting of the interrupt
> > throttling logic. When Tx or Rx reports that it has more to do,
> > interrupts are throttled or coalesced and when they both report that
> > they are done, interrupts are armed right away. If the wrong work_done
> > value is returned, the logic will start to throttle interrupts in a
> > situation where it should have just enabled them. This leads to the
> > undesired batching behavior seen in user-space.
> >
> > Fix this by returning the correct boolean value from the Tx xsk
> > zero-copy path. Return true if there is nothing to do or if we got
> > fewer packets to process than we asked for. Return false if we got as
> > many packets as the budget since there might be more packets we can
> > process.
> >
> > Fixes: 3106c580fb7c ("i40e: Use batched xsk Tx interfaces to increase performance")
> > Reported-by: Sreedevi Joshi <sreedevi.joshi@...el.com>
> > Signed-off-by: Magnus Karlsson <magnus.karlsson@...el.com>
>
> Acked-by: Maciej Fijalkowski <maciej.fijalkowski@...el.com>
>
> > ---
> >  drivers/net/ethernet/intel/i40e/i40e_xsk.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/net/ethernet/intel/i40e/i40e_xsk.c b/drivers/net/ethernet/intel/i40e/i40e_xsk.c
> > index fc32c5019b0f..12ca84113587 100644
> > --- a/drivers/net/ethernet/intel/i40e/i40e_xsk.c
> > +++ b/drivers/net/ethernet/intel/i40e/i40e_xsk.c
> > @@ -471,7 +471,7 @@ static bool i40e_xmit_zc(struct i40e_ring *xdp_ring, unsigned int budget)
> >
> >       nb_pkts = xsk_tx_peek_release_desc_batch(xdp_ring->xsk_pool, descs, budget);
> >       if (!nb_pkts)
> > -             return false;
> > +             return true;
> >
> >       if (xdp_ring->next_to_use + nb_pkts >= xdp_ring->count) {
> >               nb_processed = xdp_ring->count - xdp_ring->next_to_use;
> > @@ -488,7 +488,7 @@ static bool i40e_xmit_zc(struct i40e_ring *xdp_ring, unsigned int budget)
> >
> >       i40e_update_tx_stats(xdp_ring, nb_pkts, total_bytes);
> >
> > -     return true;
> > +     return nb_pkts < budget;
> >  }
> >
> >  /**
> >
> > base-commit: c79a707072fe3fea0e3c92edee6ca85c1e53c29f
> > --
> > 2.29.0
> >