Re: [PATCH bpf-next 05/10] ixgbe: xsk: terminate NAPI when XSK Rx queue gets full

Maciej Fijalkowski
2022-04-05T20:15:37Z

On Tue, Apr 05, 2022 at 02:36:41PM +0200, Jesper Dangaard Brouer wrote:
>
> On 05/04/2022 13.06, Maciej Fijalkowski wrote:
> > Correlate -ENOBUFS that was returned from xdp_do_redirect() with a XSK
> > Rx queue being full. In such case, terminate the softirq processing and
> > let the user space to consume descriptors from XSK Rx queue so that
> > there is room that driver can use later on.
> >
> > Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
> > ---
> > .../ethernet/intel/ixgbe/ixgbe_txrx_common.h | 1 +
> > drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c | 23 ++++++++++++-------
> > 2 files changed, 16 insertions(+), 8 deletions(-)
> >
> > diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe_txrx_common.h b/drivers/net/ethernet/intel/ixgbe/ixgbe_txrx_common.h
> > index bba3feaf3318..f1f69ce67420 100644
> > --- a/drivers/net/ethernet/intel/ixgbe/ixgbe_txrx_common.h
> > +++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_txrx_common.h
> > @@ -8,6 +8,7 @@
> > #define IXGBE_XDP_CONSUMED BIT(0)
> > #define IXGBE_XDP_TX BIT(1)
> > #define IXGBE_XDP_REDIR BIT(2)
> > +#define IXGBE_XDP_EXIT BIT(3)
> > #define IXGBE_TXD_CMD (IXGBE_TXD_CMD_EOP | \
> > IXGBE_TXD_CMD_RS)
> > diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c b/drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c
> > index dd7ff66d422f..475244a2c6e4 100644
> > --- a/drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c
> > +++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c
> > @@ -109,9 +109,10 @@ static int ixgbe_run_xdp_zc(struct ixgbe_adapter *adapter,
> > if (likely(act == XDP_REDIRECT)) {
> > err = xdp_do_redirect(rx_ring->netdev, xdp, xdp_prog);
> > - if (err)
> > - goto out_failure;
> > - return IXGBE_XDP_REDIR;
> > + if (!err)
> > + return IXGBE_XDP_REDIR;
> > + result = (err == -ENOBUFS) ? IXGBE_XDP_EXIT : IXGBE_XDP_CONSUMED;
> > + goto out_failure;
> > }
> > switch (act) {
> > @@ -130,6 +131,9 @@ static int ixgbe_run_xdp_zc(struct ixgbe_adapter *adapter,
> > if (result == IXGBE_XDP_CONSUMED)
> > goto out_failure;
> > break;
> > + case XDP_DROP:
> > + result = IXGBE_XDP_CONSUMED;
> > + break;
> > default:
> > bpf_warn_invalid_xdp_action(rx_ring->netdev, xdp_prog, act);
> > fallthrough;
> > @@ -137,9 +141,6 @@ static int ixgbe_run_xdp_zc(struct ixgbe_adapter *adapter,
> > out_failure:
> > trace_xdp_exception(rx_ring->netdev, xdp_prog, act);
> > fallthrough; /* handle aborts by dropping packet */
> > - case XDP_DROP:
> > - result = IXGBE_XDP_CONSUMED;
> > - break;
> > }
> > return result;
> > }
> > @@ -304,10 +305,16 @@ int ixgbe_clean_rx_irq_zc(struct ixgbe_q_vector *q_vector,
> > xdp_res = ixgbe_run_xdp_zc(adapter, rx_ring, bi->xdp);
> > if (xdp_res) {
> > - if (xdp_res & (IXGBE_XDP_TX | IXGBE_XDP_REDIR))
> > + if (xdp_res == IXGBE_XDP_EXIT) {
>
> Micro optimization note: Having this as the first if()-statement
> defaults the compiler to think this is the likely() case. (But compilers
> can obviously be smarter and can easily choose that the IXGBE_XDP_REDIR
> branch is so simple that it takes it as the likely case).
> Just wanted to mention this, given this is fash-path code.

Good point. Since we're 'likely-fying' redirect path in
ixgbe_run_xdp_zc(), maybe it would make sense to make the branch that does
xdp_res & IXGBE_XDP_REDIR check as the likely() one.

>
> > + failure = true;
> > + xsk_buff_free(bi->xdp);
> > + ixgbe_inc_ntc(rx_ring);
> > + break;
>
> I was wondering if we have a situation where we should set xdp_xmit bit
> to trigger the call to xdp_do_flush_map later in function, but I assume
> you have this covered.

For every previous successful redirect xdp_xmit will be set with
corresponding bits that came out of ixgbe_run_xdp_zc(), so if we got to
the point of full XSK Rx queue, xdp_do_flush_map() will be called
eventually. Before doing so, idea is to give the current buffer back to
the XSK buffer pool and increment the "next_to_clean" which acts as the
head pointer on HW Rx ring. IOW, consume the current buffer/descriptor and
yield the CPU to the user space.

>
> > + } else if (xdp_res & (IXGBE_XDP_TX | IXGBE_XDP_REDIR)) {
> > xdp_xmit |= xdp_res;
> > - else
> > + } else {
> > xsk_buff_free(bi->xdp);
> > + }
> > bi->xdp = NULL;
> > total_rx_packets++;
>