Re: [PATCH net-next v6] net: xsk: introduce XDP_MAX_TX_BUDGET set/getsockopt

[Maciej Fijalkowski <maciej.fijalkowski@intel.com>]

On Sun, Jun 29, 2025 at 06:43:05PM +0800, Jason Xing wrote:
> On Sun, Jun 29, 2025 at 10:51 AM Jason Xing <kerneljasonxing@gmail.com> wrote:
> >
> > On Fri, Jun 27, 2025 at 7:01 PM Jason Xing <kerneljasonxing@gmail.com> 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_BUDGET socket option.
> > > - Convert TX_BATCH_SIZE to tx_budget_spent.
> > > - Set tx_budget_spent to 32 by default in the initialization phase as a
> > >   per-socket granular control. 32 is also the min value for
> > >   tx_budget_spent.
> > > - Set the range of tx_budget_spent 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_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_BUDGET, 2) socket sndbuf which is 212992 decided by
> > > net.core.wmem_default. As to XDP_MAX_TX_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
> >
> > More interesting numbers are arriving here as I run some benchmarks
> > from xdp-project/bpf-examples/AF_XDP-example/ in my VM.
> >
> > Running "sudo taskset -c 2 ./xdpsock -i eth0 -q 1 -l -N -t -b 256"

do you have a patch against xdpsock that does setsockopt you're
introducing here?

-B -b 256 was for enabling busy polling and giving it 256 budget, which is
not what you wanted to achieve.

> >
> > Using the default configure 32 as the max budget iteration:
> >  sock0@eth0:1 txonly xdp-drv
> >                    pps            pkts           1.01
> > rx                 0              0
> > tx                 48,574         49,152
> >
> > Enlarging the value to 256:
> >  sock0@eth0:1 txonly xdp-drv
> >                    pps            pkts           1.00
> > rx                 0              0
> > tx                 148,277        148,736
> >
> > Enlarging the value to 512:
> >  sock0@eth0:1 txonly xdp-drv
> >                    pps            pkts           1.00
> > rx                 0              0
> > tx                 226,306        227,072
> >
> > The performance of pps goes up by 365% (with max budget set as 512)
> > which is an incredible number :)
> 
> Weird thing. I purchased another VM and didn't manage to see such a
> huge improvement.... Good luck is that I own that good machine which
> is still reproducible and I'm still digging in it. So please ignore
> this noise for now :|
> 
> Thanks,
> Jason