Re: [dpdk-dev] [PATCH v1] net/af_xdp: support need wakeup feature

[Bruce Richardson <bruce.richardson@intel.com>]

On Mon, Jun 17, 2019 at 10:51:52AM +0200, David Marchand wrote:
> On Mon, Jun 17, 2019 at 10:45 AM Ye Xiaolong <xiaolong.ye@intel.com> wrote:
> 
> > On 06/17, David Marchand wrote:
> > >On Mon, Jun 17, 2019 at 9:42 AM Xiaolong Ye <xiaolong.ye@intel.com>
> > wrote:
> > >
> > >> This patch adds a new devarg to support the need_wakeup flag for Tx and
> > >> fill rings, when this flag is set by the driver, it means that the
> > >> userspace application has to explicitly wake up the kernel Rx or kernel
> > Tx
> > >> processing by issuing a syscall. Poll() can wake up both and sendto() or
> > >> its alternatives will wake up Tx processing only.
> > >>
> > >> This feature is to provide efficient support for case that application
> > and
> > >> driver are executing on the same core.
> > >>
> > >> Signed-off-by: Xiaolong Ye <xiaolong.ye@intel.com>
> > >> ---
> > >>
> > >> Original busy poll feature has morphed into need_wakeup flag in
> > >> kernel side, the main purpose is the same, that is to support both
> > >> application and driver executing on the same core efficiently.
> > >>
> > >> kernel side patchset can be found at netdev mailing list.
> > >>
> > >>
> > https://lore.kernel.org/netdev/CAJ8uoz2szX=+JXXAMyuVmvSsMXZuDqp6a8rjDQpTioxbZwxFmQ@mail.gmail.com/T/#t
> > >>
> > >> It is targeted for v5.3
> > >>
> > >
> > >- Is this really optional? Adding too many options is just a nightmare
> > >later...
> >
> > Hmm, I think we can remove this option and alway turn the need_wakeup flag
> > on
> > since it provides better performance for 1 core case and doesn't downgrage
> > the
> > 2 core case performance.
> >
> > >
> > >- I suppose this will break compilation with kernels that have af_xdp but
> > >are < 5.3.
> >
> > Yes, that is true. It will break the compilation with early kernel, I feel
> > it's
> > sort of common issue, we enable some features in dpdk that's based on
> > kernel
> > features, then kernel side features keep evolving, we need to keep the
> > pace,
> > but it will hurt the compatiblity with the old kernel.
> >
> > What's dpdk's convention for handling this kind of case? Add some notes in
> > doc
> > to reminder the prerequisite or use the KERNEL_VERSION macro in code?
> >
> 
> Rather than a kernel version, you can check that XDP_USE_NEED_WAKEUP is
> defined (present in the uapi kernel header).
> 
+1 for this.

Also, since AF_XDP is still fairly new with ongoing development on the
kernel side, I think it is reasonable to limit out PMD to only working with
sufficiently updated kernels. Hopefully in 6 months or so, the feature set
we need should be locked down and we can specify a fixed baseline
requirement.

/Bruce