Re: xdpsock poll syscall CPU 100%

[William Tu <u9012063@gmail.com>]

On Fri, Feb 21, 2020 at 12:46 AM Magnus Karlsson
<magnus.karlsson@gmail.com> wrote:
>
> On Fri, Feb 21, 2020 at 9:40 AM Eelco Chaudron <echaudro@redhat.com> wrote:
> >
> >
> >
> > On 21 Feb 2020, at 9:33, Magnus Karlsson wrote:
> >
> > > On Fri, Feb 21, 2020 at 9:30 AM Eelco Chaudron <echaudro@redhat.com>
> > > wrote:
> > >>
> > >>
> > >>
> > >> On 20 Feb 2020, at 23:49, William Tu wrote:
> > >>
> > >>> Hi,
> > >>>
> > >>> I'm trying to save some CPU cycles when there is no packet arrives.
> > >>> I enable the poll syscall option of xdpsock, by doing
> > >>>
> > >>> $ ./xdpsock -r -p -S -i ens16
> > >>>  sock0@ens160:0 rxdrop xdp-skb poll()
> > >>>                 pps         pkts        1.00
> > >>> rx              0           0
> > >>> tx              0           0
> > >>>
> > >>> Since there is no packet coming, I though by calling poll()
> > >>> system call, the xdpsock process will be blocked and CPU utilization
> > >>> should be way under 100%. However, I'm still seeing 100%
> > >>> CPU utilization. Am I understanding this correctly?
> > >>
> > >> Hi William, I can remember I saw this in the past two with this code.
> > >> It
> > >> had something to do with the way xdpsock waits for the buffers to be
> > >> free’ ed by the kernel. What I can remember it had something to do
> > >> with the veth interfaces also.
> > >>
> > >> I do remember that I fixed it in the tutorial for AF_XDP:
> > >> https://github.com/xdp-project/xdp-tutorial/blob/master/advanced03-AF_XDP/af_xdp_user.c
> > >
> > > Eelco,
> > >
> > > Do you remember exactly what you had to fix in the xdpsock sample?
> > > Your tutorial is quite a rewrite so it is hard for me to tell exactly
> > > which of all the changes that fix this problem. The reason I ask is
> > > that it would be nice to fix this in the sample too.
> > >
> > > Thanks: Magnus
> >
> >  From an earlier email conversation we had this is where it looped in my
> > case:
>
> Thanks Eelco. Yes, the xdpsock sample is too simplistic in this case.
> I will put this on my backlog to fix so that we do not have this
> problem in the future. I might take some inspiration from your code
> :-). Hope you do not mind.
>
> /Magnus
>
Hi Magnus and Eelco,

Thanks for your reply.
I run Eelco's af_xdp_user.c and indeed, with poll syscall, the CPU is
way under 100%. But I'm still figuring out the difference and where to
fix in xdpsock_user.c.

Regards,
William