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

From: Maciej Fijalkowski <[email-masked]>
Date: Tue, 5 Apr 2022 15:52:36 +0200

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 <[email-masked]>
> > ---
> >   .../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++;
>