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

[Jason Xing <kerneljasonxing@gmail.com>]

On Fri, Jul 4, 2025 at 2:13 PM Paolo Abeni <pabeni@redhat.com> wrote:
>
> 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
> ----------------------------

Ah, I see... Sorry, I should have checked this.

I will post a v8 patch with this one and RCT issues solved.

Thanks,
Jason

>
> /P
>