Message-ID: <24044113385d101cc2c3833a01d0ca93482214d1.camel@nvidia.com>
Date:   Fri, 19 Aug 2022 08:35:29 +0000
From:   Maxim Mikityanskiy <maximmi@...dia.com>
To:     "bjorn@...nel.org" <bjorn@...nel.org>,
        "maciej.fijalkowski@...el.com" <maciej.fijalkowski@...el.com>,
        "magnus.karlsson@...el.com" <magnus.karlsson@...el.com>
CC:     "daniel@...earbox.net" <daniel@...earbox.net>,
        Saeed Mahameed <saeedm@...dia.com>,
        "alexandr.lobakin@...el.com" <alexandr.lobakin@...el.com>,
        "ast@...nel.org" <ast@...nel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "kuba@...nel.org" <kuba@...nel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>
Subject: Re: [PATCH v2 bpf-next 00/14] xsk: stop NAPI Rx processing on full
 XSK RQ

Hi Maciej,

On Wed, 2022-04-13 at 17:30 +0200, Maciej Fijalkowski wrote:
> v2:
> - add likely for internal redirect return codes in ice and ixgbe
>   (Jesper)
> - do not drop the buffer that head pointed to at full XSK RQ (Maxim)

I found an issue with this approach. If you don't drop that packet,
you'll need to run the XDP program for the same packet again. If the
XDP program is anything more complex than "redirect-everything-to-XSK",
it will get confused, for example, if it tracks any state or counts
anything.

We can't check whether there is enough space in the XSK RX ring before
running the XDP program, as we don't know in advance which XSK socket
will be selected.

We can't store bpf_redirect_info across NAPI calls to avoid running the
XDP program second time, as bpf_redirect_info is protected by RCU and
the assumption that the whole XDP_REDIRECT operation happens within one
NAPI cycle.

I see the following options:

1. Drop the packet when an overflow happens. The problem is that it
will happen systematically, but it's still better than the old behavior
(drop mulitple packets when an overflow happens and hog the CPU).

2. Make this feature opt-in. If the application opts in, it guarantees
to take measures to handle duplicate packets in XDP properly.

3. Require the XDP program to indicate it supports being called
multiple times for the same packet and pass a flag to it if it's a
repeated run. Drop the packet in the driver if the XDP program doesn't
indicate this support. The indication can be done similar to how it's
implemented for XDP multi buffer.

Thoughts? Other options?

Thanks,
Max

> - terminate Rx loop only when need_wakeup feature is enabled (Maxim)
> - reword from 'stop softirq processing' to 'stop NAPI Rx processing'
> - s/ENXIO/EINVAL in mlx5 and stmmac's ndo_xsk_wakeup to keep it
>   consitent with Intel's drivers (Maxim)
> - include Jesper's Acks
> 
> Hi!
> 
> This is a revival of Bjorn's idea [0] to break NAPI loop when XSK Rx
> queue gets full which in turn makes it impossible to keep on
> successfully producing descriptors to XSK Rx ring. By breaking out of
> the driver side immediately we will give the user space opportunity for
> consuming descriptors from XSK Rx ring and therefore provide room in the
> ring so that HW Rx -> XSK Rx redirection can be done.
> 
> Maxim asked and Jesper agreed on simplifying Bjorn's original API used
> for detecting the event of interest, so let's just simply check for
> -ENOBUFS within Intel's ZC drivers after an attempt to redirect a buffer
> to XSK Rx. No real need for redirect API extension.
> 
> One might ask why it is still relevant even after having proper busy
> poll support in place - here is the justification.
> 
> For xdpsock that was:
> - run for l2fwd scenario,
> - app/driver processing took place on the same core in busy poll
>   with 2048 budget,
> - HW ring sizes Tx 256, Rx 2048,
> 
> this work improved throughput by 78% and reduced Rx queue full statistic
> bump by 99%.
> 
> For testing ice, make sure that you have [1] present on your side.
> 
> This set, besides the work described above, carries also improvements
> around return codes in various XSK paths and lastly a minor optimization
> for xskq_cons_has_entries(), a helper that might be used when XSK Rx
> batching would make it to the kernel.
> 
> Link to v1 and discussion around it is at [2].
> 
> Thanks!
> MF
> 
> [0]: https://lore.kernel.org/bpf/20200904135332.60259-1-bjorn.topel@gmail.com/
> [1]: https://lore.kernel.org/netdev/20220317175727.340251-1-maciej.fijalkowski@intel.com/
> [2]: https://lore.kernel.org/bpf/5864171b-1e08-1b51-026e-5f09b8945076@nvidia.com/T/
> 
> Björn Töpel (1):
>   xsk: improve xdp_do_redirect() error codes
> 
> Maciej Fijalkowski (13):
>   xsk: diversify return codes in xsk_rcv_check()
>   ice: xsk: decorate ICE_XDP_REDIR with likely()
>   ixgbe: xsk: decorate IXGBE_XDP_REDIR with likely()
>   ice: xsk: terminate Rx side of NAPI when XSK Rx queue gets full
>   i40e: xsk: terminate Rx side of NAPI when XSK Rx queue gets full
>   ixgbe: xsk: terminate Rx side of NAPI when XSK Rx queue gets full
>   ice: xsk: diversify return values from xsk_wakeup call paths
>   i40e: xsk: diversify return values from xsk_wakeup call paths
>   ixgbe: xsk: diversify return values from xsk_wakeup call paths
>   mlx5: xsk: diversify return values from xsk_wakeup call paths
>   stmmac: xsk: diversify return values from xsk_wakeup call paths
>   ice: xsk: avoid refilling single Rx descriptors
>   xsk: drop ternary operator from xskq_cons_has_entries
> 
>  .../ethernet/intel/i40e/i40e_txrx_common.h    |  1 +
>  drivers/net/ethernet/intel/i40e/i40e_xsk.c    | 38 ++++++++-----
>  drivers/net/ethernet/intel/ice/ice_txrx.h     |  1 +
>  drivers/net/ethernet/intel/ice/ice_xsk.c      | 53 ++++++++++++-------
>  .../ethernet/intel/ixgbe/ixgbe_txrx_common.h  |  1 +
>  drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c  | 52 ++++++++++--------
>  .../ethernet/mellanox/mlx5/core/en/xsk/tx.c   |  2 +-
>  .../net/ethernet/stmicro/stmmac/stmmac_main.c |  4 +-
>  net/xdp/xsk.c                                 |  4 +-
>  net/xdp/xsk_queue.h                           |  4 +-
>  10 files changed, 99 insertions(+), 61 deletions(-)
> 

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