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

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Message-ID: <aGZ3mJnFSsAxv7z6@boxer>
Date: Thu, 3 Jul 2025 14:29:12 +0200
From: Maciej Fijalkowski <maciej.fijalkowski@...el.com>
To: Jason Xing <kerneljasonxing@...il.com>
CC: Paolo Abeni <pabeni@...hat.com>, <davem@...emloft.net>,
	<edumazet@...gle.com>, <kuba@...nel.org>, <bjorn@...nel.org>,
	<magnus.karlsson@...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>,
	<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 Thu, Jul 03, 2025 at 04:22:21PM +0800, Jason Xing wrote:
> On Thu, Jul 3, 2025 at 4:15 PM Paolo Abeni <pabeni@...hat.com> wrote:
> >
> > 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.
> 
> Thanks. No problem.

Hey! i did review. Jason sorry but I got confused that you need to sort
out the performance results on your side, hence the silence.

> 
> >
> > Side note: it could be useful to extend the xdp selftest to trigger the
> > new code path.
> 
> Roger that, sir. I will do it after this gets merged, maybe later this
> month, still studying for various tests in recent days :)

IMHO nothing worth testing with this patch per-se, it's rather the matter
of performance.

I would like however to ask you for follow-up with patch against xdpsock
that adds support for using this new setsockopt (once we accept this onto
kernel).

> 
> Thanks,
> Jason