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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> Acked-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com> Link: https://patch.msgid.link/20250704160138.48677-1-kerneljasonxing@gmail.com Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Why we want a copy of kernel headers in tools?
==============================================
There used to be no copies, with tools/ code using kernel headers
directly. From time to time tools/perf/ broke due to legitimate kernel
hacking. At some point Linus complained about such direct usage. Then we
adopted the current model.
The way these headers are used in perf are not restricted to just
including them to compile something.
There are sometimes used in scripts that convert defines into string
tables, etc, so some change may break one of these scripts, or new MSRs
may use some different #define pattern, etc.
E.g.:
$ ls -1 tools/perf/trace/beauty/*.sh | head -5
tools/perf/trace/beauty/arch_errno_names.sh
tools/perf/trace/beauty/drm_ioctl.sh
tools/perf/trace/beauty/fadvise.sh
tools/perf/trace/beauty/fsconfig.sh
tools/perf/trace/beauty/fsmount.sh
$
$ tools/perf/trace/beauty/fadvise.sh
static const char *fadvise_advices[] = {
[0] = "NORMAL",
[1] = "RANDOM",
[2] = "SEQUENTIAL",
[3] = "WILLNEED",
[4] = "DONTNEED",
[5] = "NOREUSE",
};
$
The tools/perf/check-headers.sh script, part of the tools/ build
process, points out changes in the original files.
So its important not to touch the copies in tools/ when doing changes in
the original kernel headers, that will be done later, when
check-headers.sh inform about the change to the perf tools hackers.
Another explanation from Ingo Molnar:
It's better than all the alternatives we tried so far:
- Symbolic links and direct #includes: this was the original approach but
was pushed back on from the kernel side, when tooling modified the
headers and broke them accidentally for kernel builds.
- Duplicate self-defined ABI headers like glibc: double the maintenance
burden, double the chance for mistakes, plus there's no tech-driven
notification mechanism to look at new kernel side changes.
What we are doing now is a third option:
- A software-enforced copy-on-write mechanism of kernel headers to
tooling, driven by non-fatal warnings on the tooling side build when
kernel headers get modified:
Warning: Kernel ABI header differences:
diff -u tools/include/uapi/drm/i915_drm.h include/uapi/drm/i915_drm.h
diff -u tools/include/uapi/linux/fs.h include/uapi/linux/fs.h
diff -u tools/include/uapi/linux/kvm.h include/uapi/linux/kvm.h
...
The tooling policy is to always pick up the kernel side headers as-is,
and integate them into the tooling build. The warnings above serve as a
notification to tooling maintainers that there's changes on the kernel
side.
We've been using this for many years now, and it might seem hacky, but
works surprisingly well.