netdev - Re: [PATCH net-next v6] net: xsk: introduce XDP_MAX_TX

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Message-ID: <CAL+tcoCSd_LA8w9ov7+_sOWLt3EU1rcqK8Sa6UF5S-xgfAGPnA@mail.gmail.com>
Date: Sun, 29 Jun 2025 10:51:57 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org, 
	pabeni@...hat.com, bjorn@...nel.org, magnus.karlsson@...el.com, 
	maciej.fijalkowski@...el.com, jonathan.lemon@...il.com, sdf@...ichev.me, 
	ast@...nel.org, daniel@...earbox.net, hawk@...nel.org, 
	john.fastabend@...il.com, joe@...a.to, willemdebruijn.kernel@...il.com
Cc: bpf@...r.kernel.org, netdev@...r.kernel.org, 
	Jason Xing <kernelxing@...cent.com>
Subject: Re: [PATCH net-next v6] net: xsk: introduce XDP_MAX_TX_BUDGET set/getsockopt

On Fri, Jun 27, 2025 at 7:01 PM Jason Xing <kerneljasonxing@...il.com> wrote:
>
> From: Jason Xing <kernelxing@...cent.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"

Using the default configure 32 as the max budget iteration:
 sock0@...0:1 txonly xdp-drv
                   pps            pkts           1.01
rx                 0              0
tx                 48,574         49,152

Enlarging the value to 256:
 sock0@...0:1 txonly xdp-drv
                   pps            pkts           1.00
rx                 0              0
tx                 148,277        148,736

Enlarging the value to 512:
 sock0@...0:1 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 :)

If the next-respin is needed, I will attach those convincing numbers
in the commit message.

Thanks,
Jason