Re: IRQ-CPU affinity for AF_XDP TX?

On Tue, Sep 10, 2024 at 9:12=E2=80=AFPM Magnus Karlsson
<[email protected]> wrote:
>
> On Tue, 10 Sept 2024 at 16:20, Srivats P <[email protected]> wrote:
> >
> > > For AF_XDP (in ZC or copy mode), the IRQ-CPU affinity plays a role in
> > > the RX direction to bind a CPU core to a particular NIC queue (mappin=
g
> > > the CPU core to the NIC queue's IRQ).
> > >
> > > My question is does the IRQ or IRQ-CPU affinity play any role in the
> > > TX direction?
> > >
> > > An xsk is specific to a NIC queue and assuming there is only one core
> > > doing TX on that xsk, is there some IRQ/CPU configuration recommended
> > > for better performance?
> > >
> > > Does the recommendation change if busy-polling mode is used or not
> > > used? ZC or non-ZC?
> >
> > Any advice or recommendation about IRQ-CPU affinity or usage for AF_XDP=
 TX path?
>
> For copy-mode, the majority of work is done in process context and
> just a minority of the work is performed in the interrupt routine.
> This is also true for zero-copy mode with busy-poll. In zero-copy mode
> without busy-poll however, it is the opposite. So irq affinity mainly
> matters in zero-copy mode without busy-poll.

Thank you Magnus - this is very useful information to know to tune the
application and system for high performance.

One follow up question - the busy-poll mode is an infra thing or is
there some driver specific work required to get the benefit of a
single core being used?

On an i40e I do see "xdpsock -t -B" using a single core, but on a mlx5
I still see "xdpcore -t -B" using 2 cores - one for the app userspace
and another for SoftIRQ.

Srivats


Srivats