Re: [PATCH net-next v7] net: xsk: introduce XDP_MAX_TX_SKB_BUDGET setsockopt

[Paolo Abeni <pabeni@redhat.com>]

On 7/3/25 4:50 PM, Jason Xing wrote:
> From: Jason Xing <kernelxing@tencent.com>
> 
> This patch provides a setsockopt method to let applications leverage to
> adjust how many descs to be handled at most in one send syscall. It
> mitigates the situation where the default value (32) that is too small
> leads to higher frequency of triggering send syscall.
> 
> Considering the prosperity/complexity the applications have, there is no
> absolutely ideal suggestion fitting all cases. So keep 32 as its default
> value like before.
> 
> The patch does the following things:
> - Add XDP_MAX_TX_SKB_BUDGET socket option.
> - Set max_tx_budget to 32 by default in the initialization phase as a
>   per-socket granular control.
> - Set the range of max_tx_budget as [32, xs->tx->nentries].
> 
> The idea behind this comes out of real workloads in production. We use a
> user-level stack with xsk support to accelerate sending packets and
> minimize triggering syscalls. When the packets are aggregated, it's not
> hard to hit the upper bound (namely, 32). The moment user-space stack
> fetches the -EAGAIN error number passed from sendto(), it will loop to try
> again until all the expected descs from tx ring are sent out to the driver.
> Enlarging the XDP_MAX_TX_SKB_BUDGET value contributes to less frequency of
> sendto() and higher throughput/PPS.
> 
> Here is what I did in production, along with some numbers as follows:
> For one application I saw lately, I suggested using 128 as max_tx_budget
> because I saw two limitations without changing any default configuration:
> 1) XDP_MAX_TX_SKB_BUDGET, 2) socket sndbuf which is 212992 decided by
> net.core.wmem_default. As to XDP_MAX_TX_SKB_BUDGET, the scenario behind
> this was I counted how many descs are transmitted to the driver at one
> time of sendto() based on [1] patch and then I calculated the
> possibility of hitting the upper bound. Finally I chose 128 as a
> suitable value because 1) it covers most of the cases, 2) a higher
> number would not bring evident results. After twisting the parameters,
> a stable improvement of around 4% for both PPS and throughput and less
> resources consumption were found to be observed by strace -c -p xxx:
> 1) %time was decreased by 7.8%
> 2) error counter was decreased from 18367 to 572
> 
> [1]: https://lore.kernel.org/all/20250619093641.70700-1-kerneljasonxing@gmail.com/
> 
> Signed-off-by: Jason Xing <kernelxing@tencent.com>
> ---
> v7
> Link: https://lore.kernel.org/all/20250627110121.73228-1-kerneljasonxing@gmail.com/
> 1. use 'copy mode' in Doc
> 2. move init of max_tx_budget to a proper position
> 3. use the max value in the if condition in setsockopt
> 4. change sockopt name to XDP_MAX_TX_SKB_BUDGET
> 5. set MAX_PER_SOCKET_BUDGET to 32 instead of TX_BATCH_SIZE because they
>    have no correlation at all.
> 
> v6
> Link: https://lore.kernel.org/all/20250625123527.98209-1-kerneljasonxing@gmail.com/
> 1. use [32, xs->tx->nentries] range
> 2. Since setsockopt may generate a different value, add getsockopt to help
>    application know what value takes effect finally.
> 
> v5
> Link: https://lore.kernel.org/all/20250623021345.69211-1-kerneljasonxing@gmail.com/
> 1. remove changes around zc mode
> 
> v4
> Link: https://lore.kernel.org/all/20250619090440.65509-1-kerneljasonxing@gmail.com/
> 1. remove getsockopt as it seems no real use case.
> 2. adjust the position of max_tx_budget to make sure it stays with other
> read-most fields in one cacheline.
> 3. set one as the lower bound of max_tx_budget
> 4. add more descriptions/performance data in Doucmentation and commit message.
> 
> V3
> Link: https://lore.kernel.org/all/20250618065553.96822-1-kerneljasonxing@gmail.com/
> 1. use a per-socket control (suggested by Stanislav)
> 2. unify both definitions into one
> 3. support setsockopt and getsockopt
> 4. add more description in commit message
> 
> V2
> Link: https://lore.kernel.org/all/20250617002236.30557-1-kerneljasonxing@gmail.com/
> 1. use a per-netns sysctl knob
> 2. use sysctl_xsk_max_tx_budget to unify both definitions.
> ---
>  Documentation/networking/af_xdp.rst |  9 +++++++++
>  include/net/xdp_sock.h              |  1 +
>  include/uapi/linux/if_xdp.h         |  1 +
>  net/xdp/xsk.c                       | 21 +++++++++++++++++++--
>  tools/include/uapi/linux/if_xdp.h   |  1 +
>  5 files changed, 31 insertions(+), 2 deletions(-)
> 
> diff --git a/Documentation/networking/af_xdp.rst b/Documentation/networking/af_xdp.rst
> index dceeb0d763aa..95ff1836e5c6 100644
> --- a/Documentation/networking/af_xdp.rst
> +++ b/Documentation/networking/af_xdp.rst
> @@ -442,6 +442,15 @@ is created by a privileged process and passed to a non-privileged one.
>  Once the option is set, kernel will refuse attempts to bind that socket
>  to a different interface.  Updating the value requires CAP_NET_RAW.
>  
> +XDP_MAX_TX_SKB_BUDGET setsockopt
> +----------------------------

I'm sorry, kdoc is not happy about the above:

/home/doc-build/testing/Documentation/networking/af_xdp.rst:442:
WARNING: Title underline too short.XDP_MAX_TX_SKB_BUDGET setsockopt
----------------------------

/P