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

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Message-ID: <af16a28a-18b9-4d45-9ab9-1b150988b7d5@redhat.com>
Date: Thu, 3 Jul 2025 10:15:36 +0200
From: Paolo Abeni <pabeni@...hat.com>
To: Jason Xing <kerneljasonxing@...il.com>, davem@...emloft.net,
 edumazet@...gle.com, kuba@...nel.org, 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 6/27/25 1:01 PM, Jason Xing 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
> 
> [1]: https://lore.kernel.org/all/20250619093641.70700-1-kerneljasonxing@gmail.com/
> 
> Signed-off-by: Jason Xing <kernelxing@...cent.com>

LGTM, waiting a little more for an explicit an ack from XDP maintainers.

Side note: it could be useful to extend the xdp selftest to trigger the
new code path.

/P